/usr/share/doc/glibc-doc/html/libc_16.html is in glibc-doc 2.15-0ubuntu10.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125 1126 1127 1128 1129 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156 1157 1158 1159 1160 1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197 1198 1199 1200 1201 1202 1203 1204 1205 1206 1207 1208 1209 1210 1211 1212 1213 1214 1215 1216 1217 1218 1219 1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233 1234 1235 1236 1237 1238 1239 1240 1241 1242 1243 1244 1245 1246 1247 1248 1249 1250 1251 1252 1253 1254 1255 1256 1257 1258 1259 1260 1261 1262 1263 1264 1265 1266 1267 1268 1269 1270 1271 1272 1273 1274 1275 1276 1277 1278 1279 1280 1281 1282 1283 1284 1285 1286 1287 1288 1289 1290 1291 1292 1293 1294 1295 1296 1297 1298 1299 1300 1301 1302 1303 1304 1305 1306 1307 1308 1309 1310 1311 1312 1313 1314 1315 1316 1317 1318 1319 1320 1321 1322 1323 1324 1325 1326 1327 1328 1329 1330 1331 1332 1333 1334 1335 1336 1337 1338 1339 1340 1341 1342 1343 1344 1345 1346 1347 1348 1349 1350 1351 1352 1353 1354 1355 1356 1357 1358 1359 1360 1361 1362 1363 1364 1365 1366 1367 1368 1369 1370 1371 1372 1373 1374 1375 1376 1377 1378 1379 1380 1381 1382 1383 1384 1385 1386 1387 1388 1389 1390 1391 1392 1393 1394 1395 1396 1397 1398 1399 1400 1401 1402 1403 1404 1405 1406 1407 1408 1409 1410 1411 1412 1413 1414 1415 1416 1417 1418 1419 1420 1421 1422 1423 1424 1425 1426 1427 1428 1429 1430 1431 1432 1433 1434 1435 1436 1437 1438 1439 1440 1441 1442 1443 1444 1445 1446 1447 1448 1449 1450 1451 1452 1453 1454 1455 1456 1457 1458 1459 1460 1461 1462 1463 1464 1465 1466 1467 1468 1469 1470 1471 1472 1473 1474 1475 1476 1477 1478 1479 1480 1481 1482 1483 1484 1485 1486 1487 1488 1489 1490 1491 1492 1493 1494 1495 1496 1497 1498 1499 1500 1501 1502 1503 1504 1505 1506 1507 1508 1509 1510 1511 1512 1513 1514 1515 1516 1517 1518 1519 1520 1521 1522 1523 1524 1525 1526 1527 1528 1529 1530 1531 1532 1533 1534 1535 1536 1537 1538 1539 1540 1541 1542 1543 1544 1545 1546 1547 1548 1549 1550 1551 1552 1553 1554 1555 1556 1557 1558 1559 1560 1561 1562 1563 1564 1565 1566 1567 1568 1569 1570 1571 1572 1573 1574 1575 1576 1577 1578 1579 1580 1581 1582 1583 1584 1585 1586 1587 1588 1589 1590 1591 1592 1593 1594 1595 1596 1597 1598 1599 1600 1601 1602 1603 1604 1605 1606 1607 1608 1609 1610 1611 1612 1613 1614 1615 1616 1617 1618 1619 1620 1621 1622 1623 1624 1625 1626 1627 1628 1629 1630 1631 1632 1633 1634 1635 1636 1637 1638 1639 1640 1641 1642 1643 1644 1645 1646 1647 1648 1649 1650 1651 1652 1653 1654 1655 1656 1657 1658 1659 1660 1661 1662 1663 1664 1665 1666 1667 1668 1669 1670 1671 1672 1673 1674 1675 1676 1677 1678 1679 1680 1681 1682 1683 1684 1685 1686 1687 1688 1689 1690 1691 1692 1693 1694 1695 1696 1697 1698 1699 1700 1701 1702 1703 1704 1705 1706 1707 1708 1709 1710 1711 1712 1713 1714 1715 1716 1717 1718 1719 1720 1721 1722 1723 1724 1725 1726 1727 1728 1729 1730 1731 1732 1733 1734 1735 1736 1737 1738 1739 1740 1741 1742 1743 1744 1745 1746 1747 1748 1749 1750 1751 1752 1753 1754 1755 1756 1757 1758 1759 1760 1761 1762 1763 1764 1765 1766 1767 1768 1769 1770 1771 1772 1773 1774 1775 1776 1777 1778 1779 1780 1781 1782 1783 1784 1785 1786 1787 1788 1789 1790 1791 1792 1793 1794 1795 1796 1797 1798 1799 1800 1801 1802 1803 1804 1805 1806 1807 1808 1809 1810 1811 1812 1813 1814 1815 1816 1817 1818 1819 1820 1821 1822 1823 1824 1825 1826 1827 1828 1829 1830 1831 1832 1833 1834 1835 1836 1837 1838 1839 1840 1841 1842 1843 1844 1845 1846 1847 1848 1849 1850 1851 1852 1853 1854 1855 1856 1857 1858 1859 1860 1861 1862 1863 1864 1865 1866 1867 1868 1869 1870 1871 1872 1873 1874 1875 1876 1877 1878 1879 1880 1881 1882 1883 1884 1885 1886 1887 1888 1889 1890 1891 1892 1893 1894 1895 1896 1897 1898 1899 1900 1901 1902 1903 1904 1905 1906 1907 1908 1909 1910 1911 1912 1913 1914 1915 1916 1917 1918 1919 1920 1921 1922 1923 1924 1925 1926 1927 1928 1929 1930 1931 1932 1933 1934 1935 1936 1937 1938 1939 1940 1941 1942 1943 1944 1945 1946 1947 1948 1949 1950 1951 1952 1953 1954 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028 2029 2030 2031 2032 2033 2034 2035 2036 2037 2038 2039 2040 2041 2042 2043 2044 2045 2046 2047 2048 2049 2050 2051 2052 2053 2054 2055 2056 2057 2058 2059 2060 2061 2062 2063 2064 2065 2066 2067 2068 2069 2070 2071 2072 2073 2074 2075 2076 2077 2078 2079 2080 2081 2082 2083 2084 2085 2086 2087 2088 2089 2090 2091 2092 2093 2094 2095 2096 2097 2098 2099 2100 2101 2102 2103 2104 2105 2106 2107 2108 2109 2110 2111 2112 2113 2114 2115 2116 2117 2118 2119 2120 2121 2122 2123 2124 2125 2126 2127 2128 2129 2130 2131 2132 2133 2134 2135 2136 2137 2138 2139 2140 2141 2142 2143 2144 2145 2146 2147 2148 2149 2150 2151 2152 2153 2154 2155 2156 2157 2158 2159 2160 2161 2162 2163 2164 2165 2166 2167 2168 2169 2170 2171 2172 2173 2174 2175 2176 2177 2178 2179 2180 2181 2182 2183 2184 2185 2186 2187 2188 2189 2190 2191 2192 2193 2194 2195 2196 2197 2198 2199 2200 2201 2202 2203 2204 2205 2206 2207 2208 2209 2210 2211 2212 2213 2214 2215 2216 2217 2218 2219 2220 2221 2222 2223 2224 2225 2226 2227 2228 2229 2230 2231 2232 2233 2234 2235 2236 2237 2238 2239 2240 2241 2242 2243 2244 2245 2246 2247 2248 2249 2250 2251 2252 2253 2254 2255 2256 2257 2258 2259 2260 2261 2262 2263 2264 2265 2266 2267 2268 2269 2270 2271 2272 2273 2274 2275 2276 2277 2278 2279 2280 2281 2282 2283 2284 2285 2286 2287 2288 2289 2290 2291 2292 2293 2294 2295 2296 2297 2298 2299 2300 2301 2302 2303 2304 2305 2306 2307 2308 2309 2310 2311 2312 2313 2314 2315 2316 2317 2318 2319 2320 2321 2322 2323 2324 2325 2326 2327 2328 2329 2330 2331 2332 2333 2334 2335 2336 2337 2338 2339 2340 2341 2342 2343 2344 2345 2346 2347 2348 2349 2350 2351 2352 2353 2354 2355 2356 2357 2358 2359 2360 2361 2362 2363 2364 2365 2366 2367 2368 2369 2370 2371 2372 2373 2374 2375 2376 2377 2378 2379 2380 2381 2382 2383 2384 2385 2386 2387 2388 2389 2390 2391 2392 2393 2394 2395 2396 2397 2398 2399 2400 2401 2402 2403 2404 2405 2406 2407 2408 2409 2410 2411 2412 2413 2414 2415 2416 2417 2418 2419 2420 2421 2422 2423 2424 2425 2426 2427 2428 2429 2430 2431 2432 2433 2434 2435 2436 2437 2438 2439 2440 2441 2442 2443 2444 2445 2446 2447 2448 2449 2450 2451 2452 2453 2454 2455 2456 2457 2458 2459 2460 2461 2462 2463 2464 2465 2466 2467 2468 2469 2470 2471 2472 2473 2474 2475 2476 2477 2478 2479 2480 2481 2482 2483 2484 2485 2486 2487 2488 2489 2490 2491 2492 2493 2494 2495 2496 2497 2498 2499 2500 2501 2502 2503 2504 2505 2506 2507 2508 2509 2510 2511 2512 2513 2514 2515 2516 2517 2518 2519 2520 2521 2522 2523 2524 2525 2526 2527 2528 2529 2530 2531 2532 2533 2534 2535 2536 2537 2538 2539 2540 2541 2542 2543 2544 2545 2546 2547 2548 2549 2550 2551 2552 2553 2554 2555 2556 2557 2558 2559 2560 2561 2562 2563 2564 2565 2566 2567 2568 2569 2570 2571 2572 2573 2574 2575 2576 2577 2578 2579 2580 2581 2582 2583 2584 2585 2586 2587 2588 2589 2590 2591 2592 2593 2594 2595 2596 2597 2598 2599 2600 2601 2602 2603 2604 2605 2606 2607 2608 2609 2610 2611 2612 2613 2614 2615 2616 2617 2618 2619 2620 2621 2622 2623 2624 2625 2626 2627 2628 2629 2630 2631 2632 2633 2634 2635 2636 2637 2638 2639 2640 2641 2642 2643 2644 2645 2646 2647 2648 2649 2650 2651 2652 2653 2654 2655 2656 2657 2658 2659 2660 2661 2662 2663 2664 2665 2666 2667 2668 2669 2670 2671 2672 2673 2674 2675 2676 2677 2678 2679 2680 2681 2682 2683 2684 2685 2686 2687 2688 2689 2690 2691 2692 2693 2694 2695 2696 2697 2698 2699 2700 2701 2702 2703 2704 2705 2706 2707 2708 2709 2710 2711 2712 2713 2714 2715 2716 2717 2718 2719 2720 2721 2722 2723 2724 2725 2726 2727 2728 2729 2730 2731 2732 2733 2734 2735 2736 2737 2738 2739 2740 2741 2742 2743 2744 2745 2746 2747 2748 2749 2750 2751 2752 2753 2754 2755 2756 2757 2758 2759 2760 2761 2762 2763 2764 2765 2766 2767 2768 2769 2770 2771 2772 2773 2774 2775 2776 2777 2778 2779 2780 2781 2782 2783 2784 2785 2786 2787 2788 2789 2790 2791 2792 2793 2794 2795 2796 2797 2798 2799 2800 2801 2802 2803 2804 2805 2806 2807 2808 2809 2810 2811 2812 2813 2814 2815 2816 2817 2818 2819 2820 2821 2822 2823 2824 2825 2826 2827 2828 2829 2830 2831 2832 2833 2834 2835 2836 2837 2838 2839 2840 2841 2842 2843 2844 2845 2846 2847 2848 2849 2850 2851 2852 2853 2854 2855 2856 2857 2858 2859 2860 2861 2862 2863 2864 2865 2866 2867 2868 2869 2870 2871 2872 2873 2874 2875 2876 2877 2878 2879 2880 2881 2882 2883 2884 2885 2886 2887 2888 2889 2890 2891 2892 2893 2894 2895 2896 2897 2898 2899 2900 2901 2902 2903 2904 2905 2906 2907 2908 2909 2910 2911 2912 2913 2914 2915 2916 2917 2918 2919 2920 2921 2922 2923 2924 2925 2926 2927 2928 2929 2930 2931 2932 2933 2934 2935 2936 2937 2938 2939 2940 2941 2942 2943 2944 2945 2946 2947 2948 2949 2950 2951 2952 2953 2954 2955 2956 2957 2958 2959 2960 2961 2962 2963 2964 2965 2966 2967 2968 2969 2970 2971 2972 2973 2974 2975 2976 2977 2978 2979 2980 2981 2982 2983 2984 2985 2986 2987 2988 2989 2990 2991 2992 2993 2994 2995 2996 2997 2998 2999 3000 3001 3002 3003 3004 3005 3006 3007 3008 3009 3010 3011 3012 3013 3014 3015 3016 3017 3018 3019 3020 3021 3022 3023 3024 3025 3026 3027 3028 3029 3030 3031 3032 3033 3034 3035 3036 3037 3038 3039 3040 3041 3042 3043 3044 3045 3046 3047 3048 3049 3050 3051 3052 3053 3054 3055 3056 3057 3058 3059 3060 3061 3062 3063 3064 3065 3066 3067 3068 3069 3070 3071 3072 3073 3074 3075 3076 3077 3078 3079 3080 3081 3082 3083 3084 3085 3086 3087 3088 3089 3090 3091 3092 3093 3094 3095 3096 3097 3098 3099 3100 3101 3102 3103 3104 3105 3106 3107 3108 3109 3110 3111 3112 3113 3114 3115 3116 3117 3118 3119 3120 3121 3122 3123 3124 3125 3126 3127 3128 3129 3130 3131 3132 3133 3134 3135 3136 3137 3138 3139 3140 3141 3142 3143 3144 3145 3146 3147 3148 3149 3150 3151 3152 3153 3154 3155 3156 3157 3158 3159 3160 3161 3162 3163 3164 3165 3166 3167 3168 3169 3170 3171 3172 3173 3174 3175 3176 3177 3178 3179 3180 3181 3182 3183 3184 3185 3186 3187 3188 3189 3190 3191 3192 3193 3194 3195 3196 3197 3198 3199 3200 3201 3202 3203 3204 3205 3206 3207 3208 3209 3210 3211 3212 3213 3214 3215 3216 3217 3218 3219 3220 3221 3222 3223 3224 3225 3226 3227 3228 3229 3230 3231 3232 3233 3234 3235 3236 3237 3238 3239 3240 3241 3242 3243 3244 3245 3246 3247 3248 3249 3250 3251 3252 3253 3254 3255 3256 3257 3258 3259 3260 3261 3262 3263 3264 3265 3266 3267 3268 3269 3270 3271 3272 3273 3274 3275 3276 3277 3278 3279 3280 3281 3282 3283 3284 3285 3286 3287 3288 3289 3290 3291 3292 3293 3294 3295 3296 3297 3298 3299 3300 3301 3302 3303 3304 3305 3306 3307 3308 3309 3310 3311 3312 3313 3314 3315 3316 3317 3318 3319 3320 3321 3322 3323 3324 3325 3326 3327 3328 3329 3330 3331 3332 3333 3334 3335 3336 3337 3338 3339 3340 3341 3342 3343 3344 3345 3346 3347 3348 3349 3350 3351 3352 3353 3354 3355 3356 3357 3358 3359 3360 3361 3362 3363 3364 3365 3366 3367 3368 3369 3370 3371 3372 3373 3374 3375 3376 3377 3378 3379 3380 3381 3382 3383 3384 3385 3386 3387 3388 3389 3390 3391 3392 3393 3394 3395 3396 3397 3398 3399 3400 3401 3402 3403 3404 3405 3406 3407 3408 3409 3410 3411 3412 3413 3414 3415 3416 3417 3418 3419 3420 3421 3422 3423 3424 3425 3426 3427 3428 3429 3430 3431 3432 3433 3434 3435 3436 3437 3438 3439 3440 3441 3442 3443 3444 3445 3446 3447 3448 3449 3450 3451 3452 3453 3454 3455 3456 3457 3458 3459 3460 3461 3462 3463 3464 3465 3466 3467 3468 3469 3470 3471 3472 3473 3474 3475 3476 3477 3478 3479 3480 3481 3482 3483 3484 3485 3486 3487 3488 3489 3490 3491 3492 3493 3494 3495 3496 3497 3498 3499 3500 3501 3502 3503 3504 3505 3506 3507 3508 3509 3510 3511 3512 3513 3514 3515 3516 3517 3518 3519 3520 3521 3522 3523 3524 3525 3526 3527 3528 3529 3530 3531 3532 3533 3534 3535 3536 3537 3538 3539 3540 3541 3542 3543 3544 3545 3546 3547 3548 3549 3550 3551 3552 3553 3554 3555 3556 3557 3558 3559 3560 3561 3562 3563 3564 3565 3566 3567 3568 3569 3570 3571 3572 3573 3574 3575 3576 3577 3578 3579 3580 3581 3582 3583 3584 3585 3586 3587 3588 3589 3590 3591 3592 3593 3594 3595 3596 3597 3598 3599 3600 3601 3602 3603 3604 3605 3606 3607 3608 3609 3610 3611 3612 3613 3614 3615 3616 3617 3618 3619 3620 3621 3622 3623 3624 3625 3626 3627 3628 3629 3630 3631 3632 3633 3634 3635 3636 3637 3638 3639 3640 3641 3642 3643 3644 3645 3646 3647 3648 3649 3650 3651 3652 3653 3654 3655 3656 3657 3658 3659 3660 3661 3662 3663 3664 3665 3666 3667 3668 3669 3670 3671 3672 3673 3674 3675 3676 3677 3678 3679 3680 3681 3682 3683 3684 3685 3686 3687 3688 3689 3690 3691 3692 3693 3694 3695 3696 3697 3698 3699 3700 3701 3702 3703 3704 3705 3706 3707 3708 3709 3710 3711 3712 3713 3714 3715 3716 3717 3718 3719 3720 3721 3722 3723 3724 3725 3726 3727 3728 3729 3730 3731 3732 3733 3734 3735 3736 3737 3738 3739 3740 3741 3742 3743 3744 3745 3746 3747 3748 3749 3750 3751 3752 3753 3754 3755 3756 3757 3758 3759 3760 3761 3762 3763 3764 3765 3766 3767 3768 3769 3770 3771 3772 3773 3774 3775 3776 3777 3778 3779 3780 3781 3782 3783 3784 3785 3786 3787 3788 3789 3790 3791 3792 3793 3794 3795 3796 3797 3798 3799 3800 3801 3802 3803 3804 3805 3806 3807 3808 3809 3810 3811 3812 3813 3814 3815 3816 3817 3818 3819 3820 3821 3822 3823 3824 3825 3826 3827 3828 3829 3830 3831 3832 3833 3834 3835 3836 3837 3838 3839 3840 3841 3842 3843 3844 3845 3846 3847 3848 3849 3850 3851 3852 3853 3854 3855 3856 3857 3858 3859 3860 3861 3862 3863 3864 3865 3866 3867 3868 3869 3870 3871 3872 3873 3874 3875 3876 3877 3878 3879 3880 3881 3882 3883 3884 3885 3886 3887 3888 3889 3890 3891 3892 3893 3894 3895 3896 3897 3898 3899 3900 3901 3902 3903 3904 3905 3906 3907 3908 3909 3910 3911 3912 3913 3914 3915 3916 3917 3918 3919 3920 3921 3922 3923 3924 3925 3926 3927 3928 3929 3930 3931 3932 3933 3934 3935 3936 3937 3938 3939 3940 3941 3942 3943 3944 3945 3946 3947 3948 3949 3950 3951 3952 3953 3954 3955 3956 3957 3958 3959 3960 3961 3962 3963 3964 3965 3966 3967 3968 3969 3970 3971 3972 3973 3974 3975 3976 3977 3978 3979 3980 3981 3982 3983 3984 3985 3986 3987 3988 3989 3990 3991 3992 3993 3994 3995 3996 3997 3998 3999 4000 4001 4002 4003 4004 4005 4006 4007 4008 4009 4010 4011 4012 4013 4014 4015 4016 4017 4018 4019 4020 4021 4022 4023 4024 4025 4026 4027 4028 4029 4030 4031 4032 4033 4034 4035 4036 4037 4038 4039 4040 4041 4042 4043 4044 4045 4046 4047 4048 4049 4050 4051 4052 4053 4054 4055 4056 4057 4058 4059 4060 4061 4062 4063 4064 4065 4066 4067 4068 4069 4070 4071 4072 4073 4074 4075 4076 4077 4078 4079 4080 4081 4082 4083 4084 4085 4086 4087 4088 4089 4090 4091 4092 4093 4094 4095 4096 4097 4098 4099 4100 4101 4102 4103 4104 4105 4106 4107 4108 4109 4110 4111 4112 4113 4114 4115 4116 4117 4118 4119 4120 4121 4122 4123 4124 4125 4126 4127 4128 4129 4130 4131 4132 4133 4134 4135 4136 4137 4138 4139 4140 4141 4142 4143 4144 4145 4146 4147 4148 4149 4150 4151 4152 4153 4154 4155 4156 4157 4158 4159 4160 4161 4162 4163 4164 4165 4166 4167 4168 4169 4170 4171 4172 4173 4174 4175 4176 4177 4178 4179 4180 4181 4182 4183 4184 4185 4186 4187 4188 4189 4190 4191 4192 4193 4194 4195 4196 4197 4198 4199 4200 4201 4202 4203 4204 4205 4206 4207 4208 4209 4210 4211 4212 4213 4214 4215 4216 4217 4218 4219 4220 4221 4222 4223 4224 4225 4226 4227 4228 4229 4230 4231 4232 4233 4234 4235 4236 4237 4238 4239 4240 4241 4242 4243 4244 4245 4246 4247 4248 4249 4250 4251 4252 4253 4254 4255 4256 4257 4258 4259 4260 4261 4262 4263 4264 4265 4266 4267 4268 4269 4270 4271 4272 4273 4274 4275 4276 4277 4278 4279 4280 4281 4282 4283 4284 4285 4286 4287 4288 4289 4290 4291 4292 4293 4294 4295 4296 4297 4298 4299 4300 4301 4302 4303 4304 4305 4306 4307 4308 4309 4310 4311 4312 4313 4314 4315 4316 4317 4318 4319 4320 4321 4322 4323 4324 4325 4326 4327 4328 4329 4330 4331 4332 4333 4334 4335 4336 4337 4338 4339 4340 4341 4342 4343 4344 4345 4346 4347 4348 4349 4350 4351 4352 4353 4354 4355 4356 4357 4358 4359 4360 4361 4362 4363 4364 4365 4366 4367 4368 4369 4370 4371 4372 4373 4374 4375 4376 4377 4378 4379 4380 4381 4382 4383 4384 4385 4386 4387 4388 4389 4390 4391 4392 4393 4394 4395 4396 4397 4398 4399 4400 4401 4402 4403 4404 4405 4406 4407 4408 4409 4410 4411 4412 4413 4414 4415 4416 4417 4418 4419 4420 4421 4422 4423 4424 4425 4426 4427 4428 4429 4430 4431 4432 4433 4434 4435 4436 4437 4438 4439 4440 4441 4442 4443 4444 4445 4446 4447 4448 4449 4450 4451 4452 4453 4454 4455 4456 4457 4458 4459 4460 4461 4462 4463 4464 4465 4466 4467 4468 4469 4470 4471 4472 4473 4474 4475 4476 4477 4478 4479 4480 4481 4482 4483 4484 4485 4486 | <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html401/loose.dtd">
<html>
<!-- This file documents the GNU C library.
This is Edition 0.13, last updated 2011-07-19,
of The GNU C Library Reference Manual, for version
2.14 (Ubuntu EGLIBC 2.15-0ubuntu10) .
Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002,
2003, 2007, 2008, 2010, 2011 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "Free Software Needs Free Documentation"
and "GNU Lesser General Public License", the Front-Cover texts being
"A GNU Manual", and with the Back-Cover Texts as in (a) below. A
copy of the license is included in the section entitled "GNU Free
Documentation License".
(a) The FSF's Back-Cover Text is: "You have the freedom to
copy and modify this GNU manual. Buying copies from the FSF
supports it in developing GNU and promoting software freedom."
-->
<!-- Created on April 20, 2012 by texi2html 1.82
texi2html was written by:
Lionel Cons <Lionel.Cons@cern.ch> (original author)
Karl Berry <karl@freefriends.org>
Olaf Bachmann <obachman@mathematik.uni-kl.de>
and many others.
Maintained by: Many creative people.
Send bugs and suggestions to <texi2html-bug@nongnu.org>
-->
<head>
<title>The GNU C Library: 16. Sockets</title>
<meta name="description" content="The GNU C Library: 16. Sockets">
<meta name="keywords" content="The GNU C Library: 16. Sockets">
<meta name="resource-type" content="document">
<meta name="distribution" content="global">
<meta name="Generator" content="texi2html 1.82">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<style type="text/css">
<!--
a.summary-letter {text-decoration: none}
blockquote.smallquotation {font-size: smaller}
pre.display {font-family: serif}
pre.format {font-family: serif}
pre.menu-comment {font-family: serif}
pre.menu-preformatted {font-family: serif}
pre.smalldisplay {font-family: serif; font-size: smaller}
pre.smallexample {font-size: smaller}
pre.smallformat {font-family: serif; font-size: smaller}
pre.smalllisp {font-size: smaller}
span.roman {font-family:serif; font-weight:normal;}
span.sansserif {font-family:sans-serif; font-weight:normal;}
ul.toc {list-style: none}
-->
</style>
</head>
<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
<a name="Sockets"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="libc_15.html#Pipe-Atomicity" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Socket-Concepts" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc_15.html#Pipes-and-FIFOs" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Sockets-1"></a>
<h1 class="chapter">16. Sockets</h1>
<p>This chapter describes the GNU facilities for interprocess
communication using sockets.
</p>
<a name="index-socket"></a>
<a name="index-interprocess-communication_002c-with-sockets"></a>
<p>A <em>socket</em> is a generalized interprocess communication channel.
Like a pipe, a socket is represented as a file descriptor. Unlike pipes
sockets support communication between unrelated processes, and even
between processes running on different machines that communicate over a
network. Sockets are the primary means of communicating with other
machines; <code>telnet</code>, <code>rlogin</code>, <code>ftp</code>, <code>talk</code> and the
other familiar network programs use sockets.
</p>
<p>Not all operating systems support sockets. In the GNU library, the
header file ‘<tt>sys/socket.h</tt>’ exists regardless of the operating
system, and the socket functions always exist, but if the system does
not really support sockets these functions always fail.
</p>
<p><strong>Incomplete:</strong> We do not currently document the facilities for
broadcast messages or for configuring Internet interfaces. The
reentrant functions and some newer functions that are related to IPv6
aren’t documented either so far.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top"><a href="#Socket-Concepts">16.1 Socket Concepts</a></td><td> </td><td align="left" valign="top"> Basic concepts you need to know about.
</td></tr>
<tr><td align="left" valign="top"><a href="#Communication-Styles">16.2 Communication Styles</a></td><td> </td><td align="left" valign="top">Stream communication, datagrams and other styles.
</td></tr>
<tr><td align="left" valign="top"><a href="#Socket-Addresses">16.3 Socket Addresses</a></td><td> </td><td align="left" valign="top"> How socket names (“addresses”) work.
</td></tr>
<tr><td align="left" valign="top"><a href="#Interface-Naming">16.4 Interface Naming</a></td><td> </td><td align="left" valign="top"> Identifying specific network interfaces.
</td></tr>
<tr><td align="left" valign="top"><a href="#Local-Namespace">16.5 The Local Namespace</a></td><td> </td><td align="left" valign="top"> Details about the local namespace.
</td></tr>
<tr><td align="left" valign="top"><a href="#Internet-Namespace">16.6 The Internet Namespace</a></td><td> </td><td align="left" valign="top"> Details about the Internet namespace.
</td></tr>
<tr><td align="left" valign="top"><a href="#Misc-Namespaces">16.7 Other Namespaces</a></td><td> </td><td align="left" valign="top"> Other namespaces not documented fully here.
</td></tr>
<tr><td align="left" valign="top"><a href="#Open_002fClose-Sockets">16.8 Opening and Closing Sockets</a></td><td> </td><td align="left" valign="top"> Creating sockets and destroying them.
</td></tr>
<tr><td align="left" valign="top"><a href="#Connections">16.9 Using Sockets with Connections</a></td><td> </td><td align="left" valign="top"> Operations on sockets with connection state.
</td></tr>
<tr><td align="left" valign="top"><a href="#Datagrams">16.10 Datagram Socket Operations</a></td><td> </td><td align="left" valign="top"> Operations on datagram sockets.
</td></tr>
<tr><td align="left" valign="top"><a href="#Inetd">16.11 The <code>inetd</code> Daemon</a></td><td> </td><td align="left" valign="top"> Inetd is a daemon that starts servers on request.
The most convenient way to write a server
is to make it work with Inetd.
</td></tr>
<tr><td align="left" valign="top"><a href="#Socket-Options">16.12 Socket Options</a></td><td> </td><td align="left" valign="top"> Miscellaneous low-level socket options.
</td></tr>
<tr><td align="left" valign="top"><a href="#Networks-Database">16.13 Networks Database</a></td><td> </td><td align="left" valign="top"> Accessing the database of network names.
</td></tr>
</table>
<hr size="6">
<a name="Socket-Concepts"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Sockets" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Communication-Styles" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Sockets" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Socket-Concepts-1"></a>
<h2 class="section">16.1 Socket Concepts</h2>
<a name="index-communication-style-_0028of-a-socket_0029"></a>
<a name="index-style-of-communication-_0028of-a-socket_0029"></a>
<p>When you create a socket, you must specify the style of communication
you want to use and the type of protocol that should implement it.
The <em>communication style</em> of a socket defines the user-level
semantics of sending and receiving data on the socket. Choosing a
communication style specifies the answers to questions such as these:
</p>
<ul>
<li>
<a name="index-packet"></a>
<a name="index-byte-stream"></a>
<a name="index-stream-_0028sockets_0029"></a>
<strong>What are the units of data transmission?</strong> Some communication
styles regard the data as a sequence of bytes with no larger
structure; others group the bytes into records (which are known in
this context as <em>packets</em>).
</li><li>
<a name="index-loss-of-data-on-sockets"></a>
<a name="index-data-loss-on-sockets"></a>
<strong>Can data be lost during normal operation?</strong> Some communication
styles guarantee that all the data sent arrives in the order it was
sent (barring system or network crashes); other styles occasionally
lose data as a normal part of operation, and may sometimes deliver
packets more than once or in the wrong order.
<p>Designing a program to use unreliable communication styles usually
involves taking precautions to detect lost or misordered packets and
to retransmit data as needed.
</p>
</li><li>
<strong>Is communication entirely with one partner?</strong> Some
communication styles are like a telephone call—you make a
<em>connection</em> with one remote socket and then exchange data
freely. Other styles are like mailing letters—you specify a
destination address for each message you send.
</li></ul>
<a name="index-namespace-_0028of-socket_0029"></a>
<a name="index-domain-_0028of-socket_0029"></a>
<a name="index-socket-namespace"></a>
<a name="index-socket-domain"></a>
<p>You must also choose a <em>namespace</em> for naming the socket. A socket
name (“address”) is meaningful only in the context of a particular
namespace. In fact, even the data type to use for a socket name may
depend on the namespace. Namespaces are also called “domains”, but we
avoid that word as it can be confused with other usage of the same
term. Each namespace has a symbolic name that starts with ‘<samp>PF_</samp>’.
A corresponding symbolic name starting with ‘<samp>AF_</samp>’ designates the
address format for that namespace.
</p>
<a name="index-network-protocol"></a>
<a name="index-protocol-_0028of-socket_0029"></a>
<a name="index-socket-protocol"></a>
<a name="index-protocol-family"></a>
<p>Finally you must choose the <em>protocol</em> to carry out the
communication. The protocol determines what low-level mechanism is used
to transmit and receive data. Each protocol is valid for a particular
namespace and communication style; a namespace is sometimes called a
<em>protocol family</em> because of this, which is why the namespace names
start with ‘<samp>PF_</samp>’.
</p>
<p>The rules of a protocol apply to the data passing between two programs,
perhaps on different computers; most of these rules are handled by the
operating system and you need not know about them. What you do need to
know about protocols is this:
</p>
<ul>
<li>
In order to have communication between two sockets, they must specify
the <em>same</em> protocol.
</li><li>
Each protocol is meaningful with particular style/namespace
combinations and cannot be used with inappropriate combinations. For
example, the TCP protocol fits only the byte stream style of
communication and the Internet namespace.
</li><li>
For each combination of style and namespace there is a <em>default
protocol</em>, which you can request by specifying 0 as the protocol
number. And that’s what you should normally do—use the default.
</li></ul>
<p>Throughout the following description at various places
variables/parameters to denote sizes are required. And here the trouble
starts. In the first implementations the type of these variables was
simply <code>int</code>. On most machines at that time an <code>int</code> was 32
bits wide, which created a <em>de facto</em> standard requiring 32-bit
variables. This is important since references to variables of this type
are passed to the kernel.
</p>
<p>Then the POSIX people came and unified the interface with the words "all
size values are of type <code>size_t</code>". On 64-bit machines
<code>size_t</code> is 64 bits wide, so pointers to variables were no longer
possible.
</p>
<p>The Unix98 specification provides a solution by introducing a type
<code>socklen_t</code>. This type is used in all of the cases that POSIX
changed to use <code>size_t</code>. The only requirement of this type is that
it be an unsigned type of at least 32 bits. Therefore, implementations
which require that references to 32-bit variables be passed can be as
happy as implementations which use 64-bit values.
</p>
<hr size="6">
<a name="Communication-Styles"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Socket-Concepts" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Socket-Addresses" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Sockets" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Communication-Styles-1"></a>
<h2 class="section">16.2 Communication Styles</h2>
<p>The GNU library includes support for several different kinds of sockets,
each with different characteristics. This section describes the
supported socket types. The symbolic constants listed here are
defined in ‘<tt>sys/socket.h</tt>’.
<a name="index-sys_002fsocket_002eh"></a>
</p>
<dl>
<dt><a name="index-SOCK_005fSTREAM"></a><u>Macro:</u> int <b>SOCK_STREAM</b></dt>
<dd><p>The <code>SOCK_STREAM</code> style is like a pipe (see section <a href="libc_15.html#Pipes-and-FIFOs">Pipes and FIFOs</a>).
It operates over a connection with a particular remote socket and
transmits data reliably as a stream of bytes.
</p>
<p>Use of this style is covered in detail in <a href="#Connections">Using Sockets with Connections</a>.
</p></dd></dl>
<dl>
<dt><a name="index-SOCK_005fDGRAM"></a><u>Macro:</u> int <b>SOCK_DGRAM</b></dt>
<dd><p>The <code>SOCK_DGRAM</code> style is used for sending
individually-addressed packets unreliably.
It is the diametrical opposite of <code>SOCK_STREAM</code>.
</p>
<p>Each time you write data to a socket of this kind, that data becomes
one packet. Since <code>SOCK_DGRAM</code> sockets do not have connections,
you must specify the recipient address with each packet.
</p>
<p>The only guarantee that the system makes about your requests to
transmit data is that it will try its best to deliver each packet you
send. It may succeed with the sixth packet after failing with the
fourth and fifth packets; the seventh packet may arrive before the
sixth, and may arrive a second time after the sixth.
</p>
<p>The typical use for <code>SOCK_DGRAM</code> is in situations where it is
acceptable to simply re-send a packet if no response is seen in a
reasonable amount of time.
</p>
<p>See section <a href="#Datagrams">Datagram Socket Operations</a>, for detailed information about how to use datagram
sockets.
</p></dd></dl>
<dl>
<dt><a name="index-SOCK_005fRAW"></a><u>Macro:</u> int <b>SOCK_RAW</b></dt>
<dd><p>This style provides access to low-level network protocols and
interfaces. Ordinary user programs usually have no need to use this
style.
</p></dd></dl>
<hr size="6">
<a name="Socket-Addresses"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Communication-Styles" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Address-Formats" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Sockets" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Socket-Addresses-1"></a>
<h2 class="section">16.3 Socket Addresses</h2>
<a name="index-address-of-socket"></a>
<a name="index-name-of-socket"></a>
<a name="index-binding-a-socket-address"></a>
<a name="index-socket-address-_0028name_0029-binding"></a>
<p>The name of a socket is normally called an <em>address</em>. The
functions and symbols for dealing with socket addresses were named
inconsistently, sometimes using the term “name” and sometimes using
“address”. You can regard these terms as synonymous where sockets
are concerned.
</p>
<p>A socket newly created with the <code>socket</code> function has no
address. Other processes can find it for communication only if you
give it an address. We call this <em>binding</em> the address to the
socket, and the way to do it is with the <code>bind</code> function.
</p>
<p>You need be concerned with the address of a socket if other processes
are to find it and start communicating with it. You can specify an
address for other sockets, but this is usually pointless; the first time
you send data from a socket, or use it to initiate a connection, the
system assigns an address automatically if you have not specified one.
</p>
<p>Occasionally a client needs to specify an address because the server
discriminates based on address; for example, the rsh and rlogin
protocols look at the client’s socket address and only bypass password
checking if it is less than <code>IPPORT_RESERVED</code> (see section <a href="#Ports">Internet Ports</a>).
</p>
<p>The details of socket addresses vary depending on what namespace you are
using. See section <a href="#Local-Namespace">The Local Namespace</a>, or <a href="#Internet-Namespace">The Internet Namespace</a>, for specific
information.
</p>
<p>Regardless of the namespace, you use the same functions <code>bind</code> and
<code>getsockname</code> to set and examine a socket’s address. These
functions use a phony data type, <code>struct sockaddr *</code>, to accept the
address. In practice, the address lives in a structure of some other
data type appropriate to the address format you are using, but you cast
its address to <code>struct sockaddr *</code> when you pass it to
<code>bind</code>.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top"><a href="#Address-Formats">16.3.1 Address Formats</a></td><td> </td><td align="left" valign="top"> About <code>struct sockaddr</code>.
</td></tr>
<tr><td align="left" valign="top"><a href="#Setting-Address">16.3.2 Setting the Address of a Socket</a></td><td> </td><td align="left" valign="top"> Binding an address to a socket.
</td></tr>
<tr><td align="left" valign="top"><a href="#Reading-Address">16.3.3 Reading the Address of a Socket</a></td><td> </td><td align="left" valign="top"> Reading the address of a socket.
</td></tr>
</table>
<hr size="6">
<a name="Address-Formats"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Socket-Addresses" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Setting-Address" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Socket-Addresses" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Address-Formats-1"></a>
<h3 class="subsection">16.3.1 Address Formats</h3>
<p>The functions <code>bind</code> and <code>getsockname</code> use the generic data
type <code>struct sockaddr *</code> to represent a pointer to a socket
address. You can’t use this data type effectively to interpret an
address or construct one; for that, you must use the proper data type
for the socket’s namespace.
</p>
<p>Thus, the usual practice is to construct an address of the proper
namespace-specific type, then cast a pointer to <code>struct sockaddr *</code>
when you call <code>bind</code> or <code>getsockname</code>.
</p>
<p>The one piece of information that you can get from the <code>struct
sockaddr</code> data type is the <em>address format designator</em>. This tells
you which data type to use to understand the address fully.
</p>
<a name="index-sys_002fsocket_002eh-1"></a>
<p>The symbols in this section are defined in the header file
‘<tt>sys/socket.h</tt>’.
</p>
<dl>
<dt><a name="index-struct-sockaddr"></a><u>Data Type:</u> <b>struct sockaddr</b></dt>
<dd><p>The <code>struct sockaddr</code> type itself has the following members:
</p>
<dl compact="compact">
<dt> <code>short int sa_family</code></dt>
<dd><p>This is the code for the address format of this address. It
identifies the format of the data which follows.
</p>
</dd>
<dt> <code>char sa_data[14]</code></dt>
<dd><p>This is the actual socket address data, which is format-dependent. Its
length also depends on the format, and may well be more than 14. The
length 14 of <code>sa_data</code> is essentially arbitrary.
</p></dd>
</dl>
</dd></dl>
<p>Each address format has a symbolic name which starts with ‘<samp>AF_</samp>’.
Each of them corresponds to a ‘<samp>PF_</samp>’ symbol which designates the
corresponding namespace. Here is a list of address format names:
</p>
<dl compact="compact">
<dt> <code>AF_LOCAL</code></dt>
<dd><a name="index-AF_005fLOCAL"></a>
<p>This designates the address format that goes with the local namespace.
(<code>PF_LOCAL</code> is the name of that namespace.) See section <a href="#Local-Namespace-Details">Details of Local Namespace</a>, for information about this address format.
</p>
</dd>
<dt> <code>AF_UNIX</code></dt>
<dd><a name="index-AF_005fUNIX"></a>
<p>This is a synonym for <code>AF_LOCAL</code>. Although <code>AF_LOCAL</code> is
mandated by POSIX.1g, <code>AF_UNIX</code> is portable to more systems.
<code>AF_UNIX</code> was the traditional name stemming from BSD, so even most
POSIX systems support it. It is also the name of choice in the Unix98
specification. (The same is true for <code>PF_UNIX</code>
vs. <code>PF_LOCAL</code>).
</p>
</dd>
<dt> <code>AF_FILE</code></dt>
<dd><a name="index-AF_005fFILE"></a>
<p>This is another synonym for <code>AF_LOCAL</code>, for compatibility.
(<code>PF_FILE</code> is likewise a synonym for <code>PF_LOCAL</code>.)
</p>
</dd>
<dt> <code>AF_INET</code></dt>
<dd><a name="index-AF_005fINET"></a>
<p>This designates the address format that goes with the Internet
namespace. (<code>PF_INET</code> is the name of that namespace.)
See section <a href="#Internet-Address-Formats">Internet Socket Address Formats</a>.
</p>
</dd>
<dt> <code>AF_INET6</code></dt>
<dd><p>This is similar to <code>AF_INET</code>, but refers to the IPv6 protocol.
(<code>PF_INET6</code> is the name of the corresponding namespace.)
</p>
</dd>
<dt> <code>AF_UNSPEC</code></dt>
<dd><a name="index-AF_005fUNSPEC"></a>
<p>This designates no particular address format. It is used only in rare
cases, such as to clear out the default destination address of a
“connected” datagram socket. See section <a href="#Sending-Datagrams">Sending Datagrams</a>.
</p>
<p>The corresponding namespace designator symbol <code>PF_UNSPEC</code> exists
for completeness, but there is no reason to use it in a program.
</p></dd>
</dl>
<p>‘<tt>sys/socket.h</tt>’ defines symbols starting with ‘<samp>AF_</samp>’ for many
different kinds of networks, most or all of which are not actually
implemented. We will document those that really work as we receive
information about how to use them.
</p>
<hr size="6">
<a name="Setting-Address"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Address-Formats" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Reading-Address" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Socket-Addresses" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Setting-the-Address-of-a-Socket"></a>
<h3 class="subsection">16.3.2 Setting the Address of a Socket</h3>
<a name="index-sys_002fsocket_002eh-2"></a>
<p>Use the <code>bind</code> function to assign an address to a socket. The
prototype for <code>bind</code> is in the header file ‘<tt>sys/socket.h</tt>’.
For examples of use, see <a href="#Local-Socket-Example">Example of Local-Namespace Sockets</a>, or see <a href="#Inet-Example">Internet Socket Example</a>.
</p>
<dl>
<dt><a name="index-bind"></a><u>Function:</u> int <b>bind</b><i> (int <var>socket</var>, struct sockaddr *<var>addr</var>, socklen_t <var>length</var>)</i></dt>
<dd><p>The <code>bind</code> function assigns an address to the socket
<var>socket</var>. The <var>addr</var> and <var>length</var> arguments specify the
address; the detailed format of the address depends on the namespace.
The first part of the address is always the format designator, which
specifies a namespace, and says that the address is in the format of
that namespace.
</p>
<p>The return value is <code>0</code> on success and <code>-1</code> on failure. The
following <code>errno</code> error conditions are defined for this function:
</p>
<dl compact="compact">
<dt> <code>EBADF</code></dt>
<dd><p>The <var>socket</var> argument is not a valid file descriptor.
</p>
</dd>
<dt> <code>ENOTSOCK</code></dt>
<dd><p>The descriptor <var>socket</var> is not a socket.
</p>
</dd>
<dt> <code>EADDRNOTAVAIL</code></dt>
<dd><p>The specified address is not available on this machine.
</p>
</dd>
<dt> <code>EADDRINUSE</code></dt>
<dd><p>Some other socket is already using the specified address.
</p>
</dd>
<dt> <code>EINVAL</code></dt>
<dd><p>The socket <var>socket</var> already has an address.
</p>
</dd>
<dt> <code>EACCES</code></dt>
<dd><p>You do not have permission to access the requested address. (In the
Internet domain, only the super-user is allowed to specify a port number
in the range 0 through <code>IPPORT_RESERVED</code> minus one; see
<a href="#Ports">Internet Ports</a>.)
</p></dd>
</dl>
<p>Additional conditions may be possible depending on the particular namespace
of the socket.
</p></dd></dl>
<hr size="6">
<a name="Reading-Address"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Setting-Address" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Interface-Naming" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Socket-Addresses" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Reading-the-Address-of-a-Socket"></a>
<h3 class="subsection">16.3.3 Reading the Address of a Socket</h3>
<a name="index-sys_002fsocket_002eh-3"></a>
<p>Use the function <code>getsockname</code> to examine the address of an
Internet socket. The prototype for this function is in the header file
‘<tt>sys/socket.h</tt>’.
</p>
<dl>
<dt><a name="index-getsockname"></a><u>Function:</u> int <b>getsockname</b><i> (int <var>socket</var>, struct sockaddr *<var>addr</var>, socklen_t *<var>length-ptr</var>)</i></dt>
<dd><p>The <code>getsockname</code> function returns information about the
address of the socket <var>socket</var> in the locations specified by the
<var>addr</var> and <var>length-ptr</var> arguments. Note that the
<var>length-ptr</var> is a pointer; you should initialize it to be the
allocation size of <var>addr</var>, and on return it contains the actual
size of the address data.
</p>
<p>The format of the address data depends on the socket namespace. The
length of the information is usually fixed for a given namespace, so
normally you can know exactly how much space is needed and can provide
that much. The usual practice is to allocate a place for the value
using the proper data type for the socket’s namespace, then cast its
address to <code>struct sockaddr *</code> to pass it to <code>getsockname</code>.
</p>
<p>The return value is <code>0</code> on success and <code>-1</code> on error. The
following <code>errno</code> error conditions are defined for this function:
</p>
<dl compact="compact">
<dt> <code>EBADF</code></dt>
<dd><p>The <var>socket</var> argument is not a valid file descriptor.
</p>
</dd>
<dt> <code>ENOTSOCK</code></dt>
<dd><p>The descriptor <var>socket</var> is not a socket.
</p>
</dd>
<dt> <code>ENOBUFS</code></dt>
<dd><p>There are not enough internal buffers available for the operation.
</p></dd>
</dl>
</dd></dl>
<p>You can’t read the address of a socket in the file namespace. This is
consistent with the rest of the system; in general, there’s no way to
find a file’s name from a descriptor for that file.
</p>
<hr size="6">
<a name="Interface-Naming"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Reading-Address" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Local-Namespace" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Sockets" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Interface-Naming-1"></a>
<h2 class="section">16.4 Interface Naming</h2>
<p>Each network interface has a name. This usually consists of a few
letters that relate to the type of interface, which may be followed by a
number if there is more than one interface of that type. Examples
might be <code>lo</code> (the loopback interface) and <code>eth0</code> (the first
Ethernet interface).
</p>
<p>Although such names are convenient for humans, it would be clumsy to
have to use them whenever a program needs to refer to an interface. In
such situations an interface is referred to by its <em>index</em>, which is
an arbitrarily-assigned small positive integer.
</p>
<p>The following functions, constants and data types are declared in the
header file ‘<tt>net/if.h</tt>’.
</p>
<dl>
<dt><a name="index-IFNAMSIZ"></a><u>Constant:</u> size_t <b>IFNAMSIZ</b></dt>
<dd><p>This constant defines the maximum buffer size needed to hold an
interface name, including its terminating zero byte.
</p></dd></dl>
<dl>
<dt><a name="index-if_005fnametoindex"></a><u>Function:</u> unsigned int <b>if_nametoindex</b><i> (const char *ifname)</i></dt>
<dd><p>This function yields the interface index corresponding to a particular
name. If no interface exists with the name given, it returns 0.
</p></dd></dl>
<dl>
<dt><a name="index-if_005findextoname"></a><u>Function:</u> char * <b>if_indextoname</b><i> (unsigned int ifindex, char *ifname)</i></dt>
<dd><p>This function maps an interface index to its corresponding name. The
returned name is placed in the buffer pointed to by <code>ifname</code>, which
must be at least <code>IFNAMSIZ</code> bytes in length. If the index was
invalid, the function’s return value is a null pointer, otherwise it is
<code>ifname</code>.
</p></dd></dl>
<dl>
<dt><a name="index-struct-if_005fnameindex"></a><u>Data Type:</u> <b>struct if_nameindex</b></dt>
<dd><p>This data type is used to hold the information about a single
interface. It has the following members:
</p>
<dl compact="compact">
<dt> <code>unsigned int if_index;</code></dt>
<dd><p>This is the interface index.
</p>
</dd>
<dt> <code>char *if_name</code></dt>
<dd><p>This is the null-terminated index name.
</p>
</dd>
</dl>
</dd></dl>
<dl>
<dt><a name="index-if_005fnameindex"></a><u>Function:</u> struct if_nameindex * <b>if_nameindex</b><i> (void)</i></dt>
<dd><p>This function returns an array of <code>if_nameindex</code> structures, one
for every interface that is present. The end of the list is indicated
by a structure with an interface of 0 and a null name pointer. If an
error occurs, this function returns a null pointer.
</p>
<p>The returned structure must be freed with <code>if_freenameindex</code> after
use.
</p></dd></dl>
<dl>
<dt><a name="index-if_005ffreenameindex"></a><u>Function:</u> void <b>if_freenameindex</b><i> (struct if_nameindex *ptr)</i></dt>
<dd><p>This function frees the structure returned by an earlier call to
<code>if_nameindex</code>.
</p></dd></dl>
<hr size="6">
<a name="Local-Namespace"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Interface-Naming" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Local-Namespace-Concepts" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Sockets" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="The-Local-Namespace"></a>
<h2 class="section">16.5 The Local Namespace</h2>
<a name="index-local-namespace_002c-for-sockets"></a>
<p>This section describes the details of the local namespace, whose
symbolic name (required when you create a socket) is <code>PF_LOCAL</code>.
The local namespace is also known as “Unix domain sockets”. Another
name is file namespace since socket addresses are normally implemented
as file names.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top"><a href="#Local-Namespace-Concepts">16.5.1 Local Namespace Concepts</a></td><td> </td><td align="left" valign="top"> What you need to understand.
</td></tr>
<tr><td align="left" valign="top"><a href="#Local-Namespace-Details">16.5.2 Details of Local Namespace</a></td><td> </td><td align="left" valign="top"> Address format, symbolic names, etc.
</td></tr>
<tr><td align="left" valign="top"><a href="#Local-Socket-Example">16.5.3 Example of Local-Namespace Sockets</a></td><td> </td><td align="left" valign="top"> Example of creating a socket.
</td></tr>
</table>
<hr size="6">
<a name="Local-Namespace-Concepts"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Local-Namespace" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Local-Namespace-Details" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Local-Namespace" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Local-Namespace-Concepts-1"></a>
<h3 class="subsection">16.5.1 Local Namespace Concepts</h3>
<p>In the local namespace socket addresses are file names. You can specify
any file name you want as the address of the socket, but you must have
write permission on the directory containing it.
It’s common to put these files in the ‘<tt>/tmp</tt>’ directory.
</p>
<p>One peculiarity of the local namespace is that the name is only used
when opening the connection; once open the address is not meaningful and
may not exist.
</p>
<p>Another peculiarity is that you cannot connect to such a socket from
another machine–not even if the other machine shares the file system
which contains the name of the socket. You can see the socket in a
directory listing, but connecting to it never succeeds. Some programs
take advantage of this, such as by asking the client to send its own
process ID, and using the process IDs to distinguish between clients.
However, we recommend you not use this method in protocols you design,
as we might someday permit connections from other machines that mount
the same file systems. Instead, send each new client an identifying
number if you want it to have one.
</p>
<p>After you close a socket in the local namespace, you should delete the
file name from the file system. Use <code>unlink</code> or <code>remove</code> to
do this; see <a href="libc_14.html#Deleting-Files">Deleting Files</a>.
</p>
<p>The local namespace supports just one protocol for any communication
style; it is protocol number <code>0</code>.
</p>
<hr size="6">
<a name="Local-Namespace-Details"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Local-Namespace-Concepts" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Local-Socket-Example" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Local-Namespace" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Details-of-Local-Namespace"></a>
<h3 class="subsection">16.5.2 Details of Local Namespace</h3>
<a name="index-sys_002fsocket_002eh-4"></a>
<p>To create a socket in the local namespace, use the constant
<code>PF_LOCAL</code> as the <var>namespace</var> argument to <code>socket</code> or
<code>socketpair</code>. This constant is defined in ‘<tt>sys/socket.h</tt>’.
</p>
<dl>
<dt><a name="index-PF_005fLOCAL"></a><u>Macro:</u> int <b>PF_LOCAL</b></dt>
<dd><p>This designates the local namespace, in which socket addresses are local
names, and its associated family of protocols. <code>PF_Local</code> is the
macro used by Posix.1g.
</p></dd></dl>
<dl>
<dt><a name="index-PF_005fUNIX"></a><u>Macro:</u> int <b>PF_UNIX</b></dt>
<dd><p>This is a synonym for <code>PF_LOCAL</code>, for compatibility’s sake.
</p></dd></dl>
<dl>
<dt><a name="index-PF_005fFILE"></a><u>Macro:</u> int <b>PF_FILE</b></dt>
<dd><p>This is a synonym for <code>PF_LOCAL</code>, for compatibility’s sake.
</p></dd></dl>
<p>The structure for specifying socket names in the local namespace is
defined in the header file ‘<tt>sys/un.h</tt>’:
<a name="index-sys_002fun_002eh"></a>
</p>
<dl>
<dt><a name="index-struct-sockaddr_005fun"></a><u>Data Type:</u> <b>struct sockaddr_un</b></dt>
<dd><p>This structure is used to specify local namespace socket addresses. It has
the following members:
</p>
<dl compact="compact">
<dt> <code>short int sun_family</code></dt>
<dd><p>This identifies the address family or format of the socket address.
You should store the value <code>AF_LOCAL</code> to designate the local
namespace. See section <a href="#Socket-Addresses">Socket Addresses</a>.
</p>
</dd>
<dt> <code>char sun_path[108]</code></dt>
<dd><p>This is the file name to use.
</p>
<p><strong>Incomplete:</strong> Why is 108 a magic number? RMS suggests making
this a zero-length array and tweaking the following example to use
<code>alloca</code> to allocate an appropriate amount of storage based on
the length of the filename.
</p></dd>
</dl>
</dd></dl>
<p>You should compute the <var>length</var> parameter for a socket address in
the local namespace as the sum of the size of the <code>sun_family</code>
component and the string length (<em>not</em> the allocation size!) of
the file name string. This can be done using the macro <code>SUN_LEN</code>:
</p>
<dl>
<dt><a name="index-SUN_005fLEN"></a><u>Macro:</u> int <b>SUN_LEN</b><i> (<em>struct sockaddr_un *</em> <var>ptr</var>)</i></dt>
<dd><p>The macro computes the length of socket address in the local namespace.
</p></dd></dl>
<hr size="6">
<a name="Local-Socket-Example"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Local-Namespace-Details" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Internet-Namespace" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Local-Namespace" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Example-of-Local_002dNamespace-Sockets"></a>
<h3 class="subsection">16.5.3 Example of Local-Namespace Sockets</h3>
<p>Here is an example showing how to create and name a socket in the local
namespace.
</p>
<table><tr><td> </td><td><pre class="smallexample">#include <stddef.h>
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/un.h>
int
make_named_socket (const char *filename)
{
struct sockaddr_un name;
int sock;
size_t size;
/* <span class="roman">Create the socket.</span> */
sock = socket (PF_LOCAL, SOCK_DGRAM, 0);
if (sock < 0)
{
perror ("socket");
exit (EXIT_FAILURE);
}
/* <span class="roman">Bind a name to the socket.</span> */
name.sun_family = AF_LOCAL;
strncpy (name.sun_path, filename, sizeof (name.sun_path));
name.sun_path[sizeof (name.sun_path) - 1] = '\0';
/* <span class="roman">The size of the address is
the offset of the start of the filename,
plus its length,
plus one for the terminating null byte.
Alternatively you can just do:
size = SUN_LEN (&name);
</span> */
size = (offsetof (struct sockaddr_un, sun_path)
+ strlen (name.sun_path) + 1);
if (bind (sock, (struct sockaddr *) &name, size) < 0)
{
perror ("bind");
exit (EXIT_FAILURE);
}
return sock;
}
</pre></td></tr></table>
<hr size="6">
<a name="Internet-Namespace"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Local-Socket-Example" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Internet-Address-Formats" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Sockets" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="The-Internet-Namespace"></a>
<h2 class="section">16.6 The Internet Namespace</h2>
<a name="index-Internet-namespace_002c-for-sockets"></a>
<p>This section describes the details of the protocols and socket naming
conventions used in the Internet namespace.
</p>
<p>Originally the Internet namespace used only IP version 4 (IPv4). With
the growing number of hosts on the Internet, a new protocol with a
larger address space was necessary: IP version 6 (IPv6). IPv6
introduces 128-bit addresses (IPv4 has 32-bit addresses) and other
features, and will eventually replace IPv4.
</p>
<p>To create a socket in the IPv4 Internet namespace, use the symbolic name
<code>PF_INET</code> of this namespace as the <var>namespace</var> argument to
<code>socket</code> or <code>socketpair</code>. For IPv6 addresses you need the
macro <code>PF_INET6</code>. These macros are defined in ‘<tt>sys/socket.h</tt>’.
<a name="index-sys_002fsocket_002eh-5"></a>
</p>
<dl>
<dt><a name="index-PF_005fINET"></a><u>Macro:</u> int <b>PF_INET</b></dt>
<dd><p>This designates the IPv4 Internet namespace and associated family of
protocols.
</p></dd></dl>
<dl>
<dt><a name="index-PF_005fINET6"></a><u>Macro:</u> int <b>PF_INET6</b></dt>
<dd><p>This designates the IPv6 Internet namespace and associated family of
protocols.
</p></dd></dl>
<p>A socket address for the Internet namespace includes the following components:
</p>
<ul>
<li>
The address of the machine you want to connect to. Internet addresses
can be specified in several ways; these are discussed in <a href="#Internet-Address-Formats">Internet Socket Address Formats</a>, <a href="#Host-Addresses">Host Addresses</a> and <a href="#Host-Names">Host Names</a>.
</li><li>
A port number for that machine. See section <a href="#Ports">Internet Ports</a>.
</li></ul>
<p>You must ensure that the address and port number are represented in a
canonical format called <em>network byte order</em>. See section <a href="#Byte-Order">Byte Order Conversion</a>,
for information about this.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top"><a href="#Internet-Address-Formats">16.6.1 Internet Socket Address Formats</a></td><td> </td><td align="left" valign="top"> How socket addresses are specified in the
Internet namespace.
</td></tr>
<tr><td align="left" valign="top"><a href="#Host-Addresses">16.6.2 Host Addresses</a></td><td> </td><td align="left" valign="top"> All about host addresses of Internet host.
</td></tr>
<tr><td align="left" valign="top"><a href="#Protocols-Database">16.6.6 Protocols Database</a></td><td> </td><td align="left" valign="top"> Referring to protocols by name.
</td></tr>
<tr><td align="left" valign="top"><a href="#Ports">16.6.3 Internet Ports</a></td><td> </td><td align="left" valign="top"> Internet port numbers.
</td></tr>
<tr><td align="left" valign="top"><a href="#Services-Database">16.6.4 The Services Database</a></td><td> </td><td align="left" valign="top"> Ports may have symbolic names.
</td></tr>
<tr><td align="left" valign="top"><a href="#Byte-Order">16.6.5 Byte Order Conversion</a></td><td> </td><td align="left" valign="top"> Different hosts may use different byte
ordering conventions; you need to
canonicalize host address and port number.
</td></tr>
<tr><td align="left" valign="top"><a href="#Inet-Example">16.6.7 Internet Socket Example</a></td><td> </td><td align="left" valign="top"> Putting it all together.
</td></tr>
</table>
<hr size="6">
<a name="Internet-Address-Formats"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Internet-Namespace" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Host-Addresses" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Internet-Namespace" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Internet-Socket-Address-Formats"></a>
<h3 class="subsection">16.6.1 Internet Socket Address Formats</h3>
<p>In the Internet namespace, for both IPv4 (<code>AF_INET</code>) and IPv6
(<code>AF_INET6</code>), a socket address consists of a host address
and a port on that host. In addition, the protocol you choose serves
effectively as a part of the address because local port numbers are
meaningful only within a particular protocol.
</p>
<p>The data types for representing socket addresses in the Internet namespace
are defined in the header file ‘<tt>netinet/in.h</tt>’.
<a name="index-netinet_002fin_002eh"></a>
</p>
<dl>
<dt><a name="index-struct-sockaddr_005fin"></a><u>Data Type:</u> <b>struct sockaddr_in</b></dt>
<dd><p>This is the data type used to represent socket addresses in the
Internet namespace. It has the following members:
</p>
<dl compact="compact">
<dt> <code>sa_family_t sin_family</code></dt>
<dd><p>This identifies the address family or format of the socket address.
You should store the value <code>AF_INET</code> in this member.
See section <a href="#Socket-Addresses">Socket Addresses</a>.
</p>
</dd>
<dt> <code>struct in_addr sin_addr</code></dt>
<dd><p>This is the Internet address of the host machine. See section <a href="#Host-Addresses">Host Addresses</a>, and <a href="#Host-Names">Host Names</a>, for how to get a value to store
here.
</p>
</dd>
<dt> <code>unsigned short int sin_port</code></dt>
<dd><p>This is the port number. See section <a href="#Ports">Internet Ports</a>.
</p></dd>
</dl>
</dd></dl>
<p>When you call <code>bind</code> or <code>getsockname</code>, you should specify
<code>sizeof (struct sockaddr_in)</code> as the <var>length</var> parameter if
you are using an IPv4 Internet namespace socket address.
</p>
<dl>
<dt><a name="index-struct-sockaddr_005fin6"></a><u>Data Type:</u> <b>struct sockaddr_in6</b></dt>
<dd><p>This is the data type used to represent socket addresses in the IPv6
namespace. It has the following members:
</p>
<dl compact="compact">
<dt> <code>sa_family_t sin6_family</code></dt>
<dd><p>This identifies the address family or format of the socket address.
You should store the value of <code>AF_INET6</code> in this member.
See section <a href="#Socket-Addresses">Socket Addresses</a>.
</p>
</dd>
<dt> <code>struct in6_addr sin6_addr</code></dt>
<dd><p>This is the IPv6 address of the host machine. See section <a href="#Host-Addresses">Host Addresses</a>, and <a href="#Host-Names">Host Names</a>, for how to get a value to store
here.
</p>
</dd>
<dt> <code>uint32_t sin6_flowinfo</code></dt>
<dd><p>This is a currently unimplemented field.
</p>
</dd>
<dt> <code>uint16_t sin6_port</code></dt>
<dd><p>This is the port number. See section <a href="#Ports">Internet Ports</a>.
</p>
</dd>
</dl>
</dd></dl>
<hr size="6">
<a name="Host-Addresses"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Internet-Address-Formats" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Abstract-Host-Addresses" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Internet-Namespace" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Host-Addresses-1"></a>
<h3 class="subsection">16.6.2 Host Addresses</h3>
<p>Each computer on the Internet has one or more <em>Internet addresses</em>,
numbers which identify that computer among all those on the Internet.
Users typically write IPv4 numeric host addresses as sequences of four
numbers, separated by periods, as in ‘<samp>128.52.46.32</samp>’, and IPv6
numeric host addresses as sequences of up to eight numbers separated by
colons, as in ‘<samp>5f03:1200:836f:c100::1</samp>’.
</p>
<p>Each computer also has one or more <em>host names</em>, which are strings
of words separated by periods, as in ‘<samp>mescaline.gnu.org</samp>’.
</p>
<p>Programs that let the user specify a host typically accept both numeric
addresses and host names. To open a connection a program needs a
numeric address, and so must convert a host name to the numeric address
it stands for.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top"><a href="#Abstract-Host-Addresses">16.6.2.1 Internet Host Addresses</a></td><td> </td><td align="left" valign="top"> What a host number consists of.
</td></tr>
<tr><td align="left" valign="top"><a href="#Host-Address-Data-Type">16.6.2.2 Host Address Data Type</a></td><td> </td><td align="left" valign="top"> Data type for a host number.
</td></tr>
<tr><td align="left" valign="top"><a href="#Host-Address-Functions">16.6.2.3 Host Address Functions</a></td><td> </td><td align="left" valign="top"> Functions to operate on them.
</td></tr>
<tr><td align="left" valign="top"><a href="#Host-Names">16.6.2.4 Host Names</a></td><td> </td><td align="left" valign="top"> Translating host names to host numbers.
</td></tr>
</table>
<hr size="6">
<a name="Abstract-Host-Addresses"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Host-Addresses" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Host-Address-Data-Type" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Host-Addresses" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Internet-Host-Addresses"></a>
<h4 class="subsubsection">16.6.2.1 Internet Host Addresses</h4>
<a name="index-host-address_002c-Internet"></a>
<a name="index-Internet-host-address"></a>
<a name="index-network-number"></a>
<a name="index-local-network-address-number"></a>
<p>An IPv4 Internet host address is a number containing four bytes of data.
Historically these are divided into two parts, a <em>network number</em> and a
<em>local network address number</em> within that network. In the
mid-1990s classless addresses were introduced which changed this
behavior. Since some functions implicitly expect the old definitions,
we first describe the class-based network and will then describe
classless addresses. IPv6 uses only classless addresses and therefore
the following paragraphs don’t apply.
</p>
<p>The class-based IPv4 network number consists of the first one, two or
three bytes; the rest of the bytes are the local address.
</p>
<p>IPv4 network numbers are registered with the Network Information Center
(NIC), and are divided into three classes—A, B and C. The local
network address numbers of individual machines are registered with the
administrator of the particular network.
</p>
<p>Class A networks have single-byte numbers in the range 0 to 127. There
are only a small number of Class A networks, but they can each support a
very large number of hosts. Medium-sized Class B networks have two-byte
network numbers, with the first byte in the range 128 to 191. Class C
networks are the smallest; they have three-byte network numbers, with
the first byte in the range 192-255. Thus, the first 1, 2, or 3 bytes
of an Internet address specify a network. The remaining bytes of the
Internet address specify the address within that network.
</p>
<p>The Class A network 0 is reserved for broadcast to all networks. In
addition, the host number 0 within each network is reserved for broadcast
to all hosts in that network. These uses are obsolete now but for
compatibility reasons you shouldn’t use network 0 and host number 0.
</p>
<p>The Class A network 127 is reserved for loopback; you can always use
the Internet address ‘<samp>127.0.0.1</samp>’ to refer to the host machine.
</p>
<p>Since a single machine can be a member of multiple networks, it can
have multiple Internet host addresses. However, there is never
supposed to be more than one machine with the same host address.
</p>
<a name="index-standard-dot-notation_002c-for-Internet-addresses"></a>
<a name="index-dot-notation_002c-for-Internet-addresses"></a>
<p>There are four forms of the <em>standard numbers-and-dots notation</em>
for Internet addresses:
</p>
<dl compact="compact">
<dt> <code><var>a</var>.<var>b</var>.<var>c</var>.<var>d</var></code></dt>
<dd><p>This specifies all four bytes of the address individually and is the
commonly used representation.
</p>
</dd>
<dt> <code><var>a</var>.<var>b</var>.<var>c</var></code></dt>
<dd><p>The last part of the address, <var>c</var>, is interpreted as a 2-byte quantity.
This is useful for specifying host addresses in a Class B network with
network address number <code><var>a</var>.<var>b</var></code>.
</p>
</dd>
<dt> <code><var>a</var>.<var>b</var></code></dt>
<dd><p>The last part of the address, <var>b</var>, is interpreted as a 3-byte quantity.
This is useful for specifying host addresses in a Class A network with
network address number <var>a</var>.
</p>
</dd>
<dt> <code><var>a</var></code></dt>
<dd><p>If only one part is given, this corresponds directly to the host address
number.
</p></dd>
</dl>
<p>Within each part of the address, the usual C conventions for specifying
the radix apply. In other words, a leading ‘<samp>0x</samp>’ or ‘<samp>0X</samp>’ implies
hexadecimal radix; a leading ‘<samp>0</samp>’ implies octal; and otherwise decimal
radix is assumed.
</p>
<a name="Classless-Addresses"></a>
<h4 class="subsubheading">Classless Addresses</h4>
<p>IPv4 addresses (and IPv6 addresses also) are now considered classless;
the distinction between classes A, B and C can be ignored. Instead an
IPv4 host address consists of a 32-bit address and a 32-bit mask. The
mask contains set bits for the network part and cleared bits for the
host part. The network part is contiguous from the left, with the
remaining bits representing the host. As a consequence, the netmask can
simply be specified as the number of set bits. Classes A, B and C are
just special cases of this general rule. For example, class A addresses
have a netmask of ‘<samp>255.0.0.0</samp>’ or a prefix length of 8.
</p>
<p>Classless IPv4 network addresses are written in numbers-and-dots
notation with the prefix length appended and a slash as separator. For
example the class A network 10 is written as ‘<samp>10.0.0.0/8</samp>’.
</p>
<a name="IPv6-Addresses"></a>
<h4 class="subsubheading">IPv6 Addresses</h4>
<p>IPv6 addresses contain 128 bits (IPv4 has 32 bits) of data. A host
address is usually written as eight 16-bit hexadecimal numbers that are
separated by colons. Two colons are used to abbreviate strings of
consecutive zeros. For example, the IPv6 loopback address
‘<samp>0:0:0:0:0:0:0:1</samp>’ can just be written as ‘<samp>::1</samp>’.
</p>
<hr size="6">
<a name="Host-Address-Data-Type"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Abstract-Host-Addresses" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Host-Address-Functions" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Host-Addresses" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Host-Address-Data-Type-1"></a>
<h4 class="subsubsection">16.6.2.2 Host Address Data Type</h4>
<p>IPv4 Internet host addresses are represented in some contexts as integers
(type <code>uint32_t</code>). In other contexts, the integer is
packaged inside a structure of type <code>struct in_addr</code>. It would
be better if the usage were made consistent, but it is not hard to extract
the integer from the structure or put the integer into a structure.
</p>
<p>You will find older code that uses <code>unsigned long int</code> for
IPv4 Internet host addresses instead of <code>uint32_t</code> or <code>struct
in_addr</code>. Historically <code>unsigned long int</code> was a 32-bit number but
with 64-bit machines this has changed. Using <code>unsigned long int</code>
might break the code if it is used on machines where this type doesn’t
have 32 bits. <code>uint32_t</code> is specified by Unix98 and guaranteed to have
32 bits.
</p>
<p>IPv6 Internet host addresses have 128 bits and are packaged inside a
structure of type <code>struct in6_addr</code>.
</p>
<p>The following basic definitions for Internet addresses are declared in
the header file ‘<tt>netinet/in.h</tt>’:
<a name="index-netinet_002fin_002eh-1"></a>
</p>
<dl>
<dt><a name="index-struct-in_005faddr"></a><u>Data Type:</u> <b>struct in_addr</b></dt>
<dd><p>This data type is used in certain contexts to contain an IPv4 Internet
host address. It has just one field, named <code>s_addr</code>, which records
the host address number as an <code>uint32_t</code>.
</p></dd></dl>
<dl>
<dt><a name="index-INADDR_005fLOOPBACK"></a><u>Macro:</u> uint32_t <b>INADDR_LOOPBACK</b></dt>
<dd><p>You can use this constant to stand for “the address of this machine,”
instead of finding its actual address. It is the IPv4 Internet address
‘<samp>127.0.0.1</samp>’, which is usually called ‘<samp>localhost</samp>’. This
special constant saves you the trouble of looking up the address of your
own machine. Also, the system usually implements <code>INADDR_LOOPBACK</code>
specially, avoiding any network traffic for the case of one machine
talking to itself.
</p></dd></dl>
<dl>
<dt><a name="index-INADDR_005fANY"></a><u>Macro:</u> uint32_t <b>INADDR_ANY</b></dt>
<dd><p>You can use this constant to stand for “any incoming address” when
binding to an address. See section <a href="#Setting-Address">Setting the Address of a Socket</a>. This is the usual
address to give in the <code>sin_addr</code> member of <code>struct
sockaddr_in</code> when you want to accept Internet connections.
</p></dd></dl>
<dl>
<dt><a name="index-INADDR_005fBROADCAST"></a><u>Macro:</u> uint32_t <b>INADDR_BROADCAST</b></dt>
<dd><p>This constant is the address you use to send a broadcast message.
</p></dd></dl>
<dl>
<dt><a name="index-INADDR_005fNONE"></a><u>Macro:</u> uint32_t <b>INADDR_NONE</b></dt>
<dd><p>This constant is returned by some functions to indicate an error.
</p></dd></dl>
<dl>
<dt><a name="index-struct-in6_005faddr"></a><u>Data Type:</u> <b>struct in6_addr</b></dt>
<dd><p>This data type is used to store an IPv6 address. It stores 128 bits of
data, which can be accessed (via a union) in a variety of ways.
</p></dd></dl>
<dl>
<dt><a name="index-in6addr_005floopback"></a><u>Constant:</u> struct in6_addr <b>in6addr_loopback</b></dt>
<dd><p>This constant is the IPv6 address ‘<samp>::1</samp>’, the loopback address. See
above for a description of what this means. The macro
<code>IN6ADDR_LOOPBACK_INIT</code> is provided to allow you to initialize your
own variables to this value.
</p></dd></dl>
<dl>
<dt><a name="index-in6addr_005fany"></a><u>Constant:</u> struct in6_addr <b>in6addr_any</b></dt>
<dd><p>This constant is the IPv6 address ‘<samp>::</samp>’, the unspecified address. See
above for a description of what this means. The macro
<code>IN6ADDR_ANY_INIT</code> is provided to allow you to initialize your
own variables to this value.
</p></dd></dl>
<hr size="6">
<a name="Host-Address-Functions"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Host-Address-Data-Type" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Host-Names" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Host-Addresses" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Host-Address-Functions-1"></a>
<h4 class="subsubsection">16.6.2.3 Host Address Functions</h4>
<a name="index-arpa_002finet_002eh"></a>
<p>These additional functions for manipulating Internet addresses are
declared in the header file ‘<tt>arpa/inet.h</tt>’. They represent Internet
addresses in network byte order, and network numbers and
local-address-within-network numbers in host byte order. See section <a href="#Byte-Order">Byte Order Conversion</a>, for an explanation of network and host byte order.
</p>
<dl>
<dt><a name="index-inet_005faton"></a><u>Function:</u> int <b>inet_aton</b><i> (const char *<var>name</var>, struct in_addr *<var>addr</var>)</i></dt>
<dd><p>This function converts the IPv4 Internet host address <var>name</var>
from the standard numbers-and-dots notation into binary data and stores
it in the <code>struct in_addr</code> that <var>addr</var> points to.
<code>inet_aton</code> returns nonzero if the address is valid, zero if not.
</p></dd></dl>
<dl>
<dt><a name="index-inet_005faddr"></a><u>Function:</u> uint32_t <b>inet_addr</b><i> (const char *<var>name</var>)</i></dt>
<dd><p>This function converts the IPv4 Internet host address <var>name</var> from the
standard numbers-and-dots notation into binary data. If the input is
not valid, <code>inet_addr</code> returns <code>INADDR_NONE</code>. This is an
obsolete interface to <code>inet_aton</code>, described immediately above. It
is obsolete because <code>INADDR_NONE</code> is a valid address
(255.255.255.255), and <code>inet_aton</code> provides a cleaner way to
indicate error return.
</p></dd></dl>
<dl>
<dt><a name="index-inet_005fnetwork"></a><u>Function:</u> uint32_t <b>inet_network</b><i> (const char *<var>name</var>)</i></dt>
<dd><p>This function extracts the network number from the address <var>name</var>,
given in the standard numbers-and-dots notation. The returned address is
in host order. If the input is not valid, <code>inet_network</code> returns
<code>-1</code>.
</p>
<p>The function works only with traditional IPv4 class A, B and C network
types. It doesn’t work with classless addresses and shouldn’t be used
anymore.
</p></dd></dl>
<dl>
<dt><a name="index-inet_005fntoa"></a><u>Function:</u> char * <b>inet_ntoa</b><i> (struct in_addr <var>addr</var>)</i></dt>
<dd><p>This function converts the IPv4 Internet host address <var>addr</var> to a
string in the standard numbers-and-dots notation. The return value is
a pointer into a statically-allocated buffer. Subsequent calls will
overwrite the same buffer, so you should copy the string if you need
to save it.
</p>
<p>In multi-threaded programs each thread has an own statically-allocated
buffer. But still subsequent calls of <code>inet_ntoa</code> in the same
thread will overwrite the result of the last call.
</p>
<p>Instead of <code>inet_ntoa</code> the newer function <code>inet_ntop</code> which is
described below should be used since it handles both IPv4 and IPv6
addresses.
</p></dd></dl>
<dl>
<dt><a name="index-inet_005fmakeaddr"></a><u>Function:</u> struct in_addr <b>inet_makeaddr</b><i> (uint32_t <var>net</var>, uint32_t <var>local</var>)</i></dt>
<dd><p>This function makes an IPv4 Internet host address by combining the network
number <var>net</var> with the local-address-within-network number
<var>local</var>.
</p></dd></dl>
<dl>
<dt><a name="index-inet_005flnaof"></a><u>Function:</u> uint32_t <b>inet_lnaof</b><i> (struct in_addr <var>addr</var>)</i></dt>
<dd><p>This function returns the local-address-within-network part of the
Internet host address <var>addr</var>.
</p>
<p>The function works only with traditional IPv4 class A, B and C network
types. It doesn’t work with classless addresses and shouldn’t be used
anymore.
</p></dd></dl>
<dl>
<dt><a name="index-inet_005fnetof"></a><u>Function:</u> uint32_t <b>inet_netof</b><i> (struct in_addr <var>addr</var>)</i></dt>
<dd><p>This function returns the network number part of the Internet host
address <var>addr</var>.
</p>
<p>The function works only with traditional IPv4 class A, B and C network
types. It doesn’t work with classless addresses and shouldn’t be used
anymore.
</p></dd></dl>
<dl>
<dt><a name="index-inet_005fpton"></a><u>Function:</u> int <b>inet_pton</b><i> (int <var>af</var>, const char *<var>cp</var>, void *<var>buf</var>)</i></dt>
<dd><p>This function converts an Internet address (either IPv4 or IPv6) from
presentation (textual) to network (binary) format. <var>af</var> should be
either <code>AF_INET</code> or <code>AF_INET6</code>, as appropriate for the type of
address being converted. <var>cp</var> is a pointer to the input string, and
<var>buf</var> is a pointer to a buffer for the result. It is the caller’s
responsibility to make sure the buffer is large enough.
</p></dd></dl>
<dl>
<dt><a name="index-inet_005fntop"></a><u>Function:</u> const char * <b>inet_ntop</b><i> (int <var>af</var>, const void *<var>cp</var>, char *<var>buf</var>, size_t <var>len</var>)</i></dt>
<dd><p>This function converts an Internet address (either IPv4 or IPv6) from
network (binary) to presentation (textual) form. <var>af</var> should be
either <code>AF_INET</code> or <code>AF_INET6</code>, as appropriate. <var>cp</var> is a
pointer to the address to be converted. <var>buf</var> should be a pointer
to a buffer to hold the result, and <var>len</var> is the length of this
buffer. The return value from the function will be this buffer address.
</p></dd></dl>
<hr size="6">
<a name="Host-Names"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Host-Address-Functions" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Ports" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Host-Addresses" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Host-Names-1"></a>
<h4 class="subsubsection">16.6.2.4 Host Names</h4>
<a name="index-hosts-database"></a>
<a name="index-converting-host-name-to-address"></a>
<a name="index-converting-host-address-to-name"></a>
<p>Besides the standard numbers-and-dots notation for Internet addresses,
you can also refer to a host by a symbolic name. The advantage of a
symbolic name is that it is usually easier to remember. For example,
the machine with Internet address ‘<samp>158.121.106.19</samp>’ is also known as
‘<samp>alpha.gnu.org</samp>’; and other machines in the ‘<samp>gnu.org</samp>’
domain can refer to it simply as ‘<samp>alpha</samp>’.
</p>
<a name="index-_002fetc_002fhosts"></a>
<a name="index-netdb_002eh"></a>
<p>Internally, the system uses a database to keep track of the mapping
between host names and host numbers. This database is usually either
the file ‘<tt>/etc/hosts</tt>’ or an equivalent provided by a name server.
The functions and other symbols for accessing this database are declared
in ‘<tt>netdb.h</tt>’. They are BSD features, defined unconditionally if
you include ‘<tt>netdb.h</tt>’.
</p>
<dl>
<dt><a name="index-struct-hostent"></a><u>Data Type:</u> <b>struct hostent</b></dt>
<dd><p>This data type is used to represent an entry in the hosts database. It
has the following members:
</p>
<dl compact="compact">
<dt> <code>char *h_name</code></dt>
<dd><p>This is the “official” name of the host.
</p>
</dd>
<dt> <code>char **h_aliases</code></dt>
<dd><p>These are alternative names for the host, represented as a null-terminated
vector of strings.
</p>
</dd>
<dt> <code>int h_addrtype</code></dt>
<dd><p>This is the host address type; in practice, its value is always either
<code>AF_INET</code> or <code>AF_INET6</code>, with the latter being used for IPv6
hosts. In principle other kinds of addresses could be represented in
the database as well as Internet addresses; if this were done, you
might find a value in this field other than <code>AF_INET</code> or
<code>AF_INET6</code>. See section <a href="#Socket-Addresses">Socket Addresses</a>.
</p>
</dd>
<dt> <code>int h_length</code></dt>
<dd><p>This is the length, in bytes, of each address.
</p>
</dd>
<dt> <code>char **h_addr_list</code></dt>
<dd><p>This is the vector of addresses for the host. (Recall that the host
might be connected to multiple networks and have different addresses on
each one.) The vector is terminated by a null pointer.
</p>
</dd>
<dt> <code>char *h_addr</code></dt>
<dd><p>This is a synonym for <code>h_addr_list[0]</code>; in other words, it is the
first host address.
</p></dd>
</dl>
</dd></dl>
<p>As far as the host database is concerned, each address is just a block
of memory <code>h_length</code> bytes long. But in other contexts there is an
implicit assumption that you can convert IPv4 addresses to a
<code>struct in_addr</code> or an <code>uint32_t</code>. Host addresses in
a <code>struct hostent</code> structure are always given in network byte
order; see <a href="#Byte-Order">Byte Order Conversion</a>.
</p>
<p>You can use <code>gethostbyname</code>, <code>gethostbyname2</code> or
<code>gethostbyaddr</code> to search the hosts database for information about
a particular host. The information is returned in a
statically-allocated structure; you must copy the information if you
need to save it across calls. You can also use <code>getaddrinfo</code> and
<code>getnameinfo</code> to obtain this information.
</p>
<dl>
<dt><a name="index-gethostbyname"></a><u>Function:</u> struct hostent * <b>gethostbyname</b><i> (const char *<var>name</var>)</i></dt>
<dd><p>The <code>gethostbyname</code> function returns information about the host
named <var>name</var>. If the lookup fails, it returns a null pointer.
</p></dd></dl>
<dl>
<dt><a name="index-gethostbyname2"></a><u>Function:</u> struct hostent * <b>gethostbyname2</b><i> (const char *<var>name</var>, int <var>af</var>)</i></dt>
<dd><p>The <code>gethostbyname2</code> function is like <code>gethostbyname</code>, but
allows the caller to specify the desired address family (e.g.
<code>AF_INET</code> or <code>AF_INET6</code>) of the result.
</p></dd></dl>
<dl>
<dt><a name="index-gethostbyaddr"></a><u>Function:</u> struct hostent * <b>gethostbyaddr</b><i> (const char *<var>addr</var>, size_t <var>length</var>, int <var>format</var>)</i></dt>
<dd><p>The <code>gethostbyaddr</code> function returns information about the host
with Internet address <var>addr</var>. The parameter <var>addr</var> is not
really a pointer to char - it can be a pointer to an IPv4 or an IPv6
address. The <var>length</var> argument is the size (in bytes) of the address
at <var>addr</var>. <var>format</var> specifies the address format; for an IPv4
Internet address, specify a value of <code>AF_INET</code>; for an IPv6
Internet address, use <code>AF_INET6</code>.
</p>
<p>If the lookup fails, <code>gethostbyaddr</code> returns a null pointer.
</p></dd></dl>
<a name="index-h_005ferrno"></a>
<p>If the name lookup by <code>gethostbyname</code> or <code>gethostbyaddr</code>
fails, you can find out the reason by looking at the value of the
variable <code>h_errno</code>. (It would be cleaner design for these
functions to set <code>errno</code>, but use of <code>h_errno</code> is compatible
with other systems.)
</p>
<p>Here are the error codes that you may find in <code>h_errno</code>:
</p>
<dl compact="compact">
<dt> <code>HOST_NOT_FOUND</code></dt>
<dd><a name="index-HOST_005fNOT_005fFOUND"></a>
<p>No such host is known in the database.
</p>
</dd>
<dt> <code>TRY_AGAIN</code></dt>
<dd><a name="index-TRY_005fAGAIN"></a>
<p>This condition happens when the name server could not be contacted. If
you try again later, you may succeed then.
</p>
</dd>
<dt> <code>NO_RECOVERY</code></dt>
<dd><a name="index-NO_005fRECOVERY"></a>
<p>A non-recoverable error occurred.
</p>
</dd>
<dt> <code>NO_ADDRESS</code></dt>
<dd><a name="index-NO_005fADDRESS"></a>
<p>The host database contains an entry for the name, but it doesn’t have an
associated Internet address.
</p></dd>
</dl>
<p>The lookup functions above all have one in common: they are not
reentrant and therefore unusable in multi-threaded applications.
Therefore provides the GNU C library a new set of functions which can be
used in this context.
</p>
<dl>
<dt><a name="index-gethostbyname_005fr"></a><u>Function:</u> int <b>gethostbyname_r</b><i> (const char *restrict <var>name</var>, struct hostent *restrict <var>result_buf</var>, char *restrict <var>buf</var>, size_t <var>buflen</var>, struct hostent **restrict <var>result</var>, int *restrict <var>h_errnop</var>)</i></dt>
<dd><p>The <code>gethostbyname_r</code> function returns information about the host
named <var>name</var>. The caller must pass a pointer to an object of type
<code>struct hostent</code> in the <var>result_buf</var> parameter. In addition
the function may need extra buffer space and the caller must pass an
pointer and the size of the buffer in the <var>buf</var> and <var>buflen</var>
parameters.
</p>
<p>A pointer to the buffer, in which the result is stored, is available in
<code>*<var>result</var></code> after the function call successfully returned. If
an error occurs or if no entry is found, the pointer <code>*<var>result</var></code>
is a null pointer. Success is signalled by a zero return value. If the
function failed the return value is an error number. In addition to the
errors defined for <code>gethostbyname</code> it can also be <code>ERANGE</code>.
In this case the call should be repeated with a larger buffer.
Additional error information is not stored in the global variable
<code>h_errno</code> but instead in the object pointed to by <var>h_errnop</var>.
</p>
<p>Here’s a small example:
</p><table><tr><td> </td><td><pre class="smallexample">struct hostent *
gethostname (char *host)
{
struct hostent hostbuf, *hp;
size_t hstbuflen;
char *tmphstbuf;
int res;
int herr;
hstbuflen = 1024;
/* Allocate buffer, remember to free it to avoid memory leakage. */
tmphstbuf = malloc (hstbuflen);
while ((res = gethostbyname_r (host, &hostbuf, tmphstbuf, hstbuflen,
&hp, &herr)) == ERANGE)
{
/* Enlarge the buffer. */
hstbuflen *= 2;
tmphstbuf = realloc (tmphstbuf, hstbuflen);
}
/* Check for errors. */
if (res || hp == NULL)
return NULL;
return hp;
}
</pre></td></tr></table>
</dd></dl>
<dl>
<dt><a name="index-gethostbyname2_005fr"></a><u>Function:</u> int <b>gethostbyname2_r</b><i> (const char *<var>name</var>, int <var>af</var>, struct hostent *restrict <var>result_buf</var>, char *restrict <var>buf</var>, size_t <var>buflen</var>, struct hostent **restrict <var>result</var>, int *restrict <var>h_errnop</var>)</i></dt>
<dd><p>The <code>gethostbyname2_r</code> function is like <code>gethostbyname_r</code>, but
allows the caller to specify the desired address family (e.g.
<code>AF_INET</code> or <code>AF_INET6</code>) for the result.
</p></dd></dl>
<dl>
<dt><a name="index-gethostbyaddr_005fr"></a><u>Function:</u> int <b>gethostbyaddr_r</b><i> (const char *<var>addr</var>, size_t <var>length</var>, int <var>format</var>, struct hostent *restrict <var>result_buf</var>, char *restrict <var>buf</var>, size_t <var>buflen</var>, struct hostent **restrict <var>result</var>, int *restrict <var>h_errnop</var>)</i></dt>
<dd><p>The <code>gethostbyaddr_r</code> function returns information about the host
with Internet address <var>addr</var>. The parameter <var>addr</var> is not
really a pointer to char - it can be a pointer to an IPv4 or an IPv6
address. The <var>length</var> argument is the size (in bytes) of the address
at <var>addr</var>. <var>format</var> specifies the address format; for an IPv4
Internet address, specify a value of <code>AF_INET</code>; for an IPv6
Internet address, use <code>AF_INET6</code>.
</p>
<p>Similar to the <code>gethostbyname_r</code> function, the caller must provide
buffers for the result and memory used internally. In case of success
the function returns zero. Otherwise the value is an error number where
<code>ERANGE</code> has the special meaning that the caller-provided buffer is
too small.
</p></dd></dl>
<p>You can also scan the entire hosts database one entry at a time using
<code>sethostent</code>, <code>gethostent</code> and <code>endhostent</code>. Be careful
when using these functions because they are not reentrant.
</p>
<dl>
<dt><a name="index-sethostent"></a><u>Function:</u> void <b>sethostent</b><i> (int <var>stayopen</var>)</i></dt>
<dd><p>This function opens the hosts database to begin scanning it. You can
then call <code>gethostent</code> to read the entries.
</p>
<p>If the <var>stayopen</var> argument is nonzero, this sets a flag so that
subsequent calls to <code>gethostbyname</code> or <code>gethostbyaddr</code> will
not close the database (as they usually would). This makes for more
efficiency if you call those functions several times, by avoiding
reopening the database for each call.
</p></dd></dl>
<dl>
<dt><a name="index-gethostent"></a><u>Function:</u> struct hostent * <b>gethostent</b><i> (void)</i></dt>
<dd><p>This function returns the next entry in the hosts database. It
returns a null pointer if there are no more entries.
</p></dd></dl>
<dl>
<dt><a name="index-endhostent"></a><u>Function:</u> void <b>endhostent</b><i> (void)</i></dt>
<dd><p>This function closes the hosts database.
</p></dd></dl>
<hr size="6">
<a name="Ports"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Host-Names" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Services-Database" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Internet-Namespace" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Internet-Ports"></a>
<h3 class="subsection">16.6.3 Internet Ports</h3>
<a name="index-port-number"></a>
<p>A socket address in the Internet namespace consists of a machine’s
Internet address plus a <em>port number</em> which distinguishes the
sockets on a given machine (for a given protocol). Port numbers range
from 0 to 65,535.
</p>
<p>Port numbers less than <code>IPPORT_RESERVED</code> are reserved for standard
servers, such as <code>finger</code> and <code>telnet</code>. There is a database
that keeps track of these, and you can use the <code>getservbyname</code>
function to map a service name onto a port number; see <a href="#Services-Database">The Services Database</a>.
</p>
<p>If you write a server that is not one of the standard ones defined in
the database, you must choose a port number for it. Use a number
greater than <code>IPPORT_USERRESERVED</code>; such numbers are reserved for
servers and won’t ever be generated automatically by the system.
Avoiding conflicts with servers being run by other users is up to you.
</p>
<p>When you use a socket without specifying its address, the system
generates a port number for it. This number is between
<code>IPPORT_RESERVED</code> and <code>IPPORT_USERRESERVED</code>.
</p>
<p>On the Internet, it is actually legitimate to have two different
sockets with the same port number, as long as they never both try to
communicate with the same socket address (host address plus port
number). You shouldn’t duplicate a port number except in special
circumstances where a higher-level protocol requires it. Normally,
the system won’t let you do it; <code>bind</code> normally insists on
distinct port numbers. To reuse a port number, you must set the
socket option <code>SO_REUSEADDR</code>. See section <a href="#Socket_002dLevel-Options">Socket-Level Options</a>.
</p>
<a name="index-netinet_002fin_002eh-2"></a>
<p>These macros are defined in the header file ‘<tt>netinet/in.h</tt>’.
</p>
<dl>
<dt><a name="index-IPPORT_005fRESERVED"></a><u>Macro:</u> int <b>IPPORT_RESERVED</b></dt>
<dd><p>Port numbers less than <code>IPPORT_RESERVED</code> are reserved for
superuser use.
</p></dd></dl>
<dl>
<dt><a name="index-IPPORT_005fUSERRESERVED"></a><u>Macro:</u> int <b>IPPORT_USERRESERVED</b></dt>
<dd><p>Port numbers greater than or equal to <code>IPPORT_USERRESERVED</code> are
reserved for explicit use; they will never be allocated automatically.
</p></dd></dl>
<hr size="6">
<a name="Services-Database"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Ports" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Byte-Order" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Internet-Namespace" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="The-Services-Database"></a>
<h3 class="subsection">16.6.4 The Services Database</h3>
<a name="index-services-database"></a>
<a name="index-converting-service-name-to-port-number"></a>
<a name="index-converting-port-number-to-service-name"></a>
<a name="index-_002fetc_002fservices"></a>
<p>The database that keeps track of “well-known” services is usually
either the file ‘<tt>/etc/services</tt>’ or an equivalent from a name server.
You can use these utilities, declared in ‘<tt>netdb.h</tt>’, to access
the services database.
<a name="index-netdb_002eh-1"></a>
</p>
<dl>
<dt><a name="index-struct-servent"></a><u>Data Type:</u> <b>struct servent</b></dt>
<dd><p>This data type holds information about entries from the services database.
It has the following members:
</p>
<dl compact="compact">
<dt> <code>char *s_name</code></dt>
<dd><p>This is the “official” name of the service.
</p>
</dd>
<dt> <code>char **s_aliases</code></dt>
<dd><p>These are alternate names for the service, represented as an array of
strings. A null pointer terminates the array.
</p>
</dd>
<dt> <code>int s_port</code></dt>
<dd><p>This is the port number for the service. Port numbers are given in
network byte order; see <a href="#Byte-Order">Byte Order Conversion</a>.
</p>
</dd>
<dt> <code>char *s_proto</code></dt>
<dd><p>This is the name of the protocol to use with this service.
See section <a href="#Protocols-Database">Protocols Database</a>.
</p></dd>
</dl>
</dd></dl>
<p>To get information about a particular service, use the
<code>getservbyname</code> or <code>getservbyport</code> functions. The information
is returned in a statically-allocated structure; you must copy the
information if you need to save it across calls.
</p>
<dl>
<dt><a name="index-getservbyname"></a><u>Function:</u> struct servent * <b>getservbyname</b><i> (const char *<var>name</var>, const char *<var>proto</var>)</i></dt>
<dd><p>The <code>getservbyname</code> function returns information about the
service named <var>name</var> using protocol <var>proto</var>. If it can’t find
such a service, it returns a null pointer.
</p>
<p>This function is useful for servers as well as for clients; servers
use it to determine which port they should listen on (see section <a href="#Listening">Listening for Connections</a>).
</p></dd></dl>
<dl>
<dt><a name="index-getservbyport"></a><u>Function:</u> struct servent * <b>getservbyport</b><i> (int <var>port</var>, const char *<var>proto</var>)</i></dt>
<dd><p>The <code>getservbyport</code> function returns information about the
service at port <var>port</var> using protocol <var>proto</var>. If it can’t
find such a service, it returns a null pointer.
</p></dd></dl>
<p>You can also scan the services database using <code>setservent</code>,
<code>getservent</code> and <code>endservent</code>. Be careful when using these
functions because they are not reentrant.
</p>
<dl>
<dt><a name="index-setservent"></a><u>Function:</u> void <b>setservent</b><i> (int <var>stayopen</var>)</i></dt>
<dd><p>This function opens the services database to begin scanning it.
</p>
<p>If the <var>stayopen</var> argument is nonzero, this sets a flag so that
subsequent calls to <code>getservbyname</code> or <code>getservbyport</code> will
not close the database (as they usually would). This makes for more
efficiency if you call those functions several times, by avoiding
reopening the database for each call.
</p></dd></dl>
<dl>
<dt><a name="index-getservent"></a><u>Function:</u> struct servent * <b>getservent</b><i> (void)</i></dt>
<dd><p>This function returns the next entry in the services database. If
there are no more entries, it returns a null pointer.
</p></dd></dl>
<dl>
<dt><a name="index-endservent"></a><u>Function:</u> void <b>endservent</b><i> (void)</i></dt>
<dd><p>This function closes the services database.
</p></dd></dl>
<hr size="6">
<a name="Byte-Order"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Services-Database" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Protocols-Database" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Internet-Namespace" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Byte-Order-Conversion"></a>
<h3 class="subsection">16.6.5 Byte Order Conversion</h3>
<a name="index-byte-order-conversion_002c-for-socket"></a>
<a name="index-converting-byte-order"></a>
<a name="index-big_002dendian"></a>
<a name="index-little_002dendian"></a>
<p>Different kinds of computers use different conventions for the
ordering of bytes within a word. Some computers put the most
significant byte within a word first (this is called “big-endian”
order), and others put it last (“little-endian” order).
</p>
<a name="index-network-byte-order"></a>
<p>So that machines with different byte order conventions can
communicate, the Internet protocols specify a canonical byte order
convention for data transmitted over the network. This is known
as <em>network byte order</em>.
</p>
<p>When establishing an Internet socket connection, you must make sure that
the data in the <code>sin_port</code> and <code>sin_addr</code> members of the
<code>sockaddr_in</code> structure are represented in network byte order.
If you are encoding integer data in the messages sent through the
socket, you should convert this to network byte order too. If you don’t
do this, your program may fail when running on or talking to other kinds
of machines.
</p>
<p>If you use <code>getservbyname</code> and <code>gethostbyname</code> or
<code>inet_addr</code> to get the port number and host address, the values are
already in network byte order, and you can copy them directly into
the <code>sockaddr_in</code> structure.
</p>
<p>Otherwise, you have to convert the values explicitly. Use <code>htons</code>
and <code>ntohs</code> to convert values for the <code>sin_port</code> member. Use
<code>htonl</code> and <code>ntohl</code> to convert IPv4 addresses for the
<code>sin_addr</code> member. (Remember, <code>struct in_addr</code> is equivalent
to <code>uint32_t</code>.) These functions are declared in
‘<tt>netinet/in.h</tt>’.
<a name="index-netinet_002fin_002eh-3"></a>
</p>
<dl>
<dt><a name="index-htons"></a><u>Function:</u> uint16_t <b>htons</b><i> (uint16_t <var>hostshort</var>)</i></dt>
<dd><p>This function converts the <code>uint16_t</code> integer <var>hostshort</var> from
host byte order to network byte order.
</p></dd></dl>
<dl>
<dt><a name="index-ntohs"></a><u>Function:</u> uint16_t <b>ntohs</b><i> (uint16_t <var>netshort</var>)</i></dt>
<dd><p>This function converts the <code>uint16_t</code> integer <var>netshort</var> from
network byte order to host byte order.
</p></dd></dl>
<dl>
<dt><a name="index-htonl"></a><u>Function:</u> uint32_t <b>htonl</b><i> (uint32_t <var>hostlong</var>)</i></dt>
<dd><p>This function converts the <code>uint32_t</code> integer <var>hostlong</var> from
host byte order to network byte order.
</p>
<p>This is used for IPv4 Internet addresses.
</p></dd></dl>
<dl>
<dt><a name="index-ntohl"></a><u>Function:</u> uint32_t <b>ntohl</b><i> (uint32_t <var>netlong</var>)</i></dt>
<dd><p>This function converts the <code>uint32_t</code> integer <var>netlong</var> from
network byte order to host byte order.
</p>
<p>This is used for IPv4 Internet addresses.
</p></dd></dl>
<hr size="6">
<a name="Protocols-Database"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Byte-Order" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Inet-Example" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Internet-Namespace" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Protocols-Database-1"></a>
<h3 class="subsection">16.6.6 Protocols Database</h3>
<a name="index-protocols-database"></a>
<p>The communications protocol used with a socket controls low-level
details of how data are exchanged. For example, the protocol implements
things like checksums to detect errors in transmissions, and routing
instructions for messages. Normal user programs have little reason to
mess with these details directly.
</p>
<a name="index-TCP-_0028Internet-protocol_0029"></a>
<p>The default communications protocol for the Internet namespace depends on
the communication style. For stream communication, the default is TCP
(“transmission control protocol”). For datagram communication, the
default is UDP (“user datagram protocol”). For reliable datagram
communication, the default is RDP (“reliable datagram protocol”).
You should nearly always use the default.
</p>
<a name="index-_002fetc_002fprotocols"></a>
<p>Internet protocols are generally specified by a name instead of a
number. The network protocols that a host knows about are stored in a
database. This is usually either derived from the file
‘<tt>/etc/protocols</tt>’, or it may be an equivalent provided by a name
server. You look up the protocol number associated with a named
protocol in the database using the <code>getprotobyname</code> function.
</p>
<p>Here are detailed descriptions of the utilities for accessing the
protocols database. These are declared in ‘<tt>netdb.h</tt>’.
<a name="index-netdb_002eh-2"></a>
</p>
<dl>
<dt><a name="index-struct-protoent"></a><u>Data Type:</u> <b>struct protoent</b></dt>
<dd><p>This data type is used to represent entries in the network protocols
database. It has the following members:
</p>
<dl compact="compact">
<dt> <code>char *p_name</code></dt>
<dd><p>This is the official name of the protocol.
</p>
</dd>
<dt> <code>char **p_aliases</code></dt>
<dd><p>These are alternate names for the protocol, specified as an array of
strings. The last element of the array is a null pointer.
</p>
</dd>
<dt> <code>int p_proto</code></dt>
<dd><p>This is the protocol number (in host byte order); use this member as the
<var>protocol</var> argument to <code>socket</code>.
</p></dd>
</dl>
</dd></dl>
<p>You can use <code>getprotobyname</code> and <code>getprotobynumber</code> to search
the protocols database for a specific protocol. The information is
returned in a statically-allocated structure; you must copy the
information if you need to save it across calls.
</p>
<dl>
<dt><a name="index-getprotobyname"></a><u>Function:</u> struct protoent * <b>getprotobyname</b><i> (const char *<var>name</var>)</i></dt>
<dd><p>The <code>getprotobyname</code> function returns information about the
network protocol named <var>name</var>. If there is no such protocol, it
returns a null pointer.
</p></dd></dl>
<dl>
<dt><a name="index-getprotobynumber"></a><u>Function:</u> struct protoent * <b>getprotobynumber</b><i> (int <var>protocol</var>)</i></dt>
<dd><p>The <code>getprotobynumber</code> function returns information about the
network protocol with number <var>protocol</var>. If there is no such
protocol, it returns a null pointer.
</p></dd></dl>
<p>You can also scan the whole protocols database one protocol at a time by
using <code>setprotoent</code>, <code>getprotoent</code> and <code>endprotoent</code>.
Be careful when using these functions because they are not reentrant.
</p>
<dl>
<dt><a name="index-setprotoent"></a><u>Function:</u> void <b>setprotoent</b><i> (int <var>stayopen</var>)</i></dt>
<dd><p>This function opens the protocols database to begin scanning it.
</p>
<p>If the <var>stayopen</var> argument is nonzero, this sets a flag so that
subsequent calls to <code>getprotobyname</code> or <code>getprotobynumber</code> will
not close the database (as they usually would). This makes for more
efficiency if you call those functions several times, by avoiding
reopening the database for each call.
</p></dd></dl>
<dl>
<dt><a name="index-getprotoent"></a><u>Function:</u> struct protoent * <b>getprotoent</b><i> (void)</i></dt>
<dd><p>This function returns the next entry in the protocols database. It
returns a null pointer if there are no more entries.
</p></dd></dl>
<dl>
<dt><a name="index-endprotoent"></a><u>Function:</u> void <b>endprotoent</b><i> (void)</i></dt>
<dd><p>This function closes the protocols database.
</p></dd></dl>
<hr size="6">
<a name="Inet-Example"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Protocols-Database" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Misc-Namespaces" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Internet-Namespace" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Internet-Socket-Example"></a>
<h3 class="subsection">16.6.7 Internet Socket Example</h3>
<p>Here is an example showing how to create and name a socket in the
Internet namespace. The newly created socket exists on the machine that
the program is running on. Rather than finding and using the machine’s
Internet address, this example specifies <code>INADDR_ANY</code> as the host
address; the system replaces that with the machine’s actual address.
</p>
<table><tr><td> </td><td><pre class="smallexample">#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>
int
make_socket (uint16_t port)
{
int sock;
struct sockaddr_in name;
/* <span class="roman">Create the socket.</span> */
sock = socket (PF_INET, SOCK_STREAM, 0);
if (sock < 0)
{
perror ("socket");
exit (EXIT_FAILURE);
}
/* <span class="roman">Give the socket a name.</span> */
name.sin_family = AF_INET;
name.sin_port = htons (port);
name.sin_addr.s_addr = htonl (INADDR_ANY);
if (bind (sock, (struct sockaddr *) &name, sizeof (name)) < 0)
{
perror ("bind");
exit (EXIT_FAILURE);
}
return sock;
}
</pre></td></tr></table>
<p>Here is another example, showing how you can fill in a <code>sockaddr_in</code>
structure, given a host name string and a port number:
</p>
<table><tr><td> </td><td><pre class="smallexample">#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
void
init_sockaddr (struct sockaddr_in *name,
const char *hostname,
uint16_t port)
{
struct hostent *hostinfo;
name->sin_family = AF_INET;
name->sin_port = htons (port);
hostinfo = gethostbyname (hostname);
if (hostinfo == NULL)
{
fprintf (stderr, "Unknown host %s.\n", hostname);
exit (EXIT_FAILURE);
}
name->sin_addr = *(struct in_addr *) hostinfo->h_addr;
}
</pre></td></tr></table>
<hr size="6">
<a name="Misc-Namespaces"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Inet-Example" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Open_002fClose-Sockets" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Sockets" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Other-Namespaces"></a>
<h2 class="section">16.7 Other Namespaces</h2>
<a name="index-PF_005fNS"></a>
<a name="index-PF_005fISO"></a>
<a name="index-PF_005fCCITT"></a>
<a name="index-PF_005fIMPLINK"></a>
<a name="index-PF_005fROUTE"></a>
<p>Certain other namespaces and associated protocol families are supported
but not documented yet because they are not often used. <code>PF_NS</code>
refers to the Xerox Network Software protocols. <code>PF_ISO</code> stands
for Open Systems Interconnect. <code>PF_CCITT</code> refers to protocols from
CCITT. ‘<tt>socket.h</tt>’ defines these symbols and others naming protocols
not actually implemented.
</p>
<p><code>PF_IMPLINK</code> is used for communicating between hosts and Internet
Message Processors. For information on this and <code>PF_ROUTE</code>, an
occasionally-used local area routing protocol, see the GNU Hurd Manual
(to appear in the future).
</p>
<hr size="6">
<a name="Open_002fClose-Sockets"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Misc-Namespaces" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Creating-a-Socket" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Sockets" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Opening-and-Closing-Sockets"></a>
<h2 class="section">16.8 Opening and Closing Sockets</h2>
<p>This section describes the actual library functions for opening and
closing sockets. The same functions work for all namespaces and
connection styles.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top"><a href="#Creating-a-Socket">16.8.1 Creating a Socket</a></td><td> </td><td align="left" valign="top"> How to open a socket.
</td></tr>
<tr><td align="left" valign="top"><a href="#Closing-a-Socket">16.8.2 Closing a Socket</a></td><td> </td><td align="left" valign="top"> How to close a socket.
</td></tr>
<tr><td align="left" valign="top"><a href="#Socket-Pairs">16.8.3 Socket Pairs</a></td><td> </td><td align="left" valign="top"> These are created like pipes.
</td></tr>
</table>
<hr size="6">
<a name="Creating-a-Socket"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Open_002fClose-Sockets" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Closing-a-Socket" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Open_002fClose-Sockets" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Creating-a-Socket-1"></a>
<h3 class="subsection">16.8.1 Creating a Socket</h3>
<a name="index-creating-a-socket"></a>
<a name="index-socket_002c-creating"></a>
<a name="index-opening-a-socket"></a>
<p>The primitive for creating a socket is the <code>socket</code> function,
declared in ‘<tt>sys/socket.h</tt>’.
<a name="index-sys_002fsocket_002eh-6"></a>
</p>
<dl>
<dt><a name="index-socket-1"></a><u>Function:</u> int <b>socket</b><i> (int <var>namespace</var>, int <var>style</var>, int <var>protocol</var>)</i></dt>
<dd><p>This function creates a socket and specifies communication style
<var>style</var>, which should be one of the socket styles listed in
<a href="#Communication-Styles">Communication Styles</a>. The <var>namespace</var> argument specifies
the namespace; it must be <code>PF_LOCAL</code> (see section <a href="#Local-Namespace">The Local Namespace</a>) or
<code>PF_INET</code> (see section <a href="#Internet-Namespace">The Internet Namespace</a>). <var>protocol</var>
designates the specific protocol (see section <a href="#Socket-Concepts">Socket Concepts</a>); zero is
usually right for <var>protocol</var>.
</p>
<p>The return value from <code>socket</code> is the file descriptor for the new
socket, or <code>-1</code> in case of error. The following <code>errno</code> error
conditions are defined for this function:
</p>
<dl compact="compact">
<dt> <code>EPROTONOSUPPORT</code></dt>
<dd><p>The <var>protocol</var> or <var>style</var> is not supported by the
<var>namespace</var> specified.
</p>
</dd>
<dt> <code>EMFILE</code></dt>
<dd><p>The process already has too many file descriptors open.
</p>
</dd>
<dt> <code>ENFILE</code></dt>
<dd><p>The system already has too many file descriptors open.
</p>
</dd>
<dt> <code>EACCES</code></dt>
<dd><p>The process does not have the privilege to create a socket of the specified
<var>style</var> or <var>protocol</var>.
</p>
</dd>
<dt> <code>ENOBUFS</code></dt>
<dd><p>The system ran out of internal buffer space.
</p></dd>
</dl>
<p>The file descriptor returned by the <code>socket</code> function supports both
read and write operations. However, like pipes, sockets do not support file
positioning operations.
</p></dd></dl>
<p>For examples of how to call the <code>socket</code> function,
see <a href="#Local-Socket-Example">Example of Local-Namespace Sockets</a>, or <a href="#Inet-Example">Internet Socket Example</a>.
</p>
<hr size="6">
<a name="Closing-a-Socket"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Creating-a-Socket" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Socket-Pairs" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Open_002fClose-Sockets" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Closing-a-Socket-1"></a>
<h3 class="subsection">16.8.2 Closing a Socket</h3>
<a name="index-socket_002c-closing"></a>
<a name="index-closing-a-socket"></a>
<a name="index-shutting-down-a-socket"></a>
<a name="index-socket-shutdown"></a>
<p>When you have finished using a socket, you can simply close its
file descriptor with <code>close</code>; see <a href="libc_13.html#Opening-and-Closing-Files">Opening and Closing Files</a>.
If there is still data waiting to be transmitted over the connection,
normally <code>close</code> tries to complete this transmission. You
can control this behavior using the <code>SO_LINGER</code> socket option to
specify a timeout period; see <a href="#Socket-Options">Socket Options</a>.
</p>
<a name="index-sys_002fsocket_002eh-7"></a>
<p>You can also shut down only reception or transmission on a
connection by calling <code>shutdown</code>, which is declared in
‘<tt>sys/socket.h</tt>’.
</p>
<dl>
<dt><a name="index-shutdown"></a><u>Function:</u> int <b>shutdown</b><i> (int <var>socket</var>, int <var>how</var>)</i></dt>
<dd><p>The <code>shutdown</code> function shuts down the connection of socket
<var>socket</var>. The argument <var>how</var> specifies what action to
perform:
</p>
<dl compact="compact">
<dt> <code>0</code></dt>
<dd><p>Stop receiving data for this socket. If further data arrives,
reject it.
</p>
</dd>
<dt> <code>1</code></dt>
<dd><p>Stop trying to transmit data from this socket. Discard any data
waiting to be sent. Stop looking for acknowledgement of data already
sent; don’t retransmit it if it is lost.
</p>
</dd>
<dt> <code>2</code></dt>
<dd><p>Stop both reception and transmission.
</p></dd>
</dl>
<p>The return value is <code>0</code> on success and <code>-1</code> on failure. The
following <code>errno</code> error conditions are defined for this function:
</p>
<dl compact="compact">
<dt> <code>EBADF</code></dt>
<dd><p><var>socket</var> is not a valid file descriptor.
</p>
</dd>
<dt> <code>ENOTSOCK</code></dt>
<dd><p><var>socket</var> is not a socket.
</p>
</dd>
<dt> <code>ENOTCONN</code></dt>
<dd><p><var>socket</var> is not connected.
</p></dd>
</dl>
</dd></dl>
<hr size="6">
<a name="Socket-Pairs"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Closing-a-Socket" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Connections" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Open_002fClose-Sockets" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Socket-Pairs-1"></a>
<h3 class="subsection">16.8.3 Socket Pairs</h3>
<a name="index-creating-a-socket-pair"></a>
<a name="index-socket-pair"></a>
<a name="index-opening-a-socket-pair"></a>
<a name="index-sys_002fsocket_002eh-8"></a>
<p>A <em>socket pair</em> consists of a pair of connected (but unnamed)
sockets. It is very similar to a pipe and is used in much the same
way. Socket pairs are created with the <code>socketpair</code> function,
declared in ‘<tt>sys/socket.h</tt>’. A socket pair is much like a pipe; the
main difference is that the socket pair is bidirectional, whereas the
pipe has one input-only end and one output-only end (see section <a href="libc_15.html#Pipes-and-FIFOs">Pipes and FIFOs</a>).
</p>
<dl>
<dt><a name="index-socketpair"></a><u>Function:</u> int <b>socketpair</b><i> (int <var>namespace</var>, int <var>style</var>, int <var>protocol</var>, int <var>filedes</var><tt>[2]</tt>)</i></dt>
<dd><p>This function creates a socket pair, returning the file descriptors in
<code><var>filedes</var>[0]</code> and <code><var>filedes</var>[1]</code>. The socket pair
is a full-duplex communications channel, so that both reading and writing
may be performed at either end.
</p>
<p>The <var>namespace</var>, <var>style</var> and <var>protocol</var> arguments are
interpreted as for the <code>socket</code> function. <var>style</var> should be
one of the communication styles listed in <a href="#Communication-Styles">Communication Styles</a>.
The <var>namespace</var> argument specifies the namespace, which must be
<code>AF_LOCAL</code> (see section <a href="#Local-Namespace">The Local Namespace</a>); <var>protocol</var> specifies the
communications protocol, but zero is the only meaningful value.
</p>
<p>If <var>style</var> specifies a connectionless communication style, then
the two sockets you get are not <em>connected</em>, strictly speaking,
but each of them knows the other as the default destination address,
so they can send packets to each other.
</p>
<p>The <code>socketpair</code> function returns <code>0</code> on success and <code>-1</code>
on failure. The following <code>errno</code> error conditions are defined
for this function:
</p>
<dl compact="compact">
<dt> <code>EMFILE</code></dt>
<dd><p>The process has too many file descriptors open.
</p>
</dd>
<dt> <code>EAFNOSUPPORT</code></dt>
<dd><p>The specified namespace is not supported.
</p>
</dd>
<dt> <code>EPROTONOSUPPORT</code></dt>
<dd><p>The specified protocol is not supported.
</p>
</dd>
<dt> <code>EOPNOTSUPP</code></dt>
<dd><p>The specified protocol does not support the creation of socket pairs.
</p></dd>
</dl>
</dd></dl>
<hr size="6">
<a name="Connections"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Socket-Pairs" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Connecting" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Sockets" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Using-Sockets-with-Connections"></a>
<h2 class="section">16.9 Using Sockets with Connections</h2>
<a name="index-connection"></a>
<a name="index-client"></a>
<a name="index-server"></a>
<p>The most common communication styles involve making a connection to a
particular other socket, and then exchanging data with that socket
over and over. Making a connection is asymmetric; one side (the
<em>client</em>) acts to request a connection, while the other side (the
<em>server</em>) makes a socket and waits for the connection request.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top"><a href="#Connecting">16.9.1 Making a Connection</a></td><td> </td><td align="left" valign="top"> What the client program must do.
</td></tr>
<tr><td align="left" valign="top"><a href="#Listening">16.9.2 Listening for Connections</a></td><td> </td><td align="left" valign="top"> How a server program waits for requests.
</td></tr>
<tr><td align="left" valign="top"><a href="#Accepting-Connections">16.9.3 Accepting Connections</a></td><td> </td><td align="left" valign="top"> What the server does when it gets a request.
</td></tr>
<tr><td align="left" valign="top"><a href="#Who-is-Connected">16.9.4 Who is Connected to Me?</a></td><td> </td><td align="left" valign="top"> Getting the address of the
other side of a connection.
</td></tr>
<tr><td align="left" valign="top"><a href="#Transferring-Data">16.9.5 Transferring Data</a></td><td> </td><td align="left" valign="top"> How to send and receive data.
</td></tr>
<tr><td align="left" valign="top"><a href="#Byte-Stream-Example">16.9.6 Byte Stream Socket Example</a></td><td> </td><td align="left" valign="top"> An example program: a client for communicating
over a byte stream socket in the Internet namespace.
</td></tr>
<tr><td align="left" valign="top"><a href="#Server-Example">16.9.7 Byte Stream Connection Server Example</a></td><td> </td><td align="left" valign="top"> A corresponding server program.
</td></tr>
<tr><td align="left" valign="top"><a href="#Out_002dof_002dBand-Data">16.9.8 Out-of-Band Data</a></td><td> </td><td align="left" valign="top"> This is an advanced feature.
</td></tr>
</table>
<hr size="6">
<a name="Connecting"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Connections" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Listening" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Connections" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Making-a-Connection"></a>
<h3 class="subsection">16.9.1 Making a Connection</h3>
<a name="index-connecting-a-socket"></a>
<a name="index-socket_002c-connecting"></a>
<a name="index-socket_002c-initiating-a-connection"></a>
<a name="index-socket_002c-client-actions"></a>
<p>In making a connection, the client makes a connection while the server
waits for and accepts the connection. Here we discuss what the client
program must do with the <code>connect</code> function, which is declared in
‘<tt>sys/socket.h</tt>’.
</p>
<dl>
<dt><a name="index-connect"></a><u>Function:</u> int <b>connect</b><i> (int <var>socket</var>, struct sockaddr *<var>addr</var>, socklen_t <var>length</var>)</i></dt>
<dd><p>The <code>connect</code> function initiates a connection from the socket
with file descriptor <var>socket</var> to the socket whose address is
specified by the <var>addr</var> and <var>length</var> arguments. (This socket
is typically on another machine, and it must be already set up as a
server.) See section <a href="#Socket-Addresses">Socket Addresses</a>, for information about how these
arguments are interpreted.
</p>
<p>Normally, <code>connect</code> waits until the server responds to the request
before it returns. You can set nonblocking mode on the socket
<var>socket</var> to make <code>connect</code> return immediately without waiting
for the response. See section <a href="libc_13.html#File-Status-Flags">File Status Flags</a>, for information about
nonblocking mode.
</p>
<p>The normal return value from <code>connect</code> is <code>0</code>. If an error
occurs, <code>connect</code> returns <code>-1</code>. The following <code>errno</code>
error conditions are defined for this function:
</p>
<dl compact="compact">
<dt> <code>EBADF</code></dt>
<dd><p>The socket <var>socket</var> is not a valid file descriptor.
</p>
</dd>
<dt> <code>ENOTSOCK</code></dt>
<dd><p>File descriptor <var>socket</var> is not a socket.
</p>
</dd>
<dt> <code>EADDRNOTAVAIL</code></dt>
<dd><p>The specified address is not available on the remote machine.
</p>
</dd>
<dt> <code>EAFNOSUPPORT</code></dt>
<dd><p>The namespace of the <var>addr</var> is not supported by this socket.
</p>
</dd>
<dt> <code>EISCONN</code></dt>
<dd><p>The socket <var>socket</var> is already connected.
</p>
</dd>
<dt> <code>ETIMEDOUT</code></dt>
<dd><p>The attempt to establish the connection timed out.
</p>
</dd>
<dt> <code>ECONNREFUSED</code></dt>
<dd><p>The server has actively refused to establish the connection.
</p>
</dd>
<dt> <code>ENETUNREACH</code></dt>
<dd><p>The network of the given <var>addr</var> isn’t reachable from this host.
</p>
</dd>
<dt> <code>EADDRINUSE</code></dt>
<dd><p>The socket address of the given <var>addr</var> is already in use.
</p>
</dd>
<dt> <code>EINPROGRESS</code></dt>
<dd><p>The socket <var>socket</var> is non-blocking and the connection could not be
established immediately. You can determine when the connection is
completely established with <code>select</code>; see section <a href="libc_13.html#Waiting-for-I_002fO">Waiting for Input or Output</a>.
Another <code>connect</code> call on the same socket, before the connection is
completely established, will fail with <code>EALREADY</code>.
</p>
</dd>
<dt> <code>EALREADY</code></dt>
<dd><p>The socket <var>socket</var> is non-blocking and already has a pending
connection in progress (see <code>EINPROGRESS</code> above).
</p></dd>
</dl>
<p>This function is defined as a cancellation point in multi-threaded
programs, so one has to be prepared for this and make sure that
allocated resources (like memory, files descriptors, semaphores or
whatever) are freed even if the thread is canceled.
</p></dd></dl>
<hr size="6">
<a name="Listening"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Connecting" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Accepting-Connections" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Connections" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Listening-for-Connections"></a>
<h3 class="subsection">16.9.2 Listening for Connections</h3>
<a name="index-listening-_0028sockets_0029"></a>
<a name="index-sockets_002c-server-actions"></a>
<a name="index-sockets_002c-listening"></a>
<p>Now let us consider what the server process must do to accept
connections on a socket. First it must use the <code>listen</code> function
to enable connection requests on the socket, and then accept each
incoming connection with a call to <code>accept</code> (see section <a href="#Accepting-Connections">Accepting Connections</a>). Once connection requests are enabled on a server socket,
the <code>select</code> function reports when the socket has a connection
ready to be accepted (see section <a href="libc_13.html#Waiting-for-I_002fO">Waiting for Input or Output</a>).
</p>
<p>The <code>listen</code> function is not allowed for sockets using
connectionless communication styles.
</p>
<p>You can write a network server that does not even start running until a
connection to it is requested. See section <a href="#Inetd-Servers"><code>inetd</code> Servers</a>.
</p>
<p>In the Internet namespace, there are no special protection mechanisms
for controlling access to a port; any process on any machine
can make a connection to your server. If you want to restrict access to
your server, make it examine the addresses associated with connection
requests or implement some other handshaking or identification
protocol.
</p>
<p>In the local namespace, the ordinary file protection bits control who has
access to connect to the socket.
</p>
<dl>
<dt><a name="index-listen"></a><u>Function:</u> int <b>listen</b><i> (int <var>socket</var>, int <var>n</var>)</i></dt>
<dd><p>The <code>listen</code> function enables the socket <var>socket</var> to accept
connections, thus making it a server socket.
</p>
<p>The argument <var>n</var> specifies the length of the queue for pending
connections. When the queue fills, new clients attempting to connect
fail with <code>ECONNREFUSED</code> until the server calls <code>accept</code> to
accept a connection from the queue.
</p>
<p>The <code>listen</code> function returns <code>0</code> on success and <code>-1</code>
on failure. The following <code>errno</code> error conditions are defined
for this function:
</p>
<dl compact="compact">
<dt> <code>EBADF</code></dt>
<dd><p>The argument <var>socket</var> is not a valid file descriptor.
</p>
</dd>
<dt> <code>ENOTSOCK</code></dt>
<dd><p>The argument <var>socket</var> is not a socket.
</p>
</dd>
<dt> <code>EOPNOTSUPP</code></dt>
<dd><p>The socket <var>socket</var> does not support this operation.
</p></dd>
</dl>
</dd></dl>
<hr size="6">
<a name="Accepting-Connections"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Listening" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Who-is-Connected" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Connections" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Accepting-Connections-1"></a>
<h3 class="subsection">16.9.3 Accepting Connections</h3>
<a name="index-sockets_002c-accepting-connections"></a>
<a name="index-accepting-connections"></a>
<p>When a server receives a connection request, it can complete the
connection by accepting the request. Use the function <code>accept</code>
to do this.
</p>
<p>A socket that has been established as a server can accept connection
requests from multiple clients. The server’s original socket
<em>does not become part of the connection</em>; instead, <code>accept</code>
makes a new socket which participates in the connection.
<code>accept</code> returns the descriptor for this socket. The server’s
original socket remains available for listening for further connection
requests.
</p>
<p>The number of pending connection requests on a server socket is finite.
If connection requests arrive from clients faster than the server can
act upon them, the queue can fill up and additional requests are refused
with an <code>ECONNREFUSED</code> error. You can specify the maximum length of
this queue as an argument to the <code>listen</code> function, although the
system may also impose its own internal limit on the length of this
queue.
</p>
<dl>
<dt><a name="index-accept"></a><u>Function:</u> int <b>accept</b><i> (int <var>socket</var>, struct sockaddr *<var>addr</var>, socklen_t *<var>length_ptr</var>)</i></dt>
<dd><p>This function is used to accept a connection request on the server
socket <var>socket</var>.
</p>
<p>The <code>accept</code> function waits if there are no connections pending,
unless the socket <var>socket</var> has nonblocking mode set. (You can use
<code>select</code> to wait for a pending connection, with a nonblocking
socket.) See section <a href="libc_13.html#File-Status-Flags">File Status Flags</a>, for information about nonblocking
mode.
</p>
<p>The <var>addr</var> and <var>length-ptr</var> arguments are used to return
information about the name of the client socket that initiated the
connection. See section <a href="#Socket-Addresses">Socket Addresses</a>, for information about the format
of the information.
</p>
<p>Accepting a connection does not make <var>socket</var> part of the
connection. Instead, it creates a new socket which becomes
connected. The normal return value of <code>accept</code> is the file
descriptor for the new socket.
</p>
<p>After <code>accept</code>, the original socket <var>socket</var> remains open and
unconnected, and continues listening until you close it. You can
accept further connections with <var>socket</var> by calling <code>accept</code>
again.
</p>
<p>If an error occurs, <code>accept</code> returns <code>-1</code>. The following
<code>errno</code> error conditions are defined for this function:
</p>
<dl compact="compact">
<dt> <code>EBADF</code></dt>
<dd><p>The <var>socket</var> argument is not a valid file descriptor.
</p>
</dd>
<dt> <code>ENOTSOCK</code></dt>
<dd><p>The descriptor <var>socket</var> argument is not a socket.
</p>
</dd>
<dt> <code>EOPNOTSUPP</code></dt>
<dd><p>The descriptor <var>socket</var> does not support this operation.
</p>
</dd>
<dt> <code>EWOULDBLOCK</code></dt>
<dd><p><var>socket</var> has nonblocking mode set, and there are no pending
connections immediately available.
</p></dd>
</dl>
<p>This function is defined as a cancellation point in multi-threaded
programs, so one has to be prepared for this and make sure that
allocated resources (like memory, files descriptors, semaphores or
whatever) are freed even if the thread is canceled.
</p></dd></dl>
<p>The <code>accept</code> function is not allowed for sockets using
connectionless communication styles.
</p>
<hr size="6">
<a name="Who-is-Connected"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Accepting-Connections" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Transferring-Data" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Connections" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Who-is-Connected-to-Me_003f"></a>
<h3 class="subsection">16.9.4 Who is Connected to Me?</h3>
<dl>
<dt><a name="index-getpeername"></a><u>Function:</u> int <b>getpeername</b><i> (int <var>socket</var>, struct sockaddr *<var>addr</var>, socklen_t *<var>length-ptr</var>)</i></dt>
<dd><p>The <code>getpeername</code> function returns the address of the socket that
<var>socket</var> is connected to; it stores the address in the memory space
specified by <var>addr</var> and <var>length-ptr</var>. It stores the length of
the address in <code>*<var>length-ptr</var></code>.
</p>
<p>See section <a href="#Socket-Addresses">Socket Addresses</a>, for information about the format of the
address. In some operating systems, <code>getpeername</code> works only for
sockets in the Internet domain.
</p>
<p>The return value is <code>0</code> on success and <code>-1</code> on error. The
following <code>errno</code> error conditions are defined for this function:
</p>
<dl compact="compact">
<dt> <code>EBADF</code></dt>
<dd><p>The argument <var>socket</var> is not a valid file descriptor.
</p>
</dd>
<dt> <code>ENOTSOCK</code></dt>
<dd><p>The descriptor <var>socket</var> is not a socket.
</p>
</dd>
<dt> <code>ENOTCONN</code></dt>
<dd><p>The socket <var>socket</var> is not connected.
</p>
</dd>
<dt> <code>ENOBUFS</code></dt>
<dd><p>There are not enough internal buffers available.
</p></dd>
</dl>
</dd></dl>
<hr size="6">
<a name="Transferring-Data"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Who-is-Connected" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Sending-Data" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Connections" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Transferring-Data-1"></a>
<h3 class="subsection">16.9.5 Transferring Data</h3>
<a name="index-reading-from-a-socket"></a>
<a name="index-writing-to-a-socket"></a>
<p>Once a socket has been connected to a peer, you can use the ordinary
<code>read</code> and <code>write</code> operations (see section <a href="libc_13.html#I_002fO-Primitives">Input and Output Primitives</a>) to
transfer data. A socket is a two-way communications channel, so read
and write operations can be performed at either end.
</p>
<p>There are also some I/O modes that are specific to socket operations.
In order to specify these modes, you must use the <code>recv</code> and
<code>send</code> functions instead of the more generic <code>read</code> and
<code>write</code> functions. The <code>recv</code> and <code>send</code> functions take
an additional argument which you can use to specify various flags to
control special I/O modes. For example, you can specify the
<code>MSG_OOB</code> flag to read or write out-of-band data, the
<code>MSG_PEEK</code> flag to peek at input, or the <code>MSG_DONTROUTE</code> flag
to control inclusion of routing information on output.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top"><a href="#Sending-Data">16.9.5.1 Sending Data</a></td><td> </td><td align="left" valign="top"> Sending data with <code>send</code>.
</td></tr>
<tr><td align="left" valign="top"><a href="#Receiving-Data">16.9.5.2 Receiving Data</a></td><td> </td><td align="left" valign="top"> Reading data with <code>recv</code>.
</td></tr>
<tr><td align="left" valign="top"><a href="#Socket-Data-Options">16.9.5.3 Socket Data Options</a></td><td> </td><td align="left" valign="top"> Using <code>send</code> and <code>recv</code>.
</td></tr>
</table>
<hr size="6">
<a name="Sending-Data"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Transferring-Data" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Receiving-Data" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Transferring-Data" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Sending-Data-1"></a>
<h4 class="subsubsection">16.9.5.1 Sending Data</h4>
<a name="index-sys_002fsocket_002eh-9"></a>
<p>The <code>send</code> function is declared in the header file
‘<tt>sys/socket.h</tt>’. If your <var>flags</var> argument is zero, you can just
as well use <code>write</code> instead of <code>send</code>; see <a href="libc_13.html#I_002fO-Primitives">Input and Output Primitives</a>. If the socket was connected but the connection has broken,
you get a <code>SIGPIPE</code> signal for any use of <code>send</code> or
<code>write</code> (see section <a href="libc_24.html#Miscellaneous-Signals">Miscellaneous Signals</a>).
</p>
<dl>
<dt><a name="index-send"></a><u>Function:</u> int <b>send</b><i> (int <var>socket</var>, void *<var>buffer</var>, size_t <var>size</var>, int <var>flags</var>)</i></dt>
<dd><p>The <code>send</code> function is like <code>write</code>, but with the additional
flags <var>flags</var>. The possible values of <var>flags</var> are described
in <a href="#Socket-Data-Options">Socket Data Options</a>.
</p>
<p>This function returns the number of bytes transmitted, or <code>-1</code> on
failure. If the socket is nonblocking, then <code>send</code> (like
<code>write</code>) can return after sending just part of the data.
See section <a href="libc_13.html#File-Status-Flags">File Status Flags</a>, for information about nonblocking mode.
</p>
<p>Note, however, that a successful return value merely indicates that
the message has been sent without error, not necessarily that it has
been received without error.
</p>
<p>The following <code>errno</code> error conditions are defined for this function:
</p>
<dl compact="compact">
<dt> <code>EBADF</code></dt>
<dd><p>The <var>socket</var> argument is not a valid file descriptor.
</p>
</dd>
<dt> <code>EINTR</code></dt>
<dd><p>The operation was interrupted by a signal before any data was sent.
See section <a href="libc_24.html#Interrupted-Primitives">Primitives Interrupted by Signals</a>.
</p>
</dd>
<dt> <code>ENOTSOCK</code></dt>
<dd><p>The descriptor <var>socket</var> is not a socket.
</p>
</dd>
<dt> <code>EMSGSIZE</code></dt>
<dd><p>The socket type requires that the message be sent atomically, but the
message is too large for this to be possible.
</p>
</dd>
<dt> <code>EWOULDBLOCK</code></dt>
<dd><p>Nonblocking mode has been set on the socket, and the write operation
would block. (Normally <code>send</code> blocks until the operation can be
completed.)
</p>
</dd>
<dt> <code>ENOBUFS</code></dt>
<dd><p>There is not enough internal buffer space available.
</p>
</dd>
<dt> <code>ENOTCONN</code></dt>
<dd><p>You never connected this socket.
</p>
</dd>
<dt> <code>EPIPE</code></dt>
<dd><p>This socket was connected but the connection is now broken. In this
case, <code>send</code> generates a <code>SIGPIPE</code> signal first; if that
signal is ignored or blocked, or if its handler returns, then
<code>send</code> fails with <code>EPIPE</code>.
</p></dd>
</dl>
<p>This function is defined as a cancellation point in multi-threaded
programs, so one has to be prepared for this and make sure that
allocated resources (like memory, files descriptors, semaphores or
whatever) are freed even if the thread is canceled.
</p></dd></dl>
<hr size="6">
<a name="Receiving-Data"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Sending-Data" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Socket-Data-Options" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Transferring-Data" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Receiving-Data-1"></a>
<h4 class="subsubsection">16.9.5.2 Receiving Data</h4>
<a name="index-sys_002fsocket_002eh-10"></a>
<p>The <code>recv</code> function is declared in the header file
‘<tt>sys/socket.h</tt>’. If your <var>flags</var> argument is zero, you can
just as well use <code>read</code> instead of <code>recv</code>; see <a href="libc_13.html#I_002fO-Primitives">Input and Output Primitives</a>.
</p>
<dl>
<dt><a name="index-recv"></a><u>Function:</u> int <b>recv</b><i> (int <var>socket</var>, void *<var>buffer</var>, size_t <var>size</var>, int <var>flags</var>)</i></dt>
<dd><p>The <code>recv</code> function is like <code>read</code>, but with the additional
flags <var>flags</var>. The possible values of <var>flags</var> are described
in <a href="#Socket-Data-Options">Socket Data Options</a>.
</p>
<p>If nonblocking mode is set for <var>socket</var>, and no data are available to
be read, <code>recv</code> fails immediately rather than waiting. See section <a href="libc_13.html#File-Status-Flags">File Status Flags</a>, for information about nonblocking mode.
</p>
<p>This function returns the number of bytes received, or <code>-1</code> on failure.
The following <code>errno</code> error conditions are defined for this function:
</p>
<dl compact="compact">
<dt> <code>EBADF</code></dt>
<dd><p>The <var>socket</var> argument is not a valid file descriptor.
</p>
</dd>
<dt> <code>ENOTSOCK</code></dt>
<dd><p>The descriptor <var>socket</var> is not a socket.
</p>
</dd>
<dt> <code>EWOULDBLOCK</code></dt>
<dd><p>Nonblocking mode has been set on the socket, and the read operation
would block. (Normally, <code>recv</code> blocks until there is input
available to be read.)
</p>
</dd>
<dt> <code>EINTR</code></dt>
<dd><p>The operation was interrupted by a signal before any data was read.
See section <a href="libc_24.html#Interrupted-Primitives">Primitives Interrupted by Signals</a>.
</p>
</dd>
<dt> <code>ENOTCONN</code></dt>
<dd><p>You never connected this socket.
</p></dd>
</dl>
<p>This function is defined as a cancellation point in multi-threaded
programs, so one has to be prepared for this and make sure that
allocated resources (like memory, files descriptors, semaphores or
whatever) are freed even if the thread is canceled.
</p></dd></dl>
<hr size="6">
<a name="Socket-Data-Options"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Receiving-Data" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Byte-Stream-Example" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Transferring-Data" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Socket-Data-Options-1"></a>
<h4 class="subsubsection">16.9.5.3 Socket Data Options</h4>
<a name="index-sys_002fsocket_002eh-11"></a>
<p>The <var>flags</var> argument to <code>send</code> and <code>recv</code> is a bit
mask. You can bitwise-OR the values of the following macros together
to obtain a value for this argument. All are defined in the header
file ‘<tt>sys/socket.h</tt>’.
</p>
<dl>
<dt><a name="index-MSG_005fOOB"></a><u>Macro:</u> int <b>MSG_OOB</b></dt>
<dd><p>Send or receive out-of-band data. See section <a href="#Out_002dof_002dBand-Data">Out-of-Band Data</a>.
</p></dd></dl>
<dl>
<dt><a name="index-MSG_005fPEEK"></a><u>Macro:</u> int <b>MSG_PEEK</b></dt>
<dd><p>Look at the data but don’t remove it from the input queue. This is
only meaningful with input functions such as <code>recv</code>, not with
<code>send</code>.
</p></dd></dl>
<dl>
<dt><a name="index-MSG_005fDONTROUTE"></a><u>Macro:</u> int <b>MSG_DONTROUTE</b></dt>
<dd><p>Don’t include routing information in the message. This is only
meaningful with output operations, and is usually only of interest for
diagnostic or routing programs. We don’t try to explain it here.
</p></dd></dl>
<hr size="6">
<a name="Byte-Stream-Example"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Socket-Data-Options" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Server-Example" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Connections" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Byte-Stream-Socket-Example"></a>
<h3 class="subsection">16.9.6 Byte Stream Socket Example</h3>
<p>Here is an example client program that makes a connection for a byte
stream socket in the Internet namespace. It doesn’t do anything
particularly interesting once it has connected to the server; it just
sends a text string to the server and exits.
</p>
<p>This program uses <code>init_sockaddr</code> to set up the socket address; see
<a href="#Inet-Example">Internet Socket Example</a>.
</p>
<table><tr><td> </td><td><pre class="smallexample">#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#define PORT 5555
#define MESSAGE "Yow!!! Are we having fun yet?!?"
#define SERVERHOST "mescaline.gnu.org"
void
write_to_server (int filedes)
{
int nbytes;
nbytes = write (filedes, MESSAGE, strlen (MESSAGE) + 1);
if (nbytes < 0)
{
perror ("write");
exit (EXIT_FAILURE);
}
}
int
main (void)
{
extern void init_sockaddr (struct sockaddr_in *name,
const char *hostname,
uint16_t port);
int sock;
struct sockaddr_in servername;
/* <span class="roman">Create the socket.</span> */
sock = socket (PF_INET, SOCK_STREAM, 0);
if (sock < 0)
{
perror ("socket (client)");
exit (EXIT_FAILURE);
}
/* <span class="roman">Connect to the server.</span> */
init_sockaddr (&servername, SERVERHOST, PORT);
if (0 > connect (sock,
(struct sockaddr *) &servername,
sizeof (servername)))
{
perror ("connect (client)");
exit (EXIT_FAILURE);
}
/* <span class="roman">Send data to the server.</span> */
write_to_server (sock);
close (sock);
exit (EXIT_SUCCESS);
}
</pre></td></tr></table>
<hr size="6">
<a name="Server-Example"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Byte-Stream-Example" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Out_002dof_002dBand-Data" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Connections" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Byte-Stream-Connection-Server-Example"></a>
<h3 class="subsection">16.9.7 Byte Stream Connection Server Example</h3>
<p>The server end is much more complicated. Since we want to allow
multiple clients to be connected to the server at the same time, it
would be incorrect to wait for input from a single client by simply
calling <code>read</code> or <code>recv</code>. Instead, the right thing to do is
to use <code>select</code> (see section <a href="libc_13.html#Waiting-for-I_002fO">Waiting for Input or Output</a>) to wait for input on
all of the open sockets. This also allows the server to deal with
additional connection requests.
</p>
<p>This particular server doesn’t do anything interesting once it has
gotten a message from a client. It does close the socket for that
client when it detects an end-of-file condition (resulting from the
client shutting down its end of the connection).
</p>
<p>This program uses <code>make_socket</code> to set up the socket address; see
<a href="#Inet-Example">Internet Socket Example</a>.
</p>
<table><tr><td> </td><td><pre class="smallexample">#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#define PORT 5555
#define MAXMSG 512
int
read_from_client (int filedes)
{
char buffer[MAXMSG];
int nbytes;
nbytes = read (filedes, buffer, MAXMSG);
if (nbytes < 0)
{
/* <span class="roman">Read error.</span> */
perror ("read");
exit (EXIT_FAILURE);
}
else if (nbytes == 0)
/* <span class="roman">End-of-file.</span> */
return -1;
else
{
/* <span class="roman">Data read.</span> */
fprintf (stderr, "Server: got message: `%s'\n", buffer);
return 0;
}
}
int
main (void)
{
extern int make_socket (uint16_t port);
int sock;
fd_set active_fd_set, read_fd_set;
int i;
struct sockaddr_in clientname;
size_t size;
/* <span class="roman">Create the socket and set it up to accept connections.</span> */
sock = make_socket (PORT);
if (listen (sock, 1) < 0)
{
perror ("listen");
exit (EXIT_FAILURE);
}
/* <span class="roman">Initialize the set of active sockets.</span> */
FD_ZERO (&active_fd_set);
FD_SET (sock, &active_fd_set);
while (1)
{
/* <span class="roman">Block until input arrives on one or more active sockets.</span> */
read_fd_set = active_fd_set;
if (select (FD_SETSIZE, &read_fd_set, NULL, NULL, NULL) < 0)
{
perror ("select");
exit (EXIT_FAILURE);
}
/* <span class="roman">Service all the sockets with input pending.</span> */
for (i = 0; i < FD_SETSIZE; ++i)
if (FD_ISSET (i, &read_fd_set))
{
if (i == sock)
{
/* <span class="roman">Connection request on original socket.</span> */
int new;
size = sizeof (clientname);
new = accept (sock,
(struct sockaddr *) &clientname,
&size);
if (new < 0)
{
perror ("accept");
exit (EXIT_FAILURE);
}
fprintf (stderr,
"Server: connect from host %s, port %hd.\n",
inet_ntoa (clientname.sin_addr),
ntohs (clientname.sin_port));
FD_SET (new, &active_fd_set);
}
else
{
/* <span class="roman">Data arriving on an already-connected socket.</span> */
if (read_from_client (i) < 0)
{
close (i);
FD_CLR (i, &active_fd_set);
}
}
}
}
}
</pre></td></tr></table>
<hr size="6">
<a name="Out_002dof_002dBand-Data"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Server-Example" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Datagrams" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Connections" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Out_002dof_002dBand-Data-1"></a>
<h3 class="subsection">16.9.8 Out-of-Band Data</h3>
<a name="index-out_002dof_002dband-data"></a>
<a name="index-high_002dpriority-data"></a>
<p>Streams with connections permit <em>out-of-band</em> data that is
delivered with higher priority than ordinary data. Typically the
reason for sending out-of-band data is to send notice of an
exceptional condition. To send out-of-band data use
<code>send</code>, specifying the flag <code>MSG_OOB</code> (see section <a href="#Sending-Data">Sending Data</a>).
</p>
<p>Out-of-band data are received with higher priority because the
receiving process need not read it in sequence; to read the next
available out-of-band data, use <code>recv</code> with the <code>MSG_OOB</code>
flag (see section <a href="#Receiving-Data">Receiving Data</a>). Ordinary read operations do not read
out-of-band data; they read only ordinary data.
</p>
<a name="index-urgent-socket-condition"></a>
<p>When a socket finds that out-of-band data are on their way, it sends a
<code>SIGURG</code> signal to the owner process or process group of the
socket. You can specify the owner using the <code>F_SETOWN</code> command
to the <code>fcntl</code> function; see <a href="libc_13.html#Interrupt-Input">Interrupt-Driven Input</a>. You must
also establish a handler for this signal, as described in <a href="libc_24.html#Signal-Handling">Signal Handling</a>, in order to take appropriate action such as reading the
out-of-band data.
</p>
<p>Alternatively, you can test for pending out-of-band data, or wait
until there is out-of-band data, using the <code>select</code> function; it
can wait for an exceptional condition on the socket. See section <a href="libc_13.html#Waiting-for-I_002fO">Waiting for Input or Output</a>, for more information about <code>select</code>.
</p>
<p>Notification of out-of-band data (whether with <code>SIGURG</code> or with
<code>select</code>) indicates that out-of-band data are on the way; the data
may not actually arrive until later. If you try to read the
out-of-band data before it arrives, <code>recv</code> fails with an
<code>EWOULDBLOCK</code> error.
</p>
<p>Sending out-of-band data automatically places a “mark” in the stream
of ordinary data, showing where in the sequence the out-of-band data
“would have been”. This is useful when the meaning of out-of-band
data is “cancel everything sent so far”. Here is how you can test,
in the receiving process, whether any ordinary data was sent before
the mark:
</p>
<table><tr><td> </td><td><pre class="smallexample">success = ioctl (socket, SIOCATMARK, &atmark);
</pre></td></tr></table>
<p>The <code>integer</code> variable <var>atmark</var> is set to a nonzero value if
the socket’s read pointer has reached the “mark”.
</p>
<p>Here’s a function to discard any ordinary data preceding the
out-of-band mark:
</p>
<table><tr><td> </td><td><pre class="smallexample">int
discard_until_mark (int socket)
{
while (1)
{
/* <span class="roman">This is not an arbitrary limit; any size will do.</span> */
char buffer[1024];
int atmark, success;
/* <span class="roman">If we have reached the mark, return.</span> */
success = ioctl (socket, SIOCATMARK, &atmark);
if (success < 0)
perror ("ioctl");
if (result)
return;
/* <span class="roman">Otherwise, read a bunch of ordinary data and discard it.</span>
<span class="roman">This is guaranteed not to read past the mark</span>
<span class="roman">if it starts before the mark.</span> */
success = read (socket, buffer, sizeof buffer);
if (success < 0)
perror ("read");
}
}
</pre></td></tr></table>
<p>If you don’t want to discard the ordinary data preceding the mark, you
may need to read some of it anyway, to make room in internal system
buffers for the out-of-band data. If you try to read out-of-band data
and get an <code>EWOULDBLOCK</code> error, try reading some ordinary data
(saving it so that you can use it when you want it) and see if that
makes room. Here is an example:
</p>
<table><tr><td> </td><td><pre class="smallexample">struct buffer
{
char *buf;
int size;
struct buffer *next;
};
/* <span class="roman">Read the out-of-band data from SOCKET and return it</span>
<span class="roman">as a `struct buffer', which records the address of the data</span>
<span class="roman">and its size.</span>
<span class="roman">It may be necessary to read some ordinary data</span>
<span class="roman">in order to make room for the out-of-band data.</span>
<span class="roman">If so, the ordinary data are saved as a chain of buffers</span>
<span class="roman">found in the `next' field of the value.</span> */
struct buffer *
read_oob (int socket)
{
struct buffer *tail = 0;
struct buffer *list = 0;
while (1)
{
/* <span class="roman">This is an arbitrary limit.</span>
<span class="roman">Does anyone know how to do this without a limit?</span> */
#define BUF_SZ 1024
char *buf = (char *) xmalloc (BUF_SZ);
int success;
int atmark;
/* <span class="roman">Try again to read the out-of-band data.</span> */
success = recv (socket, buf, BUF_SZ, MSG_OOB);
if (success >= 0)
{
/* <span class="roman">We got it, so return it.</span> */
struct buffer *link
= (struct buffer *) xmalloc (sizeof (struct buffer));
link->buf = buf;
link->size = success;
link->next = list;
return link;
}
/* <span class="roman">If we fail, see if we are at the mark.</span> */
success = ioctl (socket, SIOCATMARK, &atmark);
if (success < 0)
perror ("ioctl");
if (atmark)
{
/* <span class="roman">At the mark; skipping past more ordinary data cannot help.</span>
<span class="roman">So just wait a while.</span> */
sleep (1);
continue;
}
/* <span class="roman">Otherwise, read a bunch of ordinary data and save it.</span>
<span class="roman">This is guaranteed not to read past the mark</span>
<span class="roman">if it starts before the mark.</span> */
success = read (socket, buf, BUF_SZ);
if (success < 0)
perror ("read");
/* <span class="roman">Save this data in the buffer list.</span> */
{
struct buffer *link
= (struct buffer *) xmalloc (sizeof (struct buffer));
link->buf = buf;
link->size = success;
/* <span class="roman">Add the new link to the end of the list.</span> */
if (tail)
tail->next = link;
else
list = link;
tail = link;
}
}
}
</pre></td></tr></table>
<hr size="6">
<a name="Datagrams"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Out_002dof_002dBand-Data" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Sending-Datagrams" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Sockets" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Datagram-Socket-Operations"></a>
<h2 class="section">16.10 Datagram Socket Operations</h2>
<a name="index-datagram-socket"></a>
<p>This section describes how to use communication styles that don’t use
connections (styles <code>SOCK_DGRAM</code> and <code>SOCK_RDM</code>). Using
these styles, you group data into packets and each packet is an
independent communication. You specify the destination for each
packet individually.
</p>
<p>Datagram packets are like letters: you send each one independently
with its own destination address, and they may arrive in the wrong
order or not at all.
</p>
<p>The <code>listen</code> and <code>accept</code> functions are not allowed for
sockets using connectionless communication styles.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top"><a href="#Sending-Datagrams">16.10.1 Sending Datagrams</a></td><td> </td><td align="left" valign="top"> Sending packets on a datagram socket.
</td></tr>
<tr><td align="left" valign="top"><a href="#Receiving-Datagrams">16.10.2 Receiving Datagrams</a></td><td> </td><td align="left" valign="top"> Receiving packets on a datagram socket.
</td></tr>
<tr><td align="left" valign="top"><a href="#Datagram-Example">16.10.3 Datagram Socket Example</a></td><td> </td><td align="left" valign="top"> An example program: packets sent over a
datagram socket in the local namespace.
</td></tr>
<tr><td align="left" valign="top"><a href="#Example-Receiver">16.10.4 Example of Reading Datagrams</a></td><td> </td><td align="left" valign="top"> Another program, that receives those packets.
</td></tr>
</table>
<hr size="6">
<a name="Sending-Datagrams"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Datagrams" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Receiving-Datagrams" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Datagrams" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Sending-Datagrams-1"></a>
<h3 class="subsection">16.10.1 Sending Datagrams</h3>
<a name="index-sending-a-datagram"></a>
<a name="index-transmitting-datagrams"></a>
<a name="index-datagrams_002c-transmitting"></a>
<a name="index-sys_002fsocket_002eh-12"></a>
<p>The normal way of sending data on a datagram socket is by using the
<code>sendto</code> function, declared in ‘<tt>sys/socket.h</tt>’.
</p>
<p>You can call <code>connect</code> on a datagram socket, but this only
specifies a default destination for further data transmission on the
socket. When a socket has a default destination you can use
<code>send</code> (see section <a href="#Sending-Data">Sending Data</a>) or even <code>write</code> (see section <a href="libc_13.html#I_002fO-Primitives">Input and Output Primitives</a>) to send a packet there. You can cancel the default
destination by calling <code>connect</code> using an address format of
<code>AF_UNSPEC</code> in the <var>addr</var> argument. See section <a href="#Connecting">Making a Connection</a>, for
more information about the <code>connect</code> function.
</p>
<dl>
<dt><a name="index-sendto"></a><u>Function:</u> int <b>sendto</b><i> (int <var>socket</var>, void *<var>buffer</var>. size_t <var>size</var>, int <var>flags</var>, struct sockaddr *<var>addr</var>, socklen_t <var>length</var>)</i></dt>
<dd><p>The <code>sendto</code> function transmits the data in the <var>buffer</var>
through the socket <var>socket</var> to the destination address specified
by the <var>addr</var> and <var>length</var> arguments. The <var>size</var> argument
specifies the number of bytes to be transmitted.
</p>
<p>The <var>flags</var> are interpreted the same way as for <code>send</code>; see
<a href="#Socket-Data-Options">Socket Data Options</a>.
</p>
<p>The return value and error conditions are also the same as for
<code>send</code>, but you cannot rely on the system to detect errors and
report them; the most common error is that the packet is lost or there
is no-one at the specified address to receive it, and the operating
system on your machine usually does not know this.
</p>
<p>It is also possible for one call to <code>sendto</code> to report an error
owing to a problem related to a previous call.
</p>
<p>This function is defined as a cancellation point in multi-threaded
programs, so one has to be prepared for this and make sure that
allocated resources (like memory, files descriptors, semaphores or
whatever) are freed even if the thread is canceled.
</p></dd></dl>
<hr size="6">
<a name="Receiving-Datagrams"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Sending-Datagrams" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Datagram-Example" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Datagrams" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Receiving-Datagrams-1"></a>
<h3 class="subsection">16.10.2 Receiving Datagrams</h3>
<a name="index-receiving-datagrams"></a>
<p>The <code>recvfrom</code> function reads a packet from a datagram socket and
also tells you where it was sent from. This function is declared in
‘<tt>sys/socket.h</tt>’.
</p>
<dl>
<dt><a name="index-recvfrom"></a><u>Function:</u> int <b>recvfrom</b><i> (int <var>socket</var>, void *<var>buffer</var>, size_t <var>size</var>, int <var>flags</var>, struct sockaddr *<var>addr</var>, socklen_t *<var>length-ptr</var>)</i></dt>
<dd><p>The <code>recvfrom</code> function reads one packet from the socket
<var>socket</var> into the buffer <var>buffer</var>. The <var>size</var> argument
specifies the maximum number of bytes to be read.
</p>
<p>If the packet is longer than <var>size</var> bytes, then you get the first
<var>size</var> bytes of the packet and the rest of the packet is lost.
There’s no way to read the rest of the packet. Thus, when you use a
packet protocol, you must always know how long a packet to expect.
</p>
<p>The <var>addr</var> and <var>length-ptr</var> arguments are used to return the
address where the packet came from. See section <a href="#Socket-Addresses">Socket Addresses</a>. For a
socket in the local domain the address information won’t be meaningful,
since you can’t read the address of such a socket (see section <a href="#Local-Namespace">The Local Namespace</a>). You can specify a null pointer as the <var>addr</var> argument
if you are not interested in this information.
</p>
<p>The <var>flags</var> are interpreted the same way as for <code>recv</code>
(see section <a href="#Socket-Data-Options">Socket Data Options</a>). The return value and error conditions
are also the same as for <code>recv</code>.
</p>
<p>This function is defined as a cancellation point in multi-threaded
programs, so one has to be prepared for this and make sure that
allocated resources (like memory, files descriptors, semaphores or
whatever) are freed even if the thread is canceled.
</p></dd></dl>
<p>You can use plain <code>recv</code> (see section <a href="#Receiving-Data">Receiving Data</a>) instead of
<code>recvfrom</code> if you don’t need to find out who sent the packet
(either because you know where it should come from or because you
treat all possible senders alike). Even <code>read</code> can be used if
you don’t want to specify <var>flags</var> (see section <a href="libc_13.html#I_002fO-Primitives">Input and Output Primitives</a>).
</p>
<hr size="6">
<a name="Datagram-Example"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Receiving-Datagrams" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Example-Receiver" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Datagrams" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Datagram-Socket-Example"></a>
<h3 class="subsection">16.10.3 Datagram Socket Example</h3>
<p>Here is a set of example programs that send messages over a datagram
stream in the local namespace. Both the client and server programs use
the <code>make_named_socket</code> function that was presented in <a href="#Local-Socket-Example">Example of Local-Namespace Sockets</a>, to create and name their sockets.
</p>
<p>First, here is the server program. It sits in a loop waiting for
messages to arrive, bouncing each message back to the sender.
Obviously this isn’t a particularly useful program, but it does show
the general ideas involved.
</p>
<table><tr><td> </td><td><pre class="smallexample">#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/un.h>
#define SERVER "/tmp/serversocket"
#define MAXMSG 512
int
main (void)
{
int sock;
char message[MAXMSG];
struct sockaddr_un name;
size_t size;
int nbytes;
/* <span class="roman">Remove the filename first, it's ok if the call fails</span> */
unlink (SERVER);
/* <span class="roman">Make the socket, then loop endlessly.</span> */
sock = make_named_socket (SERVER);
while (1)
{
/* <span class="roman">Wait for a datagram.</span> */
size = sizeof (name);
nbytes = recvfrom (sock, message, MAXMSG, 0,
(struct sockaddr *) & name, &size);
if (nbytes < 0)
{
perror ("recfrom (server)");
exit (EXIT_FAILURE);
}
/* <span class="roman">Give a diagnostic message.</span> */
fprintf (stderr, "Server: got message: %s\n", message);
/* <span class="roman">Bounce the message back to the sender.</span> */
nbytes = sendto (sock, message, nbytes, 0,
(struct sockaddr *) & name, size);
if (nbytes < 0)
{
perror ("sendto (server)");
exit (EXIT_FAILURE);
}
}
}
</pre></td></tr></table>
<hr size="6">
<a name="Example-Receiver"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Datagram-Example" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Inetd" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Datagrams" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Example-of-Reading-Datagrams"></a>
<h3 class="subsection">16.10.4 Example of Reading Datagrams</h3>
<p>Here is the client program corresponding to the server above.
</p>
<p>It sends a datagram to the server and then waits for a reply. Notice
that the socket for the client (as well as for the server) in this
example has to be given a name. This is so that the server can direct
a message back to the client. Since the socket has no associated
connection state, the only way the server can do this is by
referencing the name of the client.
</p>
<table><tr><td> </td><td><pre class="smallexample">#include <stdio.h>
#include <errno.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/un.h>
#define SERVER "/tmp/serversocket"
#define CLIENT "/tmp/mysocket"
#define MAXMSG 512
#define MESSAGE "Yow!!! Are we having fun yet?!?"
int
main (void)
{
extern int make_named_socket (const char *name);
int sock;
char message[MAXMSG];
struct sockaddr_un name;
size_t size;
int nbytes;
/* <span class="roman">Make the socket.</span> */
sock = make_named_socket (CLIENT);
/* <span class="roman">Initialize the server socket address.</span> */
name.sun_family = AF_LOCAL;
strcpy (name.sun_path, SERVER);
size = strlen (name.sun_path) + sizeof (name.sun_family);
/* <span class="roman">Send the datagram.</span> */
nbytes = sendto (sock, MESSAGE, strlen (MESSAGE) + 1, 0,
(struct sockaddr *) & name, size);
if (nbytes < 0)
{
perror ("sendto (client)");
exit (EXIT_FAILURE);
}
/* <span class="roman">Wait for a reply.</span> */
nbytes = recvfrom (sock, message, MAXMSG, 0, NULL, 0);
if (nbytes < 0)
{
perror ("recfrom (client)");
exit (EXIT_FAILURE);
}
/* <span class="roman">Print a diagnostic message.</span> */
fprintf (stderr, "Client: got message: %s\n", message);
/* <span class="roman">Clean up.</span> */
remove (CLIENT);
close (sock);
}
</pre></td></tr></table>
<p>Keep in mind that datagram socket communications are unreliable. In
this example, the client program waits indefinitely if the message
never reaches the server or if the server’s response never comes
back. It’s up to the user running the program to kill and restart
it if desired. A more automatic solution could be to use
<code>select</code> (see section <a href="libc_13.html#Waiting-for-I_002fO">Waiting for Input or Output</a>) to establish a timeout period
for the reply, and in case of timeout either re-send the message or
shut down the socket and exit.
</p>
<hr size="6">
<a name="Inetd"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Example-Receiver" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Inetd-Servers" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Sockets" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="The-inetd-Daemon"></a>
<h2 class="section">16.11 The <code>inetd</code> Daemon</h2>
<p>We’ve explained above how to write a server program that does its own
listening. Such a server must already be running in order for anyone
to connect to it.
</p>
<p>Another way to provide a service on an Internet port is to let the daemon
program <code>inetd</code> do the listening. <code>inetd</code> is a program that
runs all the time and waits (using <code>select</code>) for messages on a
specified set of ports. When it receives a message, it accepts the
connection (if the socket style calls for connections) and then forks a
child process to run the corresponding server program. You specify the
ports and their programs in the file ‘<tt>/etc/inetd.conf</tt>’.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top"><a href="#Inetd-Servers">16.11.1 <code>inetd</code> Servers</a></td><td> </td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top"><a href="#Configuring-Inetd">16.11.2 Configuring <code>inetd</code></a></td><td> </td><td align="left" valign="top">
</td></tr>
</table>
<hr size="6">
<a name="Inetd-Servers"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Inetd" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Configuring-Inetd" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Inetd" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="inetd-Servers"></a>
<h3 class="subsection">16.11.1 <code>inetd</code> Servers</h3>
<p>Writing a server program to be run by <code>inetd</code> is very simple. Each time
someone requests a connection to the appropriate port, a new server
process starts. The connection already exists at this time; the
socket is available as the standard input descriptor and as the
standard output descriptor (descriptors 0 and 1) in the server
process. Thus the server program can begin reading and writing data
right away. Often the program needs only the ordinary I/O facilities;
in fact, a general-purpose filter program that knows nothing about
sockets can work as a byte stream server run by <code>inetd</code>.
</p>
<p>You can also use <code>inetd</code> for servers that use connectionless
communication styles. For these servers, <code>inetd</code> does not try to accept
a connection since no connection is possible. It just starts the
server program, which can read the incoming datagram packet from
descriptor 0. The server program can handle one request and then
exit, or you can choose to write it to keep reading more requests
until no more arrive, and then exit. You must specify which of these
two techniques the server uses when you configure <code>inetd</code>.
</p>
<hr size="6">
<a name="Configuring-Inetd"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Inetd-Servers" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Socket-Options" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Inetd" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Configuring-inetd"></a>
<h3 class="subsection">16.11.2 Configuring <code>inetd</code></h3>
<p>The file ‘<tt>/etc/inetd.conf</tt>’ tells <code>inetd</code> which ports to listen to
and what server programs to run for them. Normally each entry in the
file is one line, but you can split it onto multiple lines provided
all but the first line of the entry start with whitespace. Lines that
start with ‘<samp>#</samp>’ are comments.
</p>
<p>Here are two standard entries in ‘<tt>/etc/inetd.conf</tt>’:
</p>
<table><tr><td> </td><td><pre class="smallexample">ftp stream tcp nowait root /libexec/ftpd ftpd
talk dgram udp wait root /libexec/talkd talkd
</pre></td></tr></table>
<p>An entry has this format:
</p>
<table><tr><td> </td><td><pre class="smallexample"><var>service</var> <var>style</var> <var>protocol</var> <var>wait</var> <var>username</var> <var>program</var> <var>arguments</var>
</pre></td></tr></table>
<p>The <var>service</var> field says which service this program provides. It
should be the name of a service defined in ‘<tt>/etc/services</tt>’.
<code>inetd</code> uses <var>service</var> to decide which port to listen on for
this entry.
</p>
<p>The fields <var>style</var> and <var>protocol</var> specify the communication
style and the protocol to use for the listening socket. The style
should be the name of a communication style, converted to lower case
and with ‘<samp>SOCK_</samp>’ deleted—for example, ‘<samp>stream</samp>’ or
‘<samp>dgram</samp>’. <var>protocol</var> should be one of the protocols listed in
‘<tt>/etc/protocols</tt>’. The typical protocol names are ‘<samp>tcp</samp>’ for
byte stream connections and ‘<samp>udp</samp>’ for unreliable datagrams.
</p>
<p>The <var>wait</var> field should be either ‘<samp>wait</samp>’ or ‘<samp>nowait</samp>’.
Use ‘<samp>wait</samp>’ if <var>style</var> is a connectionless style and the
server, once started, handles multiple requests as they come in.
Use ‘<samp>nowait</samp>’ if <code>inetd</code> should start a new process for each message
or request that comes in. If <var>style</var> uses connections, then
<var>wait</var> <strong>must</strong> be ‘<samp>nowait</samp>’.
</p>
<p><var>user</var> is the user name that the server should run as. <code>inetd</code> runs
as root, so it can set the user ID of its children arbitrarily. It’s
best to avoid using ‘<samp>root</samp>’ for <var>user</var> if you can; but some
servers, such as Telnet and FTP, read a username and password
themselves. These servers need to be root initially so they can log
in as commanded by the data coming over the network.
</p>
<p><var>program</var> together with <var>arguments</var> specifies the command to
run to start the server. <var>program</var> should be an absolute file
name specifying the executable file to run. <var>arguments</var> consists
of any number of whitespace-separated words, which become the
command-line arguments of <var>program</var>. The first word in
<var>arguments</var> is argument zero, which should by convention be the
program name itself (sans directories).
</p>
<p>If you edit ‘<tt>/etc/inetd.conf</tt>’, you can tell <code>inetd</code> to reread the
file and obey its new contents by sending the <code>inetd</code> process the
<code>SIGHUP</code> signal. You’ll have to use <code>ps</code> to determine the
process ID of the <code>inetd</code> process as it is not fixed.
</p>
<hr size="6">
<a name="Socket-Options"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Configuring-Inetd" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Socket-Option-Functions" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Sockets" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Socket-Options-1"></a>
<h2 class="section">16.12 Socket Options</h2>
<a name="index-socket-options"></a>
<p>This section describes how to read or set various options that modify
the behavior of sockets and their underlying communications protocols.
</p>
<a name="index-level_002c-for-socket-options"></a>
<a name="index-socket-option-level"></a>
<p>When you are manipulating a socket option, you must specify which
<em>level</em> the option pertains to. This describes whether the option
applies to the socket interface, or to a lower-level communications
protocol interface.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top"><a href="#Socket-Option-Functions">16.12.1 Socket Option Functions</a></td><td> </td><td align="left" valign="top"> The basic functions for setting and getting
socket options.
</td></tr>
<tr><td align="left" valign="top"><a href="#Socket_002dLevel-Options">16.12.2 Socket-Level Options</a></td><td> </td><td align="left" valign="top"> Details of the options at the socket level.
</td></tr>
</table>
<hr size="6">
<a name="Socket-Option-Functions"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Socket-Options" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Socket_002dLevel-Options" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Socket-Options" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Socket-Option-Functions-1"></a>
<h3 class="subsection">16.12.1 Socket Option Functions</h3>
<a name="index-sys_002fsocket_002eh-13"></a>
<p>Here are the functions for examining and modifying socket options.
They are declared in ‘<tt>sys/socket.h</tt>’.
</p>
<dl>
<dt><a name="index-getsockopt"></a><u>Function:</u> int <b>getsockopt</b><i> (int <var>socket</var>, int <var>level</var>, int <var>optname</var>, void *<var>optval</var>, socklen_t *<var>optlen-ptr</var>)</i></dt>
<dd><p>The <code>getsockopt</code> function gets information about the value of
option <var>optname</var> at level <var>level</var> for socket <var>socket</var>.
</p>
<p>The option value is stored in a buffer that <var>optval</var> points to.
Before the call, you should supply in <code>*<var>optlen-ptr</var></code> the
size of this buffer; on return, it contains the number of bytes of
information actually stored in the buffer.
</p>
<p>Most options interpret the <var>optval</var> buffer as a single <code>int</code>
value.
</p>
<p>The actual return value of <code>getsockopt</code> is <code>0</code> on success
and <code>-1</code> on failure. The following <code>errno</code> error conditions
are defined:
</p>
<dl compact="compact">
<dt> <code>EBADF</code></dt>
<dd><p>The <var>socket</var> argument is not a valid file descriptor.
</p>
</dd>
<dt> <code>ENOTSOCK</code></dt>
<dd><p>The descriptor <var>socket</var> is not a socket.
</p>
</dd>
<dt> <code>ENOPROTOOPT</code></dt>
<dd><p>The <var>optname</var> doesn’t make sense for the given <var>level</var>.
</p></dd>
</dl>
</dd></dl>
<dl>
<dt><a name="index-setsockopt"></a><u>Function:</u> int <b>setsockopt</b><i> (int <var>socket</var>, int <var>level</var>, int <var>optname</var>, void *<var>optval</var>, socklen_t <var>optlen</var>)</i></dt>
<dd><p>This function is used to set the socket option <var>optname</var> at level
<var>level</var> for socket <var>socket</var>. The value of the option is passed
in the buffer <var>optval</var> of size <var>optlen</var>.
</p>
</dd></dl>
<hr size="6">
<a name="Socket_002dLevel-Options"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Socket-Option-Functions" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Networks-Database" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Socket-Options" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Socket_002dLevel-Options-1"></a>
<h3 class="subsection">16.12.2 Socket-Level Options</h3>
<dl>
<dt><a name="index-SOL_005fSOCKET"></a><u>Constant:</u> int <b>SOL_SOCKET</b></dt>
<dd><p>Use this constant as the <var>level</var> argument to <code>getsockopt</code> or
<code>setsockopt</code> to manipulate the socket-level options described in
this section.
</p></dd></dl>
<a name="index-sys_002fsocket_002eh-14"></a>
<p>Here is a table of socket-level option names; all are defined in the
header file ‘<tt>sys/socket.h</tt>’.
</p>
<dl compact="compact">
<dt> <code>SO_DEBUG</code></dt>
<dd>
<p>This option toggles recording of debugging information in the underlying
protocol modules. The value has type <code>int</code>; a nonzero value means
“yes”.
</p>
</dd>
<dt> <code>SO_REUSEADDR</code></dt>
<dd><p>This option controls whether <code>bind</code> (see section <a href="#Setting-Address">Setting the Address of a Socket</a>)
should permit reuse of local addresses for this socket. If you enable
this option, you can actually have two sockets with the same Internet
port number; but the system won’t allow you to use the two
identically-named sockets in a way that would confuse the Internet. The
reason for this option is that some higher-level Internet protocols,
including FTP, require you to keep reusing the same port number.
</p>
<p>The value has type <code>int</code>; a nonzero value means “yes”.
</p>
</dd>
<dt> <code>SO_KEEPALIVE</code></dt>
<dd><p>This option controls whether the underlying protocol should
periodically transmit messages on a connected socket. If the peer
fails to respond to these messages, the connection is considered
broken. The value has type <code>int</code>; a nonzero value means
“yes”.
</p>
</dd>
<dt> <code>SO_DONTROUTE</code></dt>
<dd><p>This option controls whether outgoing messages bypass the normal
message routing facilities. If set, messages are sent directly to the
network interface instead. The value has type <code>int</code>; a nonzero
value means “yes”.
</p>
</dd>
<dt> <code>SO_LINGER</code></dt>
<dd><p>This option specifies what should happen when the socket of a type
that promises reliable delivery still has untransmitted messages when
it is closed; see <a href="#Closing-a-Socket">Closing a Socket</a>. The value has type
<code>struct linger</code>.
</p>
<dl>
<dt><a name="index-struct-linger"></a><u>Data Type:</u> <b>struct linger</b></dt>
<dd><p>This structure type has the following members:
</p>
<dl compact="compact">
<dt> <code>int l_onoff</code></dt>
<dd><p>This field is interpreted as a boolean. If nonzero, <code>close</code>
blocks until the data are transmitted or the timeout period has expired.
</p>
</dd>
<dt> <code>int l_linger</code></dt>
<dd><p>This specifies the timeout period, in seconds.
</p></dd>
</dl>
</dd></dl>
</dd>
<dt> <code>SO_BROADCAST</code></dt>
<dd><p>This option controls whether datagrams may be broadcast from the socket.
The value has type <code>int</code>; a nonzero value means “yes”.
</p>
</dd>
<dt> <code>SO_OOBINLINE</code></dt>
<dd><p>If this option is set, out-of-band data received on the socket is
placed in the normal input queue. This permits it to be read using
<code>read</code> or <code>recv</code> without specifying the <code>MSG_OOB</code>
flag. See section <a href="#Out_002dof_002dBand-Data">Out-of-Band Data</a>. The value has type <code>int</code>; a
nonzero value means “yes”.
</p>
</dd>
<dt> <code>SO_SNDBUF</code></dt>
<dd><p>This option gets or sets the size of the output buffer. The value is a
<code>size_t</code>, which is the size in bytes.
</p>
</dd>
<dt> <code>SO_RCVBUF</code></dt>
<dd><p>This option gets or sets the size of the input buffer. The value is a
<code>size_t</code>, which is the size in bytes.
</p>
</dd>
<dt> <code>SO_STYLE</code></dt>
<dt> <code>SO_TYPE</code></dt>
<dd><p>This option can be used with <code>getsockopt</code> only. It is used to
get the socket’s communication style. <code>SO_TYPE</code> is the
historical name, and <code>SO_STYLE</code> is the preferred name in GNU.
The value has type <code>int</code> and its value designates a communication
style; see <a href="#Communication-Styles">Communication Styles</a>.
</p>
</dd>
<dt> <code>SO_ERROR</code></dt>
<dd>
<p>This option can be used with <code>getsockopt</code> only. It is used to reset
the error status of the socket. The value is an <code>int</code>, which represents
the previous error status.
</p></dd>
</dl>
<hr size="6">
<a name="Networks-Database"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Socket_002dLevel-Options" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Sockets" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Networks-Database-1"></a>
<h2 class="section">16.13 Networks Database</h2>
<a name="index-networks-database"></a>
<a name="index-converting-network-number-to-network-name"></a>
<a name="index-converting-network-name-to-network-number"></a>
<a name="index-_002fetc_002fnetworks"></a>
<a name="index-netdb_002eh-3"></a>
<p>Many systems come with a database that records a list of networks known
to the system developer. This is usually kept either in the file
‘<tt>/etc/networks</tt>’ or in an equivalent from a name server. This data
base is useful for routing programs such as <code>route</code>, but it is not
useful for programs that simply communicate over the network. We
provide functions to access this database, which are declared in
‘<tt>netdb.h</tt>’.
</p>
<dl>
<dt><a name="index-struct-netent"></a><u>Data Type:</u> <b>struct netent</b></dt>
<dd><p>This data type is used to represent information about entries in the
networks database. It has the following members:
</p>
<dl compact="compact">
<dt> <code>char *n_name</code></dt>
<dd><p>This is the “official” name of the network.
</p>
</dd>
<dt> <code>char **n_aliases</code></dt>
<dd><p>These are alternative names for the network, represented as a vector
of strings. A null pointer terminates the array.
</p>
</dd>
<dt> <code>int n_addrtype</code></dt>
<dd><p>This is the type of the network number; this is always equal to
<code>AF_INET</code> for Internet networks.
</p>
</dd>
<dt> <code>unsigned long int n_net</code></dt>
<dd><p>This is the network number. Network numbers are returned in host
byte order; see <a href="#Byte-Order">Byte Order Conversion</a>.
</p></dd>
</dl>
</dd></dl>
<p>Use the <code>getnetbyname</code> or <code>getnetbyaddr</code> functions to search
the networks database for information about a specific network. The
information is returned in a statically-allocated structure; you must
copy the information if you need to save it.
</p>
<dl>
<dt><a name="index-getnetbyname"></a><u>Function:</u> struct netent * <b>getnetbyname</b><i> (const char *<var>name</var>)</i></dt>
<dd><p>The <code>getnetbyname</code> function returns information about the network
named <var>name</var>. It returns a null pointer if there is no such
network.
</p></dd></dl>
<dl>
<dt><a name="index-getnetbyaddr"></a><u>Function:</u> struct netent * <b>getnetbyaddr</b><i> (unsigned long int <var>net</var>, int <var>type</var>)</i></dt>
<dd><p>The <code>getnetbyaddr</code> function returns information about the network
of type <var>type</var> with number <var>net</var>. You should specify a value of
<code>AF_INET</code> for the <var>type</var> argument for Internet networks.
</p>
<p><code>getnetbyaddr</code> returns a null pointer if there is no such
network.
</p></dd></dl>
<p>You can also scan the networks database using <code>setnetent</code>,
<code>getnetent</code> and <code>endnetent</code>. Be careful when using these
functions because they are not reentrant.
</p>
<dl>
<dt><a name="index-setnetent"></a><u>Function:</u> void <b>setnetent</b><i> (int <var>stayopen</var>)</i></dt>
<dd><p>This function opens and rewinds the networks database.
</p>
<p>If the <var>stayopen</var> argument is nonzero, this sets a flag so that
subsequent calls to <code>getnetbyname</code> or <code>getnetbyaddr</code> will
not close the database (as they usually would). This makes for more
efficiency if you call those functions several times, by avoiding
reopening the database for each call.
</p></dd></dl>
<dl>
<dt><a name="index-getnetent"></a><u>Function:</u> struct netent * <b>getnetent</b><i> (void)</i></dt>
<dd><p>This function returns the next entry in the networks database. It
returns a null pointer if there are no more entries.
</p></dd></dl>
<dl>
<dt><a name="index-endnetent"></a><u>Function:</u> void <b>endnetent</b><i> (void)</i></dt>
<dd><p>This function closes the networks database.
</p></dd></dl>
<hr size="6">
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Sockets" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="libc_17.html#Low_002dLevel-Terminal-Interface" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="libc.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="libc_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="libc_42.html#Concept-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<p>
<font size="-1">
This document was generated by <em>root</em> on <em>April 20, 2012</em> using <a href="http://www.nongnu.org/texi2html/"><em>texi2html 1.82</em></a>.
</font>
<br>
</p>
</body>
</html>
|