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<a name="String-and-Array-Utilities-1"></a>
<h1 class="chapter">5. String and Array Utilities</h1>

<p>Operations on strings (or arrays of characters) are an important part of
many programs.  The GNU C library provides an extensive set of string
utility functions, including functions for copying, concatenating,
comparing, and searching strings.  Many of these functions can also
operate on arbitrary regions of storage; for example, the <code>memcpy</code>
function can be used to copy the contents of any kind of array.
</p>
<p>It&rsquo;s fairly common for beginning C programmers to &ldquo;reinvent the wheel&rdquo;
by duplicating this functionality in their own code, but it pays to
become familiar with the library functions and to make use of them,
since this offers benefits in maintenance, efficiency, and portability.
</p>
<p>For instance, you could easily compare one string to another in two
lines of C code, but if you use the built-in <code>strcmp</code> function,
you&rsquo;re less likely to make a mistake.  And, since these library
functions are typically highly optimized, your program may run faster
too.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top"><a href="#Representation-of-Strings">5.1 Representation of Strings</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">   Introduction to basic concepts.
</td></tr>
<tr><td align="left" valign="top"><a href="#String_002fArray-Conventions">5.2 String and Array Conventions</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">    Whether to use a string function or an
				 arbitrary array function.
</td></tr>
<tr><td align="left" valign="top"><a href="#String-Length">5.3 String Length</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">               Determining the length of a string.
</td></tr>
<tr><td align="left" valign="top"><a href="#Copying-and-Concatenation">5.4 Copying and Concatenation</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">   Functions to copy the contents of strings
				 and arrays.
</td></tr>
<tr><td align="left" valign="top"><a href="#String_002fArray-Comparison">5.5 String/Array Comparison</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">     Functions for byte-wise and character-wise
				 comparison.
</td></tr>
<tr><td align="left" valign="top"><a href="#Collation-Functions">5.6 Collation Functions</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">         Functions for collating strings.
</td></tr>
<tr><td align="left" valign="top"><a href="#Search-Functions">5.7 Search Functions</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">            Searching for a specific element or substring.
</td></tr>
<tr><td align="left" valign="top"><a href="#Finding-Tokens-in-a-String">5.8 Finding Tokens in a String</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">  Splitting a string into tokens by looking
				 for delimiters.
</td></tr>
<tr><td align="left" valign="top"><a href="#strfry">5.9 strfry</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">                      Function for flash-cooking a string.
</td></tr>
<tr><td align="left" valign="top"><a href="#Trivial-Encryption">5.10 Trivial Encryption</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">          Obscuring data.
</td></tr>
<tr><td align="left" valign="top"><a href="#Encode-Binary-Data">5.11 Encode Binary Data</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">          Encoding and Decoding of Binary Data.
</td></tr>
<tr><td align="left" valign="top"><a href="#Argz-and-Envz-Vectors">5.12 Argz and Envz Vectors</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">       Null-separated string vectors.
</td></tr>
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<a name="Representation-of-Strings-1"></a>
<h2 class="section">5.1 Representation of Strings</h2>
<a name="index-string_002c-representation-of"></a>

<p>This section is a quick summary of string concepts for beginning C
programmers.  It describes how character strings are represented in C
and some common pitfalls.  If you are already familiar with this
material, you can skip this section.
</p>
<a name="index-string"></a>
<a name="index-multibyte-character-string"></a>
<p>A <em>string</em> is an array of <code>char</code> objects.  But string-valued
variables are usually declared to be pointers of type <code>char *</code>.
Such variables do not include space for the text of a string; that has
to be stored somewhere else&mdash;in an array variable, a string constant,
or dynamically allocated memory (see section <a href="libc_3.html#Memory-Allocation">Allocating Storage For Program Data</a>).  It&rsquo;s up to
you to store the address of the chosen memory space into the pointer
variable.  Alternatively you can store a <em>null pointer</em> in the
pointer variable.  The null pointer does not point anywhere, so
attempting to reference the string it points to gets an error.
</p>
<a name="index-wide-character-string"></a>
<p>&ldquo;string&rdquo; normally refers to multibyte character strings as opposed to
wide character strings.  Wide character strings are arrays of type
<code>wchar_t</code> and as for multibyte character strings usually pointers
of type <code>wchar_t *</code> are used.
</p>
<a name="index-null-character"></a>
<a name="index-null-wide-character"></a>
<p>By convention, a <em>null character</em>, <code>'\0'</code>, marks the end of a
multibyte character string and the <em>null wide character</em>,
<code>L'\0'</code>, marks the end of a wide character string.  For example, in
testing to see whether the <code>char *</code> variable <var>p</var> points to a
null character marking the end of a string, you can write
<code>!*<var>p</var></code> or <code>*<var>p</var> == '\0'</code>.
</p>
<p>A null character is quite different conceptually from a null pointer,
although both are represented by the integer <code>0</code>.
</p>
<a name="index-string-literal"></a>
<p><em>String literals</em> appear in C program source as strings of
characters between double-quote characters (&lsquo;<samp>&quot;</samp>&rsquo;) where the initial
double-quote character is immediately preceded by a capital &lsquo;<samp>L</samp>&rsquo;
(ell) character (as in <code>L&quot;foo&quot;</code>).  In ISO C, string literals
can also be formed by <em>string concatenation</em>: <code>&quot;a&quot; &quot;b&quot;</code> is the
same as <code>&quot;ab&quot;</code>.  For wide character strings one can either use
<code>L&quot;a&quot; L&quot;b&quot;</code> or <code>L&quot;a&quot; &quot;b&quot;</code>.  Modification of string literals is
not allowed by the GNU C compiler, because literals are placed in
read-only storage.
</p>
<p>Character arrays that are declared <code>const</code> cannot be modified
either.  It&rsquo;s generally good style to declare non-modifiable string
pointers to be of type <code>const char *</code>, since this often allows the
C compiler to detect accidental modifications as well as providing some
amount of documentation about what your program intends to do with the
string.
</p>
<p>The amount of memory allocated for the character array may extend past
the null character that normally marks the end of the string.  In this
document, the term <em>allocated size</em> is always used to refer to the
total amount of memory allocated for the string, while the term
<em>length</em> refers to the number of characters up to (but not
including) the terminating null character.
<a name="index-length-of-string"></a>
<a name="index-allocation-size-of-string"></a>
<a name="index-size-of-string"></a>
<a name="index-string-length"></a>
<a name="index-string-allocation"></a>
</p>
<p>A notorious source of program bugs is trying to put more characters in a
string than fit in its allocated size.  When writing code that extends
strings or moves characters into a pre-allocated array, you should be
very careful to keep track of the length of the text and make explicit
checks for overflowing the array.  Many of the library functions
<em>do not</em> do this for you!  Remember also that you need to allocate
an extra byte to hold the null character that marks the end of the
string.
</p>
<a name="index-single_002dbyte-string"></a>
<a name="index-multibyte-string"></a>
<p>Originally strings were sequences of bytes where each byte represents a
single character.  This is still true today if the strings are encoded
using a single-byte character encoding.  Things are different if the
strings are encoded using a multibyte encoding (for more information on
encodings see <a href="libc_6.html#Extended-Char-Intro">Introduction to Extended Characters</a>).  There is no difference in
the programming interface for these two kind of strings; the programmer
has to be aware of this and interpret the byte sequences accordingly.
</p>
<p>But since there is no separate interface taking care of these
differences the byte-based string functions are sometimes hard to use.
Since the count parameters of these functions specify bytes a call to
<code>strncpy</code> could cut a multibyte character in the middle and put an
incomplete (and therefore unusable) byte sequence in the target buffer.
</p>
<a name="index-wide-character-string-1"></a>
<p>To avoid these problems later versions of the ISO C standard
introduce a second set of functions which are operating on <em>wide
characters</em> (see section <a href="libc_6.html#Extended-Char-Intro">Introduction to Extended Characters</a>).  These functions don&rsquo;t have
the problems the single-byte versions have since every wide character is
a legal, interpretable value.  This does not mean that cutting wide
character strings at arbitrary points is without problems.  It normally
is for alphabet-based languages (except for non-normalized text) but
languages based on syllables still have the problem that more than one
wide character is necessary to complete a logical unit.  This is a
higher level problem which the C library functions are not designed
to solve.  But it is at least good that no invalid byte sequences can be
created.  Also, the higher level functions can also much easier operate
on wide character than on multibyte characters so that a general advise
is to use wide characters internally whenever text is more than simply
copied.
</p>
<p>The remaining of this chapter will discuss the functions for handling
wide character strings in parallel with the discussion of the multibyte
character strings since there is almost always an exact equivalent
available.
</p>
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<a name="String-and-Array-Conventions"></a>
<h2 class="section">5.2 String and Array Conventions</h2>

<p>This chapter describes both functions that work on arbitrary arrays or
blocks of memory, and functions that are specific to null-terminated
arrays of characters and wide characters.
</p>
<p>Functions that operate on arbitrary blocks of memory have names
beginning with &lsquo;<samp>mem</samp>&rsquo; and &lsquo;<samp>wmem</samp>&rsquo; (such as <code>memcpy</code> and
<code>wmemcpy</code>) and invariably take an argument which specifies the size
(in bytes and wide characters respectively) of the block of memory to
operate on.  The array arguments and return values for these functions
have type <code>void *</code> or <code>wchar_t</code>.  As a matter of style, the
elements of the arrays used with the &lsquo;<samp>mem</samp>&rsquo; functions are referred
to as &ldquo;bytes&rdquo;.  You can pass any kind of pointer to these functions,
and the <code>sizeof</code> operator is useful in computing the value for the
size argument.  Parameters to the &lsquo;<samp>wmem</samp>&rsquo; functions must be of type
<code>wchar_t *</code>.  These functions are not really usable with anything
but arrays of this type.
</p>
<p>In contrast, functions that operate specifically on strings and wide
character strings have names beginning with &lsquo;<samp>str</samp>&rsquo; and &lsquo;<samp>wcs</samp>&rsquo;
respectively (such as <code>strcpy</code> and <code>wcscpy</code>) and look for a
null character to terminate the string instead of requiring an explicit
size argument to be passed.  (Some of these functions accept a specified
maximum length, but they also check for premature termination with a
null character.)  The array arguments and return values for these
functions have type <code>char *</code> and <code>wchar_t *</code> respectively, and
the array elements are referred to as &ldquo;characters&rdquo; and &ldquo;wide
characters&rdquo;.
</p>
<p>In many cases, there are both &lsquo;<samp>mem</samp>&rsquo; and &lsquo;<samp>str</samp>&rsquo;/&lsquo;<samp>wcs</samp>&rsquo;
versions of a function.  The one that is more appropriate to use depends
on the exact situation.  When your program is manipulating arbitrary
arrays or blocks of storage, then you should always use the &lsquo;<samp>mem</samp>&rsquo;
functions.  On the other hand, when you are manipulating null-terminated
strings it is usually more convenient to use the &lsquo;<samp>str</samp>&rsquo;/&lsquo;<samp>wcs</samp>&rsquo;
functions, unless you already know the length of the string in advance.
The &lsquo;<samp>wmem</samp>&rsquo; functions should be used for wide character arrays with
known size.
</p>
<a name="index-wint_005ft"></a>
<a name="index-parameter-promotion"></a>
<p>Some of the memory and string functions take single characters as
arguments.  Since a value of type <code>char</code> is automatically promoted
into an value of type <code>int</code> when used as a parameter, the functions
are declared with <code>int</code> as the type of the parameter in question.
In case of the wide character function the situation is similarly: the
parameter type for a single wide character is <code>wint_t</code> and not
<code>wchar_t</code>.  This would for many implementations not be necessary
since the <code>wchar_t</code> is large enough to not be automatically
promoted, but since the ISO C standard does not require such a
choice of types the <code>wint_t</code> type is used.
</p>
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<a name="String-Length-1"></a>
<h2 class="section">5.3 String Length</h2>

<p>You can get the length of a string using the <code>strlen</code> function.
This function is declared in the header file &lsquo;<tt>string.h</tt>&rsquo;.
<a name="index-string_002eh"></a>
</p>
<dl>
<dt><a name="index-strlen"></a><u>Function:</u> size_t <b>strlen</b><i> (const char *<var>s</var>)</i></dt>
<dd><p>The <code>strlen</code> function returns the length of the null-terminated
string <var>s</var> in bytes.  (In other words, it returns the offset of the
terminating null character within the array.)
</p>
<p>For example,
</p><table><tr><td>&nbsp;</td><td><pre class="smallexample">strlen (&quot;hello, world&quot;)
    &rArr; 12
</pre></td></tr></table>

<p>When applied to a character array, the <code>strlen</code> function returns
the length of the string stored there, not its allocated size.  You can
get the allocated size of the character array that holds a string using
the <code>sizeof</code> operator:
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">char string[32] = &quot;hello, world&quot;;
sizeof (string)
    &rArr; 32
strlen (string)
    &rArr; 12
</pre></td></tr></table>

<p>But beware, this will not work unless <var>string</var> is the character
array itself, not a pointer to it.  For example:
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">char string[32] = &quot;hello, world&quot;;
char *ptr = string;
sizeof (string)
    &rArr; 32
sizeof (ptr)
    &rArr; 4  /* <span class="roman">(on a machine with 4 byte pointers)</span> */
</pre></td></tr></table>

<p>This is an easy mistake to make when you are working with functions that
take string arguments; those arguments are always pointers, not arrays.
</p>
<p>It must also be noted that for multibyte encoded strings the return
value does not have to correspond to the number of characters in the
string.  To get this value the string can be converted to wide
characters and <code>wcslen</code> can be used or something like the following
code can be used:
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">/* <span class="roman">The input is in <code>string</code>.</span>
   <span class="roman">The length is expected in <code>n</code>.</span>  */
{
  mbstate_t t;
  char *scopy = string;
  /* In initial state.  */
  memset (&amp;t, '\0', sizeof (t));
  /* Determine number of characters.  */
  n = mbsrtowcs (NULL, &amp;scopy, strlen (scopy), &amp;t);
}
</pre></td></tr></table>

<p>This is cumbersome to do so if the number of characters (as opposed to
bytes) is needed often it is better to work with wide characters.
</p></dd></dl>

<p>The wide character equivalent is declared in &lsquo;<tt>wchar.h</tt>&rsquo;.
</p>
<dl>
<dt><a name="index-wcslen"></a><u>Function:</u> size_t <b>wcslen</b><i> (const wchar_t *<var>ws</var>)</i></dt>
<dd><p>The <code>wcslen</code> function is the wide character equivalent to
<code>strlen</code>.  The return value is the number of wide characters in the
wide character string pointed to by <var>ws</var> (this is also the offset of
the terminating null wide character of <var>ws</var>).
</p>
<p>Since there are no multi wide character sequences making up one
character the return value is not only the offset in the array, it is
also the number of wide characters.
</p>
<p>This function was introduced in Amendment 1 to ISO C90.
</p></dd></dl>

<dl>
<dt><a name="index-strnlen"></a><u>Function:</u> size_t <b>strnlen</b><i> (const char *<var>s</var>, size_t <var>maxlen</var>)</i></dt>
<dd><p>The <code>strnlen</code> function returns the length of the string <var>s</var> in
bytes if this length is smaller than <var>maxlen</var> bytes.  Otherwise it
returns <var>maxlen</var>.  Therefore this function is equivalent to
<code>(strlen (<var>s</var>) &lt; n ? strlen (<var>s</var>) : <var>maxlen</var>)</code> but it
is more efficient and works even if the string <var>s</var> is not
null-terminated.
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">char string[32] = &quot;hello, world&quot;;
strnlen (string, 32)
    &rArr; 12
strnlen (string, 5)
    &rArr; 5
</pre></td></tr></table>

<p>This function is a GNU extension and is declared in &lsquo;<tt>string.h</tt>&rsquo;.
</p></dd></dl>

<dl>
<dt><a name="index-wcsnlen"></a><u>Function:</u> size_t <b>wcsnlen</b><i> (const wchar_t *<var>ws</var>, size_t <var>maxlen</var>)</i></dt>
<dd><p><code>wcsnlen</code> is the wide character equivalent to <code>strnlen</code>.  The
<var>maxlen</var> parameter specifies the maximum number of wide characters.
</p>
<p>This function is a GNU extension and is declared in &lsquo;<tt>wchar.h</tt>&rsquo;.
</p></dd></dl>

<hr size="6">
<a name="Copying-and-Concatenation"></a>
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<td valign="middle" align="left">[<a href="libc_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Copying-and-Concatenation-1"></a>
<h2 class="section">5.4 Copying and Concatenation</h2>

<p>You can use the functions described in this section to copy the contents
of strings and arrays, or to append the contents of one string to
another.  The &lsquo;<samp>str</samp>&rsquo; and &lsquo;<samp>mem</samp>&rsquo; functions are declared in the
header file &lsquo;<tt>string.h</tt>&rsquo; while the &lsquo;<samp>wstr</samp>&rsquo; and &lsquo;<samp>wmem</samp>&rsquo;
functions are declared in the file &lsquo;<tt>wchar.h</tt>&rsquo;.
<a name="index-string_002eh-1"></a>
<a name="index-wchar_002eh"></a>
<a name="index-copying-strings-and-arrays"></a>
<a name="index-string-copy-functions"></a>
<a name="index-array-copy-functions"></a>
<a name="index-concatenating-strings"></a>
<a name="index-string-concatenation-functions"></a>
</p>
<p>A helpful way to remember the ordering of the arguments to the functions
in this section is that it corresponds to an assignment expression, with
the destination array specified to the left of the source array.  All
of these functions return the address of the destination array.
</p>
<p>Most of these functions do not work properly if the source and
destination arrays overlap.  For example, if the beginning of the
destination array overlaps the end of the source array, the original
contents of that part of the source array may get overwritten before it
is copied.  Even worse, in the case of the string functions, the null
character marking the end of the string may be lost, and the copy
function might get stuck in a loop trashing all the memory allocated to
your program.
</p>
<p>All functions that have problems copying between overlapping arrays are
explicitly identified in this manual.  In addition to functions in this
section, there are a few others like <code>sprintf</code> (see section <a href="libc_12.html#Formatted-Output-Functions">Formatted Output Functions</a>) and <code>scanf</code> (see section <a href="libc_12.html#Formatted-Input-Functions">Formatted Input Functions</a>).
</p>
<dl>
<dt><a name="index-memcpy"></a><u>Function:</u> void * <b>memcpy</b><i> (void *restrict <var>to</var>, const void *restrict <var>from</var>, size_t <var>size</var>)</i></dt>
<dd><p>The <code>memcpy</code> function copies <var>size</var> bytes from the object
beginning at <var>from</var> into the object beginning at <var>to</var>.  The
behavior of this function is undefined if the two arrays <var>to</var> and
<var>from</var> overlap; use <code>memmove</code> instead if overlapping is possible.
</p>
<p>The value returned by <code>memcpy</code> is the value of <var>to</var>.
</p>
<p>Here is an example of how you might use <code>memcpy</code> to copy the
contents of an array:
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">struct foo *oldarray, *newarray;
int arraysize;
&hellip;
memcpy (new, old, arraysize * sizeof (struct foo));
</pre></td></tr></table>
</dd></dl>

<dl>
<dt><a name="index-wmemcpy"></a><u>Function:</u> wchar_t * <b>wmemcpy</b><i> (wchar_t *restrict <var>wto</var>, const wchar_t *restrict <var>wfrom</var>, size_t <var>size</var>)</i></dt>
<dd><p>The <code>wmemcpy</code> function copies <var>size</var> wide characters from the object
beginning at <var>wfrom</var> into the object beginning at <var>wto</var>.  The
behavior of this function is undefined if the two arrays <var>wto</var> and
<var>wfrom</var> overlap; use <code>wmemmove</code> instead if overlapping is possible.
</p>
<p>The following is a possible implementation of <code>wmemcpy</code> but there
are more optimizations possible.
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">wchar_t *
wmemcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
         size_t size)
{
  return (wchar_t *) memcpy (wto, wfrom, size * sizeof (wchar_t));
}
</pre></td></tr></table>

<p>The value returned by <code>wmemcpy</code> is the value of <var>wto</var>.
</p>
<p>This function was introduced in Amendment 1 to ISO C90.
</p></dd></dl>

<dl>
<dt><a name="index-mempcpy"></a><u>Function:</u> void * <b>mempcpy</b><i> (void *restrict <var>to</var>, const void *restrict <var>from</var>, size_t <var>size</var>)</i></dt>
<dd><p>The <code>mempcpy</code> function is nearly identical to the <code>memcpy</code>
function.  It copies <var>size</var> bytes from the object beginning at
<code>from</code> into the object pointed to by <var>to</var>.  But instead of
returning the value of <var>to</var> it returns a pointer to the byte
following the last written byte in the object beginning at <var>to</var>.
I.e., the value is <code>((void *) ((char *) <var>to</var> + <var>size</var>))</code>.
</p>
<p>This function is useful in situations where a number of objects shall be
copied to consecutive memory positions.
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">void *
combine (void *o1, size_t s1, void *o2, size_t s2)
{
  void *result = malloc (s1 + s2);
  if (result != NULL)
    mempcpy (mempcpy (result, o1, s1), o2, s2);
  return result;
}
</pre></td></tr></table>

<p>This function is a GNU extension.
</p></dd></dl>

<dl>
<dt><a name="index-wmempcpy"></a><u>Function:</u> wchar_t * <b>wmempcpy</b><i> (wchar_t *restrict <var>wto</var>, const wchar_t *restrict <var>wfrom</var>, size_t <var>size</var>)</i></dt>
<dd><p>The <code>wmempcpy</code> function is nearly identical to the <code>wmemcpy</code>
function.  It copies <var>size</var> wide characters from the object
beginning at <code>wfrom</code> into the object pointed to by <var>wto</var>.  But
instead of returning the value of <var>wto</var> it returns a pointer to the
wide character following the last written wide character in the object
beginning at <var>wto</var>.  I.e., the value is <code><var>wto</var> + <var>size</var></code>.
</p>
<p>This function is useful in situations where a number of objects shall be
copied to consecutive memory positions.
</p>
<p>The following is a possible implementation of <code>wmemcpy</code> but there
are more optimizations possible.
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">wchar_t *
wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
          size_t size)
{
  return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t));
}
</pre></td></tr></table>

<p>This function is a GNU extension.
</p></dd></dl>

<dl>
<dt><a name="index-memmove"></a><u>Function:</u> void * <b>memmove</b><i> (void *<var>to</var>, const void *<var>from</var>, size_t <var>size</var>)</i></dt>
<dd><p><code>memmove</code> copies the <var>size</var> bytes at <var>from</var> into the
<var>size</var> bytes at <var>to</var>, even if those two blocks of space
overlap.  In the case of overlap, <code>memmove</code> is careful to copy the
original values of the bytes in the block at <var>from</var>, including those
bytes which also belong to the block at <var>to</var>.
</p>
<p>The value returned by <code>memmove</code> is the value of <var>to</var>.
</p></dd></dl>

<dl>
<dt><a name="index-wmemmove"></a><u>Function:</u> wchar_t * <b>wmemmove</b><i> (wchar *<var>wto</var>, const wchar_t *<var>wfrom</var>, size_t <var>size</var>)</i></dt>
<dd><p><code>wmemmove</code> copies the <var>size</var> wide characters at <var>wfrom</var>
into the <var>size</var> wide characters at <var>wto</var>, even if those two
blocks of space overlap.  In the case of overlap, <code>memmove</code> is
careful to copy the original values of the wide characters in the block
at <var>wfrom</var>, including those wide characters which also belong to the
block at <var>wto</var>.
</p>
<p>The following is a possible implementation of <code>wmemcpy</code> but there
are more optimizations possible.
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">wchar_t *
wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
          size_t size)
{
  return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t));
}
</pre></td></tr></table>

<p>The value returned by <code>wmemmove</code> is the value of <var>wto</var>.
</p>
<p>This function is a GNU extension.
</p></dd></dl>

<dl>
<dt><a name="index-memccpy"></a><u>Function:</u> void * <b>memccpy</b><i> (void *restrict <var>to</var>, const void *restrict <var>from</var>, int <var>c</var>, size_t <var>size</var>)</i></dt>
<dd><p>This function copies no more than <var>size</var> bytes from <var>from</var> to
<var>to</var>, stopping if a byte matching <var>c</var> is found.  The return
value is a pointer into <var>to</var> one byte past where <var>c</var> was copied,
or a null pointer if no byte matching <var>c</var> appeared in the first
<var>size</var> bytes of <var>from</var>.
</p></dd></dl>

<dl>
<dt><a name="index-memset"></a><u>Function:</u> void * <b>memset</b><i> (void *<var>block</var>, int <var>c</var>, size_t <var>size</var>)</i></dt>
<dd><p>This function copies the value of <var>c</var> (converted to an
<code>unsigned char</code>) into each of the first <var>size</var> bytes of the
object beginning at <var>block</var>.  It returns the value of <var>block</var>.
</p></dd></dl>

<dl>
<dt><a name="index-wmemset"></a><u>Function:</u> wchar_t * <b>wmemset</b><i> (wchar_t *<var>block</var>, wchar_t <var>wc</var>, size_t <var>size</var>)</i></dt>
<dd><p>This function copies the value of <var>wc</var> into each of the first
<var>size</var> wide characters of the object beginning at <var>block</var>.  It
returns the value of <var>block</var>.
</p></dd></dl>

<dl>
<dt><a name="index-strcpy"></a><u>Function:</u> char * <b>strcpy</b><i> (char *restrict <var>to</var>, const char *restrict <var>from</var>)</i></dt>
<dd><p>This copies characters from the string <var>from</var> (up to and including
the terminating null character) into the string <var>to</var>.  Like
<code>memcpy</code>, this function has undefined results if the strings
overlap.  The return value is the value of <var>to</var>.
</p></dd></dl>

<dl>
<dt><a name="index-wcscpy"></a><u>Function:</u> wchar_t * <b>wcscpy</b><i> (wchar_t *restrict <var>wto</var>, const wchar_t *restrict <var>wfrom</var>)</i></dt>
<dd><p>This copies wide characters from the string <var>wfrom</var> (up to and
including the terminating null wide character) into the string
<var>wto</var>.  Like <code>wmemcpy</code>, this function has undefined results if
the strings overlap.  The return value is the value of <var>wto</var>.
</p></dd></dl>

<dl>
<dt><a name="index-strncpy"></a><u>Function:</u> char * <b>strncpy</b><i> (char *restrict <var>to</var>, const char *restrict <var>from</var>, size_t <var>size</var>)</i></dt>
<dd><p>This function is similar to <code>strcpy</code> but always copies exactly
<var>size</var> characters into <var>to</var>.
</p>
<p>If the length of <var>from</var> is more than <var>size</var>, then <code>strncpy</code>
copies just the first <var>size</var> characters.  Note that in this case
there is no null terminator written into <var>to</var>.
</p>
<p>If the length of <var>from</var> is less than <var>size</var>, then <code>strncpy</code>
copies all of <var>from</var>, followed by enough null characters to add up
to <var>size</var> characters in all.  This behavior is rarely useful, but it
is specified by the ISO C standard.
</p>
<p>The behavior of <code>strncpy</code> is undefined if the strings overlap.
</p>
<p>Using <code>strncpy</code> as opposed to <code>strcpy</code> is a way to avoid bugs
relating to writing past the end of the allocated space for <var>to</var>.
However, it can also make your program much slower in one common case:
copying a string which is probably small into a potentially large buffer.
In this case, <var>size</var> may be large, and when it is, <code>strncpy</code> will
waste a considerable amount of time copying null characters.
</p></dd></dl>

<dl>
<dt><a name="index-wcsncpy"></a><u>Function:</u> wchar_t * <b>wcsncpy</b><i> (wchar_t *restrict <var>wto</var>, const wchar_t *restrict <var>wfrom</var>, size_t <var>size</var>)</i></dt>
<dd><p>This function is similar to <code>wcscpy</code> but always copies exactly
<var>size</var> wide characters into <var>wto</var>.
</p>
<p>If the length of <var>wfrom</var> is more than <var>size</var>, then
<code>wcsncpy</code> copies just the first <var>size</var> wide characters.  Note
that in this case there is no null terminator written into <var>wto</var>.
</p>
<p>If the length of <var>wfrom</var> is less than <var>size</var>, then
<code>wcsncpy</code> copies all of <var>wfrom</var>, followed by enough null wide
characters to add up to <var>size</var> wide characters in all.  This
behavior is rarely useful, but it is specified by the ISO C
standard.
</p>
<p>The behavior of <code>wcsncpy</code> is undefined if the strings overlap.
</p>
<p>Using <code>wcsncpy</code> as opposed to <code>wcscpy</code> is a way to avoid bugs
relating to writing past the end of the allocated space for <var>wto</var>.
However, it can also make your program much slower in one common case:
copying a string which is probably small into a potentially large buffer.
In this case, <var>size</var> may be large, and when it is, <code>wcsncpy</code> will
waste a considerable amount of time copying null wide characters.
</p></dd></dl>

<dl>
<dt><a name="index-strdup"></a><u>Function:</u> char * <b>strdup</b><i> (const char *<var>s</var>)</i></dt>
<dd><p>This function copies the null-terminated string <var>s</var> into a newly
allocated string.  The string is allocated using <code>malloc</code>; see
<a href="libc_3.html#Unconstrained-Allocation">Unconstrained Allocation</a>.  If <code>malloc</code> cannot allocate space
for the new string, <code>strdup</code> returns a null pointer.  Otherwise it
returns a pointer to the new string.
</p></dd></dl>

<dl>
<dt><a name="index-wcsdup"></a><u>Function:</u> wchar_t * <b>wcsdup</b><i> (const wchar_t *<var>ws</var>)</i></dt>
<dd><p>This function copies the null-terminated wide character string <var>ws</var>
into a newly allocated string.  The string is allocated using
<code>malloc</code>; see <a href="libc_3.html#Unconstrained-Allocation">Unconstrained Allocation</a>.  If <code>malloc</code>
cannot allocate space for the new string, <code>wcsdup</code> returns a null
pointer.  Otherwise it returns a pointer to the new wide character
string.
</p>
<p>This function is a GNU extension.
</p></dd></dl>

<dl>
<dt><a name="index-strndup"></a><u>Function:</u> char * <b>strndup</b><i> (const char *<var>s</var>, size_t <var>size</var>)</i></dt>
<dd><p>This function is similar to <code>strdup</code> but always copies at most
<var>size</var> characters into the newly allocated string.
</p>
<p>If the length of <var>s</var> is more than <var>size</var>, then <code>strndup</code>
copies just the first <var>size</var> characters and adds a closing null
terminator.  Otherwise all characters are copied and the string is
terminated.
</p>
<p>This function is different to <code>strncpy</code> in that it always
terminates the destination string.
</p>
<p><code>strndup</code> is a GNU extension.
</p></dd></dl>

<dl>
<dt><a name="index-stpcpy"></a><u>Function:</u> char * <b>stpcpy</b><i> (char *restrict <var>to</var>, const char *restrict <var>from</var>)</i></dt>
<dd><p>This function is like <code>strcpy</code>, except that it returns a pointer to
the end of the string <var>to</var> (that is, the address of the terminating
null character <code>to + strlen (from)</code>) rather than the beginning.
</p>
<p>For example, this program uses <code>stpcpy</code> to concatenate &lsquo;<samp>foo</samp>&rsquo;
and &lsquo;<samp>bar</samp>&rsquo; to produce &lsquo;<samp>foobar</samp>&rsquo;, which it then prints.
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">#include &lt;string.h&gt;
#include &lt;stdio.h&gt;

int
main (void)
{
  char buffer[10];
  char *to = buffer;
  to = stpcpy (to, &quot;foo&quot;);
  to = stpcpy (to, &quot;bar&quot;);
  puts (buffer);
  return 0;
}
</pre></td></tr></table>

<p>This function is not part of the ISO or POSIX standards, and is not
customary on Unix systems, but we did not invent it either.  Perhaps it
comes from MS-DOG.
</p>
<p>Its behavior is undefined if the strings overlap.  The function is
declared in &lsquo;<tt>string.h</tt>&rsquo;.
</p></dd></dl>

<dl>
<dt><a name="index-wcpcpy"></a><u>Function:</u> wchar_t * <b>wcpcpy</b><i> (wchar_t *restrict <var>wto</var>, const wchar_t *restrict <var>wfrom</var>)</i></dt>
<dd><p>This function is like <code>wcscpy</code>, except that it returns a pointer to
the end of the string <var>wto</var> (that is, the address of the terminating
null character <code>wto + strlen (wfrom)</code>) rather than the beginning.
</p>
<p>This function is not part of ISO or POSIX but was found useful while
developing the GNU C Library itself.
</p>
<p>The behavior of <code>wcpcpy</code> is undefined if the strings overlap.
</p>
<p><code>wcpcpy</code> is a GNU extension and is declared in &lsquo;<tt>wchar.h</tt>&rsquo;.
</p></dd></dl>

<dl>
<dt><a name="index-stpncpy"></a><u>Function:</u> char * <b>stpncpy</b><i> (char *restrict <var>to</var>, const char *restrict <var>from</var>, size_t <var>size</var>)</i></dt>
<dd><p>This function is similar to <code>stpcpy</code> but copies always exactly
<var>size</var> characters into <var>to</var>.
</p>
<p>If the length of <var>from</var> is more then <var>size</var>, then <code>stpncpy</code>
copies just the first <var>size</var> characters and returns a pointer to the
character directly following the one which was copied last.  Note that in
this case there is no null terminator written into <var>to</var>.
</p>
<p>If the length of <var>from</var> is less than <var>size</var>, then <code>stpncpy</code>
copies all of <var>from</var>, followed by enough null characters to add up
to <var>size</var> characters in all.  This behavior is rarely useful, but it
is implemented to be useful in contexts where this behavior of the
<code>strncpy</code> is used.  <code>stpncpy</code> returns a pointer to the
<em>first</em> written null character.
</p>
<p>This function is not part of ISO or POSIX but was found useful while
developing the GNU C Library itself.
</p>
<p>Its behavior is undefined if the strings overlap.  The function is
declared in &lsquo;<tt>string.h</tt>&rsquo;.
</p></dd></dl>

<dl>
<dt><a name="index-wcpncpy"></a><u>Function:</u> wchar_t * <b>wcpncpy</b><i> (wchar_t *restrict <var>wto</var>, const wchar_t *restrict <var>wfrom</var>, size_t <var>size</var>)</i></dt>
<dd><p>This function is similar to <code>wcpcpy</code> but copies always exactly
<var>wsize</var> characters into <var>wto</var>.
</p>
<p>If the length of <var>wfrom</var> is more then <var>size</var>, then
<code>wcpncpy</code> copies just the first <var>size</var> wide characters and
returns a pointer to the wide character directly following the last
non-null wide character which was copied last.  Note that in this case
there is no null terminator written into <var>wto</var>.
</p>
<p>If the length of <var>wfrom</var> is less than <var>size</var>, then <code>wcpncpy</code>
copies all of <var>wfrom</var>, followed by enough null characters to add up
to <var>size</var> characters in all.  This behavior is rarely useful, but it
is implemented to be useful in contexts where this behavior of the
<code>wcsncpy</code> is used.  <code>wcpncpy</code> returns a pointer to the
<em>first</em> written null character.
</p>
<p>This function is not part of ISO or POSIX but was found useful while
developing the GNU C Library itself.
</p>
<p>Its behavior is undefined if the strings overlap.
</p>
<p><code>wcpncpy</code> is a GNU extension and is declared in &lsquo;<tt>wchar.h</tt>&rsquo;.
</p></dd></dl>

<dl>
<dt><a name="index-strdupa"></a><u>Macro:</u> char * <b>strdupa</b><i> (const char *<var>s</var>)</i></dt>
<dd><p>This macro is similar to <code>strdup</code> but allocates the new string
using <code>alloca</code> instead of <code>malloc</code> (see section <a href="libc_3.html#Variable-Size-Automatic">Automatic Storage with Variable Size</a>).  This means of course the returned string has the same
limitations as any block of memory allocated using <code>alloca</code>.
</p>
<p>For obvious reasons <code>strdupa</code> is implemented only as a macro;
you cannot get the address of this function.  Despite this limitation
it is a useful function.  The following code shows a situation where
using <code>malloc</code> would be a lot more expensive.
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">#include &lt;paths.h&gt;
#include &lt;string.h&gt;
#include &lt;stdio.h&gt;

const char path[] = _PATH_STDPATH;

int
main (void)
{
  char *wr_path = strdupa (path);
  char *cp = strtok (wr_path, &quot;:&quot;);

  while (cp != NULL)
    {
      puts (cp);
      cp = strtok (NULL, &quot;:&quot;);
    }
  return 0;
}
</pre></td></tr></table>

<p>Please note that calling <code>strtok</code> using <var>path</var> directly is
invalid.  It is also not allowed to call <code>strdupa</code> in the argument
list of <code>strtok</code> since <code>strdupa</code> uses <code>alloca</code>
(see section <a href="libc_3.html#Variable-Size-Automatic">Automatic Storage with Variable Size</a>) can interfere with the parameter
passing.
</p>
<p>This function is only available if GNU CC is used.
</p></dd></dl>

<dl>
<dt><a name="index-strndupa"></a><u>Macro:</u> char * <b>strndupa</b><i> (const char *<var>s</var>, size_t <var>size</var>)</i></dt>
<dd><p>This function is similar to <code>strndup</code> but like <code>strdupa</code> it
allocates the new string using <code>alloca</code>
see section <a href="libc_3.html#Variable-Size-Automatic">Automatic Storage with Variable Size</a>.  The same advantages and limitations
of <code>strdupa</code> are valid for <code>strndupa</code>, too.
</p>
<p>This function is implemented only as a macro, just like <code>strdupa</code>.
Just as <code>strdupa</code> this macro also must not be used inside the
parameter list in a function call.
</p>
<p><code>strndupa</code> is only available if GNU CC is used.
</p></dd></dl>

<dl>
<dt><a name="index-strcat"></a><u>Function:</u> char * <b>strcat</b><i> (char *restrict <var>to</var>, const char *restrict <var>from</var>)</i></dt>
<dd><p>The <code>strcat</code> function is similar to <code>strcpy</code>, except that the
characters from <var>from</var> are concatenated or appended to the end of
<var>to</var>, instead of overwriting it.  That is, the first character from
<var>from</var> overwrites the null character marking the end of <var>to</var>.
</p>
<p>An equivalent definition for <code>strcat</code> would be:
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">char *
strcat (char *restrict to, const char *restrict from)
{
  strcpy (to + strlen (to), from);
  return to;
}
</pre></td></tr></table>

<p>This function has undefined results if the strings overlap.
</p></dd></dl>

<dl>
<dt><a name="index-wcscat"></a><u>Function:</u> wchar_t * <b>wcscat</b><i> (wchar_t *restrict <var>wto</var>, const wchar_t *restrict <var>wfrom</var>)</i></dt>
<dd><p>The <code>wcscat</code> function is similar to <code>wcscpy</code>, except that the
characters from <var>wfrom</var> are concatenated or appended to the end of
<var>wto</var>, instead of overwriting it.  That is, the first character from
<var>wfrom</var> overwrites the null character marking the end of <var>wto</var>.
</p>
<p>An equivalent definition for <code>wcscat</code> would be:
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">wchar_t *
wcscat (wchar_t *wto, const wchar_t *wfrom)
{
  wcscpy (wto + wcslen (wto), wfrom);
  return wto;
}
</pre></td></tr></table>

<p>This function has undefined results if the strings overlap.
</p></dd></dl>

<p>Programmers using the <code>strcat</code> or <code>wcscat</code> function (or the
following <code>strncat</code> or <code>wcsncar</code> functions for that matter)
can easily be recognized as lazy and reckless.  In almost all situations
the lengths of the participating strings are known (it better should be
since how can one otherwise ensure the allocated size of the buffer is
sufficient?)  Or at least, one could know them if one keeps track of the
results of the various function calls.  But then it is very inefficient
to use <code>strcat</code>/<code>wcscat</code>.  A lot of time is wasted finding the
end of the destination string so that the actual copying can start.
This is a common example:
</p>
<a name="index-_005f_005fva_005fcopy"></a>
<a name="index-va_005fcopy"></a>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">/* <span class="roman">This function concatenates arbitrarily many strings.  The last</span>
   <span class="roman">parameter must be <code>NULL</code>.</span>  */
char *
concat (const char *str, &hellip;)
{
  va_list ap, ap2;
  size_t total = 1;
  const char *s;
  char *result;

  va_start (ap, str);
  /* <span class="roman">Actually <code>va_copy</code>, but this is the name more gcc versions</span>
     <span class="roman">understand.</span>  */
  __va_copy (ap2, ap);

  /* <span class="roman">Determine how much space we need.</span>  */
  for (s = str; s != NULL; s = va_arg (ap, const char *))
    total += strlen (s);

  va_end (ap);

  result = (char *) malloc (total);
  if (result != NULL)
    {
      result[0] = '\0';

      /* <span class="roman">Copy the strings.</span>  */
      for (s = str; s != NULL; s = va_arg (ap2, const char *))
        strcat (result, s);
    }

  va_end (ap2);

  return result;
}
</pre></td></tr></table>

<p>This looks quite simple, especially the second loop where the strings
are actually copied.  But these innocent lines hide a major performance
penalty.  Just imagine that ten strings of 100 bytes each have to be
concatenated.  For the second string we search the already stored 100
bytes for the end of the string so that we can append the next string.
For all strings in total the comparisons necessary to find the end of
the intermediate results sums up to 5500!  If we combine the copying
with the search for the allocation we can write this function more
efficient:
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">char *
concat (const char *str, &hellip;)
{
  va_list ap;
  size_t allocated = 100;
  char *result = (char *) malloc (allocated);

  if (result != NULL)
    {
      char *newp;
      char *wp;

      va_start (ap, str);

      wp = result;
      for (s = str; s != NULL; s = va_arg (ap, const char *))
        {
          size_t len = strlen (s);

          /* <span class="roman">Resize the allocated memory if necessary.</span>  */
          if (wp + len + 1 &gt; result + allocated)
            {
              allocated = (allocated + len) * 2;
              newp = (char *) realloc (result, allocated);
              if (newp == NULL)
                {
                  free (result);
                  return NULL;
                }
              wp = newp + (wp - result);
              result = newp;
            }

          wp = mempcpy (wp, s, len);
        }

      /* <span class="roman">Terminate the result string.</span>  */
      *wp++ = '\0';

      /* <span class="roman">Resize memory to the optimal size.</span>  */
      newp = realloc (result, wp - result);
      if (newp != NULL)
        result = newp;

      va_end (ap);
    }

  return result;
}
</pre></td></tr></table>

<p>With a bit more knowledge about the input strings one could fine-tune
the memory allocation.  The difference we are pointing to here is that
we don&rsquo;t use <code>strcat</code> anymore.  We always keep track of the length
of the current intermediate result so we can safe us the search for the
end of the string and use <code>mempcpy</code>.  Please note that we also
don&rsquo;t use <code>stpcpy</code> which might seem more natural since we handle
with strings.  But this is not necessary since we already know the
length of the string and therefore can use the faster memory copying
function.  The example would work for wide characters the same way.
</p>
<p>Whenever a programmer feels the need to use <code>strcat</code> she or he
should think twice and look through the program whether the code cannot
be rewritten to take advantage of already calculated results.  Again: it
is almost always unnecessary to use <code>strcat</code>.
</p>
<dl>
<dt><a name="index-strncat"></a><u>Function:</u> char * <b>strncat</b><i> (char *restrict <var>to</var>, const char *restrict <var>from</var>, size_t <var>size</var>)</i></dt>
<dd><p>This function is like <code>strcat</code> except that not more than <var>size</var>
characters from <var>from</var> are appended to the end of <var>to</var>.  A
single null character is also always appended to <var>to</var>, so the total
allocated size of <var>to</var> must be at least <code><var>size</var> + 1</code> bytes
longer than its initial length.
</p>
<p>The <code>strncat</code> function could be implemented like this:
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">char *
strncat (char *to, const char *from, size_t size)
{
  to[strlen (to) + size] = '\0';
  strncpy (to + strlen (to), from, size);
  return to;
}
</pre></td></tr></table>

<p>The behavior of <code>strncat</code> is undefined if the strings overlap.
</p></dd></dl>

<dl>
<dt><a name="index-wcsncat"></a><u>Function:</u> wchar_t * <b>wcsncat</b><i> (wchar_t *restrict <var>wto</var>, const wchar_t *restrict <var>wfrom</var>, size_t <var>size</var>)</i></dt>
<dd><p>This function is like <code>wcscat</code> except that not more than <var>size</var>
characters from <var>from</var> are appended to the end of <var>to</var>.  A
single null character is also always appended to <var>to</var>, so the total
allocated size of <var>to</var> must be at least <code><var>size</var> + 1</code> bytes
longer than its initial length.
</p>
<p>The <code>wcsncat</code> function could be implemented like this:
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">wchar_t *
wcsncat (wchar_t *restrict wto, const wchar_t *restrict wfrom,
         size_t size)
{
  wto[wcslen (to) + size] = L'\0';
  wcsncpy (wto + wcslen (wto), wfrom, size);
  return wto;
}
</pre></td></tr></table>

<p>The behavior of <code>wcsncat</code> is undefined if the strings overlap.
</p></dd></dl>

<p>Here is an example showing the use of <code>strncpy</code> and <code>strncat</code>
(the wide character version is equivalent).  Notice how, in the call to
<code>strncat</code>, the <var>size</var> parameter is computed to avoid
overflowing the character array <code>buffer</code>.
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">#include &lt;string.h&gt;
#include &lt;stdio.h&gt;

#define SIZE 10

static char buffer[SIZE];

main ()
{
  strncpy (buffer, &quot;hello&quot;, SIZE);
  puts (buffer);
  strncat (buffer, &quot;, world&quot;, SIZE - strlen (buffer) - 1);
  puts (buffer);
}
</pre></td></tr></table>

<p>The output produced by this program looks like:
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">hello
hello, wo
</pre></td></tr></table>

<dl>
<dt><a name="index-bcopy"></a><u>Function:</u> void <b>bcopy</b><i> (const void *<var>from</var>, void *<var>to</var>, size_t <var>size</var>)</i></dt>
<dd><p>This is a partially obsolete alternative for <code>memmove</code>, derived from
BSD.  Note that it is not quite equivalent to <code>memmove</code>, because the
arguments are not in the same order and there is no return value.
</p></dd></dl>

<dl>
<dt><a name="index-bzero"></a><u>Function:</u> void <b>bzero</b><i> (void *<var>block</var>, size_t <var>size</var>)</i></dt>
<dd><p>This is a partially obsolete alternative for <code>memset</code>, derived from
BSD.  Note that it is not as general as <code>memset</code>, because the only
value it can store is zero.
</p></dd></dl>

<hr size="6">
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</tr></table>
<a name="String_002fArray-Comparison-1"></a>
<h2 class="section">5.5 String/Array Comparison</h2>
<a name="index-comparing-strings-and-arrays"></a>
<a name="index-string-comparison-functions"></a>
<a name="index-array-comparison-functions"></a>
<a name="index-predicates-on-strings"></a>
<a name="index-predicates-on-arrays"></a>

<p>You can use the functions in this section to perform comparisons on the
contents of strings and arrays.  As well as checking for equality, these
functions can also be used as the ordering functions for sorting
operations.  See section <a href="libc_9.html#Searching-and-Sorting">Searching and Sorting</a>, for an example of this.
</p>
<p>Unlike most comparison operations in C, the string comparison functions
return a nonzero value if the strings are <em>not</em> equivalent rather
than if they are.  The sign of the value indicates the relative ordering
of the first characters in the strings that are not equivalent:  a
negative value indicates that the first string is &ldquo;less&rdquo; than the
second, while a positive value indicates that the first string is
&ldquo;greater&rdquo;.
</p>
<p>The most common use of these functions is to check only for equality.
This is canonically done with an expression like &lsquo;<samp>! strcmp (s1, s2)</samp>&rsquo;.
</p>
<p>All of these functions are declared in the header file &lsquo;<tt>string.h</tt>&rsquo;.
<a name="index-string_002eh-2"></a>
</p>
<dl>
<dt><a name="index-memcmp"></a><u>Function:</u> int <b>memcmp</b><i> (const void *<var>a1</var>, const void *<var>a2</var>, size_t <var>size</var>)</i></dt>
<dd><p>The function <code>memcmp</code> compares the <var>size</var> bytes of memory
beginning at <var>a1</var> against the <var>size</var> bytes of memory beginning
at <var>a2</var>.  The value returned has the same sign as the difference
between the first differing pair of bytes (interpreted as <code>unsigned
char</code> objects, then promoted to <code>int</code>).
</p>
<p>If the contents of the two blocks are equal, <code>memcmp</code> returns
<code>0</code>.
</p></dd></dl>

<dl>
<dt><a name="index-wmemcmp"></a><u>Function:</u> int <b>wmemcmp</b><i> (const wchar_t *<var>a1</var>, const wchar_t *<var>a2</var>, size_t <var>size</var>)</i></dt>
<dd><p>The function <code>wmemcmp</code> compares the <var>size</var> wide characters
beginning at <var>a1</var> against the <var>size</var> wide characters beginning
at <var>a2</var>.  The value returned is smaller than or larger than zero
depending on whether the first differing wide character is <var>a1</var> is
smaller or larger than the corresponding character in <var>a2</var>.
</p>
<p>If the contents of the two blocks are equal, <code>wmemcmp</code> returns
<code>0</code>.
</p></dd></dl>

<p>On arbitrary arrays, the <code>memcmp</code> function is mostly useful for
testing equality.  It usually isn&rsquo;t meaningful to do byte-wise ordering
comparisons on arrays of things other than bytes.  For example, a
byte-wise comparison on the bytes that make up floating-point numbers
isn&rsquo;t likely to tell you anything about the relationship between the
values of the floating-point numbers.
</p>
<p><code>wmemcmp</code> is really only useful to compare arrays of type
<code>wchar_t</code> since the function looks at <code>sizeof (wchar_t)</code> bytes
at a time and this number of bytes is system dependent.
</p>
<p>You should also be careful about using <code>memcmp</code> to compare objects
that can contain &ldquo;holes&rdquo;, such as the padding inserted into structure
objects to enforce alignment requirements, extra space at the end of
unions, and extra characters at the ends of strings whose length is less
than their allocated size.  The contents of these &ldquo;holes&rdquo; are
indeterminate and may cause strange behavior when performing byte-wise
comparisons.  For more predictable results, perform an explicit
component-wise comparison.
</p>
<p>For example, given a structure type definition like:
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">struct foo
  {
    unsigned char tag;
    union
      {
        double f;
        long i;
        char *p;
      } value;
  };
</pre></td></tr></table>

<p>you are better off writing a specialized comparison function to compare
<code>struct foo</code> objects instead of comparing them with <code>memcmp</code>.
</p>
<dl>
<dt><a name="index-strcmp"></a><u>Function:</u> int <b>strcmp</b><i> (const char *<var>s1</var>, const char *<var>s2</var>)</i></dt>
<dd><p>The <code>strcmp</code> function compares the string <var>s1</var> against
<var>s2</var>, returning a value that has the same sign as the difference
between the first differing pair of characters (interpreted as
<code>unsigned char</code> objects, then promoted to <code>int</code>).
</p>
<p>If the two strings are equal, <code>strcmp</code> returns <code>0</code>.
</p>
<p>A consequence of the ordering used by <code>strcmp</code> is that if <var>s1</var>
is an initial substring of <var>s2</var>, then <var>s1</var> is considered to be
&ldquo;less than&rdquo; <var>s2</var>.
</p>
<p><code>strcmp</code> does not take sorting conventions of the language the
strings are written in into account.  To get that one has to use
<code>strcoll</code>.
</p></dd></dl>

<dl>
<dt><a name="index-wcscmp"></a><u>Function:</u> int <b>wcscmp</b><i> (const wchar_t *<var>ws1</var>, const wchar_t *<var>ws2</var>)</i></dt>
<dd>
<p>The <code>wcscmp</code> function compares the wide character string <var>ws1</var>
against <var>ws2</var>.  The value returned is smaller than or larger than zero
depending on whether the first differing wide character is <var>ws1</var> is
smaller or larger than the corresponding character in <var>ws2</var>.
</p>
<p>If the two strings are equal, <code>wcscmp</code> returns <code>0</code>.
</p>
<p>A consequence of the ordering used by <code>wcscmp</code> is that if <var>ws1</var>
is an initial substring of <var>ws2</var>, then <var>ws1</var> is considered to be
&ldquo;less than&rdquo; <var>ws2</var>.
</p>
<p><code>wcscmp</code> does not take sorting conventions of the language the
strings are written in into account.  To get that one has to use
<code>wcscoll</code>.
</p></dd></dl>

<dl>
<dt><a name="index-strcasecmp"></a><u>Function:</u> int <b>strcasecmp</b><i> (const char *<var>s1</var>, const char *<var>s2</var>)</i></dt>
<dd><p>This function is like <code>strcmp</code>, except that differences in case are
ignored.  How uppercase and lowercase characters are related is
determined by the currently selected locale.  In the standard <code>&quot;C&quot;</code>
locale the characters &Auml; and &auml; do not match but in a locale which
regards these characters as parts of the alphabet they do match.
</p>
<p><code>strcasecmp</code> is derived from BSD.
</p></dd></dl>

<dl>
<dt><a name="index-wcscasecmp"></a><u>Function:</u> int <b>wcscasecmp</b><i> (const wchar_t *<var>ws1</var>, const wchar_T *<var>ws2</var>)</i></dt>
<dd><p>This function is like <code>wcscmp</code>, except that differences in case are
ignored.  How uppercase and lowercase characters are related is
determined by the currently selected locale.  In the standard <code>&quot;C&quot;</code>
locale the characters &Auml; and &auml; do not match but in a locale which
regards these characters as parts of the alphabet they do match.
</p>
<p><code>wcscasecmp</code> is a GNU extension.
</p></dd></dl>

<dl>
<dt><a name="index-strncmp"></a><u>Function:</u> int <b>strncmp</b><i> (const char *<var>s1</var>, const char *<var>s2</var>, size_t <var>size</var>)</i></dt>
<dd><p>This function is the similar to <code>strcmp</code>, except that no more than
<var>size</var> characters are compared.  In other words, if the two
strings are the same in their first <var>size</var> characters, the
return value is zero.
</p></dd></dl>

<dl>
<dt><a name="index-wcsncmp"></a><u>Function:</u> int <b>wcsncmp</b><i> (const wchar_t *<var>ws1</var>, const wchar_t *<var>ws2</var>, size_t <var>size</var>)</i></dt>
<dd><p>This function is the similar to <code>wcscmp</code>, except that no more than
<var>size</var> wide characters are compared.  In other words, if the two
strings are the same in their first <var>size</var> wide characters, the
return value is zero.
</p></dd></dl>

<dl>
<dt><a name="index-strncasecmp"></a><u>Function:</u> int <b>strncasecmp</b><i> (const char *<var>s1</var>, const char *<var>s2</var>, size_t <var>n</var>)</i></dt>
<dd><p>This function is like <code>strncmp</code>, except that differences in case
are ignored.  Like <code>strcasecmp</code>, it is locale dependent how
uppercase and lowercase characters are related.
</p>
<p><code>strncasecmp</code> is a GNU extension.
</p></dd></dl>

<dl>
<dt><a name="index-wcsncasecmp"></a><u>Function:</u> int <b>wcsncasecmp</b><i> (const wchar_t *<var>ws1</var>, const wchar_t *<var>s2</var>, size_t <var>n</var>)</i></dt>
<dd><p>This function is like <code>wcsncmp</code>, except that differences in case
are ignored.  Like <code>wcscasecmp</code>, it is locale dependent how
uppercase and lowercase characters are related.
</p>
<p><code>wcsncasecmp</code> is a GNU extension.
</p></dd></dl>

<p>Here are some examples showing the use of <code>strcmp</code> and
<code>strncmp</code> (equivalent examples can be constructed for the wide
character functions).  These examples assume the use of the ASCII
character set.  (If some other character set&mdash;say, EBCDIC&mdash;is used
instead, then the glyphs are associated with different numeric codes,
and the return values and ordering may differ.)
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">strcmp (&quot;hello&quot;, &quot;hello&quot;)
    &rArr; 0    /* <span class="roman">These two strings are the same.</span> */
strcmp (&quot;hello&quot;, &quot;Hello&quot;)
    &rArr; 32   /* <span class="roman">Comparisons are case-sensitive.</span> */
strcmp (&quot;hello&quot;, &quot;world&quot;)
    &rArr; -15  /* <span class="roman">The character <code>'h'</code> comes before <code>'w'</code>.</span> */
strcmp (&quot;hello&quot;, &quot;hello, world&quot;)
    &rArr; -44  /* <span class="roman">Comparing a null character against a comma.</span> */
strncmp (&quot;hello&quot;, &quot;hello, world&quot;, 5)
    &rArr; 0    /* <span class="roman">The initial 5 characters are the same.</span> */
strncmp (&quot;hello, world&quot;, &quot;hello, stupid world!!!&quot;, 5)
    &rArr; 0    /* <span class="roman">The initial 5 characters are the same.</span> */
</pre></td></tr></table>

<dl>
<dt><a name="index-strverscmp"></a><u>Function:</u> int <b>strverscmp</b><i> (const char *<var>s1</var>, const char *<var>s2</var>)</i></dt>
<dd><p>The <code>strverscmp</code> function compares the string <var>s1</var> against
<var>s2</var>, considering them as holding indices/version numbers.  The
return value follows the same conventions as found in the
<code>strcmp</code> function.  In fact, if <var>s1</var> and <var>s2</var> contain no
digits, <code>strverscmp</code> behaves like <code>strcmp</code>.
</p>
<p>Basically, we compare strings normally (character by character), until
we find a digit in each string - then we enter a special comparison
mode, where each sequence of digits is taken as a whole.  If we reach the
end of these two parts without noticing a difference, we return to the
standard comparison mode.  There are two types of numeric parts:
&quot;integral&quot; and &quot;fractional&quot; (those  begin with a &rsquo;0&rsquo;). The types
of the numeric parts affect the way we sort them:
</p>
<ul>
<li>
integral/integral: we compare values as you would expect.

</li><li>
fractional/integral: the fractional part is less than the integral one.
Again, no surprise.

</li><li>
fractional/fractional: the things become a bit more complex.
If the common prefix contains only leading zeroes, the longest part is less
than the other one; else the comparison behaves normally.
</li></ul>

<table><tr><td>&nbsp;</td><td><pre class="smallexample">strverscmp (&quot;no digit&quot;, &quot;no digit&quot;)
    &rArr; 0    /* <span class="roman">same behavior as strcmp.</span> */
strverscmp (&quot;item#99&quot;, &quot;item#100&quot;)
    &rArr; &lt;0   /* <span class="roman">same prefix, but 99 &lt; 100.</span> */
strverscmp (&quot;alpha1&quot;, &quot;alpha001&quot;)
    &rArr; &gt;0   /* <span class="roman">fractional part inferior to integral one.</span> */
strverscmp (&quot;part1_f012&quot;, &quot;part1_f01&quot;)
    &rArr; &gt;0   /* <span class="roman">two fractional parts.</span> */
strverscmp (&quot;foo.009&quot;, &quot;foo.0&quot;)
    &rArr; &lt;0   /* <span class="roman">idem, but with leading zeroes only.</span> */
</pre></td></tr></table>

<p>This function is especially useful when dealing with filename sorting,
because filenames frequently hold indices/version numbers.
</p>
<p><code>strverscmp</code> is a GNU extension.
</p></dd></dl>

<dl>
<dt><a name="index-bcmp"></a><u>Function:</u> int <b>bcmp</b><i> (const void *<var>a1</var>, const void *<var>a2</var>, size_t <var>size</var>)</i></dt>
<dd><p>This is an obsolete alias for <code>memcmp</code>, derived from BSD.
</p></dd></dl>

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<a name="Collation-Functions-1"></a>
<h2 class="section">5.6 Collation Functions</h2>

<a name="index-collating-strings"></a>
<a name="index-string-collation-functions"></a>

<p>In some locales, the conventions for lexicographic ordering differ from
the strict numeric ordering of character codes.  For example, in Spanish
most glyphs with diacritical marks such as accents are not considered
distinct letters for the purposes of collation.  On the other hand, the
two-character sequence &lsquo;<samp>ll</samp>&rsquo; is treated as a single letter that is
collated immediately after &lsquo;<samp>l</samp>&rsquo;.
</p>
<p>You can use the functions <code>strcoll</code> and <code>strxfrm</code> (declared in
the headers file &lsquo;<tt>string.h</tt>&rsquo;) and <code>wcscoll</code> and <code>wcsxfrm</code>
(declared in the headers file &lsquo;<tt>wchar</tt>&rsquo;) to compare strings using a
collation ordering appropriate for the current locale.  The locale used
by these functions in particular can be specified by setting the locale
for the <code>LC_COLLATE</code> category; see <a href="libc_7.html#Locales">Locales and Internationalization</a>.
<a name="index-string_002eh-3"></a>
<a name="index-wchar_002eh-1"></a>
</p>
<p>In the standard C locale, the collation sequence for <code>strcoll</code> is
the same as that for <code>strcmp</code>.  Similarly, <code>wcscoll</code> and
<code>wcscmp</code> are the same in this situation.
</p>
<p>Effectively, the way these functions work is by applying a mapping to
transform the characters in a string to a byte sequence that represents
the string&rsquo;s position in the collating sequence of the current locale.
Comparing two such byte sequences in a simple fashion is equivalent to
comparing the strings with the locale&rsquo;s collating sequence.
</p>
<p>The functions <code>strcoll</code> and <code>wcscoll</code> perform this translation
implicitly, in order to do one comparison.  By contrast, <code>strxfrm</code>
and <code>wcsxfrm</code> perform the mapping explicitly.  If you are making
multiple comparisons using the same string or set of strings, it is
likely to be more efficient to use <code>strxfrm</code> or <code>wcsxfrm</code> to
transform all the strings just once, and subsequently compare the
transformed strings with <code>strcmp</code> or <code>wcscmp</code>.
</p>
<dl>
<dt><a name="index-strcoll"></a><u>Function:</u> int <b>strcoll</b><i> (const char *<var>s1</var>, const char *<var>s2</var>)</i></dt>
<dd><p>The <code>strcoll</code> function is similar to <code>strcmp</code> but uses the
collating sequence of the current locale for collation (the
<code>LC_COLLATE</code> locale).
</p></dd></dl>

<dl>
<dt><a name="index-wcscoll"></a><u>Function:</u> int <b>wcscoll</b><i> (const wchar_t *<var>ws1</var>, const wchar_t *<var>ws2</var>)</i></dt>
<dd><p>The <code>wcscoll</code> function is similar to <code>wcscmp</code> but uses the
collating sequence of the current locale for collation (the
<code>LC_COLLATE</code> locale).
</p></dd></dl>

<p>Here is an example of sorting an array of strings, using <code>strcoll</code>
to compare them.  The actual sort algorithm is not written here; it
comes from <code>qsort</code> (see section <a href="libc_9.html#Array-Sort-Function">Array Sort Function</a>).  The job of the
code shown here is to say how to compare the strings while sorting them.
(Later on in this section, we will show a way to do this more
efficiently using <code>strxfrm</code>.)
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">/* <span class="roman">This is the comparison function used with <code>qsort</code>.</span> */

int
compare_elements (char **p1, char **p2)
{
  return strcoll (*p1, *p2);
}

/* <span class="roman">This is the entry point---the function to sort</span>
   <span class="roman">strings using the locale's collating sequence.</span> */

void
sort_strings (char **array, int nstrings)
{
  /* <span class="roman">Sort <code>temp_array</code> by comparing the strings.</span> */
  qsort (array, nstrings,
         sizeof (char *), compare_elements);
}
</pre></td></tr></table>

<a name="index-converting-string-to-collation-order"></a>
<dl>
<dt><a name="index-strxfrm"></a><u>Function:</u> size_t <b>strxfrm</b><i> (char *restrict <var>to</var>, const char *restrict <var>from</var>, size_t <var>size</var>)</i></dt>
<dd><p>The function <code>strxfrm</code> transforms the string <var>from</var> using the
collation transformation determined by the locale currently selected for
collation, and stores the transformed string in the array <var>to</var>.  Up
to <var>size</var> characters (including a terminating null character) are
stored.
</p>
<p>The behavior is undefined if the strings <var>to</var> and <var>from</var>
overlap; see <a href="#Copying-and-Concatenation">Copying and Concatenation</a>.
</p>
<p>The return value is the length of the entire transformed string.  This
value is not affected by the value of <var>size</var>, but if it is greater
or equal than <var>size</var>, it means that the transformed string did not
entirely fit in the array <var>to</var>.  In this case, only as much of the
string as actually fits was stored.  To get the whole transformed
string, call <code>strxfrm</code> again with a bigger output array.
</p>
<p>The transformed string may be longer than the original string, and it
may also be shorter.
</p>
<p>If <var>size</var> is zero, no characters are stored in <var>to</var>.  In this
case, <code>strxfrm</code> simply returns the number of characters that would
be the length of the transformed string.  This is useful for determining
what size the allocated array should be.  It does not matter what
<var>to</var> is if <var>size</var> is zero; <var>to</var> may even be a null pointer.
</p></dd></dl>

<dl>
<dt><a name="index-wcsxfrm"></a><u>Function:</u> size_t <b>wcsxfrm</b><i> (wchar_t *restrict <var>wto</var>, const wchar_t *<var>wfrom</var>, size_t <var>size</var>)</i></dt>
<dd><p>The function <code>wcsxfrm</code> transforms wide character string <var>wfrom</var>
using the collation transformation determined by the locale currently
selected for collation, and stores the transformed string in the array
<var>wto</var>.  Up to <var>size</var> wide characters (including a terminating null
character) are stored.
</p>
<p>The behavior is undefined if the strings <var>wto</var> and <var>wfrom</var>
overlap; see <a href="#Copying-and-Concatenation">Copying and Concatenation</a>.
</p>
<p>The return value is the length of the entire transformed wide character
string.  This value is not affected by the value of <var>size</var>, but if
it is greater or equal than <var>size</var>, it means that the transformed
wide character string did not entirely fit in the array <var>wto</var>.  In
this case, only as much of the wide character string as actually fits
was stored.  To get the whole transformed wide character string, call
<code>wcsxfrm</code> again with a bigger output array.
</p>
<p>The transformed wide character string may be longer than the original
wide character string, and it may also be shorter.
</p>
<p>If <var>size</var> is zero, no characters are stored in <var>to</var>.  In this
case, <code>wcsxfrm</code> simply returns the number of wide characters that
would be the length of the transformed wide character string.  This is
useful for determining what size the allocated array should be (remember
to multiply with <code>sizeof (wchar_t)</code>).  It does not matter what
<var>wto</var> is if <var>size</var> is zero; <var>wto</var> may even be a null pointer.
</p></dd></dl>

<p>Here is an example of how you can use <code>strxfrm</code> when
you plan to do many comparisons.  It does the same thing as the previous
example, but much faster, because it has to transform each string only
once, no matter how many times it is compared with other strings.  Even
the time needed to allocate and free storage is much less than the time
we save, when there are many strings.
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">struct sorter { char *input; char *transformed; };

/* <span class="roman">This is the comparison function used with <code>qsort</code></span>
   <span class="roman">to sort an array of <code>struct sorter</code>.</span> */

int
compare_elements (struct sorter *p1, struct sorter *p2)
{
  return strcmp (p1-&gt;transformed, p2-&gt;transformed);
}

/* <span class="roman">This is the entry point---the function to sort</span>
   <span class="roman">strings using the locale's collating sequence.</span> */

void
sort_strings_fast (char **array, int nstrings)
{
  struct sorter temp_array[nstrings];
  int i;

  /* <span class="roman">Set up <code>temp_array</code>.  Each element contains</span>
     <span class="roman">one input string and its transformed string.</span> */
  for (i = 0; i &lt; nstrings; i++)
    {
      size_t length = strlen (array[i]) * 2;
      char *transformed;
      size_t transformed_length;

      temp_array[i].input = array[i];

      /* <span class="roman">First try a buffer perhaps big enough.</span>  */
      transformed = (char *) xmalloc (length);

      /* <span class="roman">Transform <code>array[i]</code>.</span>  */
      transformed_length = strxfrm (transformed, array[i], length);

      /* <span class="roman">If the buffer was not large enough, resize it</span>
         <span class="roman">and try again.</span>  */
      if (transformed_length &gt;= length)
        {
          /* <span class="roman">Allocate the needed space. +1 for terminating</span>
             <span class="roman"><code>NUL</code> character.</span>  */
          transformed = (char *) xrealloc (transformed,
                                           transformed_length + 1);

          /* <span class="roman">The return value is not interesting because we know</span>
             <span class="roman">how long the transformed string is.</span>  */
          (void) strxfrm (transformed, array[i],
                          transformed_length + 1);
        }

      temp_array[i].transformed = transformed;
    }

  /* <span class="roman">Sort <code>temp_array</code> by comparing transformed strings.</span> */
  qsort (temp_array, sizeof (struct sorter),
         nstrings, compare_elements);

  /* <span class="roman">Put the elements back in the permanent array</span>
     <span class="roman">in their sorted order.</span> */
  for (i = 0; i &lt; nstrings; i++)
    array[i] = temp_array[i].input;

  /* <span class="roman">Free the strings we allocated.</span> */
  for (i = 0; i &lt; nstrings; i++)
    free (temp_array[i].transformed);
}
</pre></td></tr></table>

<p>The interesting part of this code for the wide character version would
look like this:
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">void
sort_strings_fast (wchar_t **array, int nstrings)
{
  &hellip;
      /* <span class="roman">Transform <code>array[i]</code>.</span>  */
      transformed_length = wcsxfrm (transformed, array[i], length);

      /* <span class="roman">If the buffer was not large enough, resize it</span>
         <span class="roman">and try again.</span>  */
      if (transformed_length &gt;= length)
        {
          /* <span class="roman">Allocate the needed space. +1 for terminating</span>
             <span class="roman"><code>NUL</code> character.</span>  */
          transformed = (wchar_t *) xrealloc (transformed,
                                              (transformed_length + 1)
                                              * sizeof (wchar_t));

          /* <span class="roman">The return value is not interesting because we know</span>
             <span class="roman">how long the transformed string is.</span>  */
          (void) wcsxfrm (transformed, array[i],
                          transformed_length + 1);
        }
  &hellip;
</pre></td></tr></table>

<p>Note the additional multiplication with <code>sizeof (wchar_t)</code> in the
<code>realloc</code> call.
</p>
<p><strong>Compatibility Note:</strong> The string collation functions are a new
feature of ISO C90.  Older C dialects have no equivalent feature.
The wide character versions were introduced in Amendment 1 to ISO
C90.
</p>
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<a name="Search-Functions-1"></a>
<h2 class="section">5.7 Search Functions</h2>

<p>This section describes library functions which perform various kinds
of searching operations on strings and arrays.  These functions are
declared in the header file &lsquo;<tt>string.h</tt>&rsquo;.
<a name="index-string_002eh-4"></a>
<a name="index-search-functions-_0028for-strings_0029"></a>
<a name="index-string-search-functions"></a>
</p>
<dl>
<dt><a name="index-memchr"></a><u>Function:</u> void * <b>memchr</b><i> (const void *<var>block</var>, int <var>c</var>, size_t <var>size</var>)</i></dt>
<dd><p>This function finds the first occurrence of the byte <var>c</var> (converted
to an <code>unsigned char</code>) in the initial <var>size</var> bytes of the
object beginning at <var>block</var>.  The return value is a pointer to the
located byte, or a null pointer if no match was found.
</p></dd></dl>

<dl>
<dt><a name="index-wmemchr"></a><u>Function:</u> wchar_t * <b>wmemchr</b><i> (const wchar_t *<var>block</var>, wchar_t <var>wc</var>, size_t <var>size</var>)</i></dt>
<dd><p>This function finds the first occurrence of the wide character <var>wc</var>
in the initial <var>size</var> wide characters of the object beginning at
<var>block</var>.  The return value is a pointer to the located wide
character, or a null pointer if no match was found.
</p></dd></dl>

<dl>
<dt><a name="index-rawmemchr"></a><u>Function:</u> void * <b>rawmemchr</b><i> (const void *<var>block</var>, int <var>c</var>)</i></dt>
<dd><p>Often the <code>memchr</code> function is used with the knowledge that the
byte <var>c</var> is available in the memory block specified by the
parameters.  But this means that the <var>size</var> parameter is not really
needed and that the tests performed with it at runtime (to check whether
the end of the block is reached) are not needed.
</p>
<p>The <code>rawmemchr</code> function exists for just this situation which is
surprisingly frequent.  The interface is similar to <code>memchr</code> except
that the <var>size</var> parameter is missing.  The function will look beyond
the end of the block pointed to by <var>block</var> in case the programmer
made an error in assuming that the byte <var>c</var> is present in the block.
In this case the result is unspecified.  Otherwise the return value is a
pointer to the located byte.
</p>
<p>This function is of special interest when looking for the end of a
string.  Since all strings are terminated by a null byte a call like
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">   rawmemchr (str, '\0')
</pre></td></tr></table>

<p>will never go beyond the end of the string.
</p>
<p>This function is a GNU extension.
</p></dd></dl>

<dl>
<dt><a name="index-memrchr"></a><u>Function:</u> void * <b>memrchr</b><i> (const void *<var>block</var>, int <var>c</var>, size_t <var>size</var>)</i></dt>
<dd><p>The function <code>memrchr</code> is like <code>memchr</code>, except that it searches
backwards from the end of the block defined by <var>block</var> and <var>size</var>
(instead of forwards from the front).
</p>
<p>This function is a GNU extension.
</p></dd></dl>

<dl>
<dt><a name="index-strchr"></a><u>Function:</u> char * <b>strchr</b><i> (const char *<var>string</var>, int <var>c</var>)</i></dt>
<dd><p>The <code>strchr</code> function finds the first occurrence of the character
<var>c</var> (converted to a <code>char</code>) in the null-terminated string
beginning at <var>string</var>.  The return value is a pointer to the located
character, or a null pointer if no match was found.
</p>
<p>For example,
</p><table><tr><td>&nbsp;</td><td><pre class="smallexample">strchr (&quot;hello, world&quot;, 'l')
    &rArr; &quot;llo, world&quot;
strchr (&quot;hello, world&quot;, '?')
    &rArr; NULL
</pre></td></tr></table>

<p>The terminating null character is considered to be part of the string,
so you can use this function get a pointer to the end of a string by
specifying a null character as the value of the <var>c</var> argument.  It
would be better (but less portable) to use <code>strchrnul</code> in this
case, though.
</p></dd></dl>

<dl>
<dt><a name="index-wcschr"></a><u>Function:</u> wchar_t * <b>wcschr</b><i> (const wchar_t *<var>wstring</var>, int <var>wc</var>)</i></dt>
<dd><p>The <code>wcschr</code> function finds the first occurrence of the wide
character <var>wc</var> in the null-terminated wide character string
beginning at <var>wstring</var>.  The return value is a pointer to the
located wide character, or a null pointer if no match was found.
</p>
<p>The terminating null character is considered to be part of the wide
character string, so you can use this function get a pointer to the end
of a wide character string by specifying a null wude character as the
value of the <var>wc</var> argument.  It would be better (but less portable)
to use <code>wcschrnul</code> in this case, though.
</p></dd></dl>

<dl>
<dt><a name="index-strchrnul"></a><u>Function:</u> char * <b>strchrnul</b><i> (const char *<var>string</var>, int <var>c</var>)</i></dt>
<dd><p><code>strchrnul</code> is the same as <code>strchr</code> except that if it does
not find the character, it returns a pointer to string&rsquo;s terminating
null character rather than a null pointer.
</p>
<p>This function is a GNU extension.
</p></dd></dl>

<dl>
<dt><a name="index-wcschrnul"></a><u>Function:</u> wchar_t * <b>wcschrnul</b><i> (const wchar_t *<var>wstring</var>, wchar_t <var>wc</var>)</i></dt>
<dd><p><code>wcschrnul</code> is the same as <code>wcschr</code> except that if it does not
find the wide character, it returns a pointer to wide character string&rsquo;s
terminating null wide character rather than a null pointer.
</p>
<p>This function is a GNU extension.
</p></dd></dl>

<p>One useful, but unusual, use of the <code>strchr</code>
function is when one wants to have a pointer pointing to the NUL byte
terminating a string.  This is often written in this way:
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">  s += strlen (s);
</pre></td></tr></table>

<p>This is almost optimal but the addition operation duplicated a bit of
the work already done in the <code>strlen</code> function.  A better solution
is this:
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">  s = strchr (s, '\0');
</pre></td></tr></table>

<p>There is no restriction on the second parameter of <code>strchr</code> so it
could very well also be the NUL character.  Those readers thinking very
hard about this might now point out that the <code>strchr</code> function is
more expensive than the <code>strlen</code> function since we have two abort
criteria.  This is right.  But in the GNU C library the implementation of
<code>strchr</code> is optimized in a special way so that <code>strchr</code>
actually is faster.
</p>
<dl>
<dt><a name="index-strrchr"></a><u>Function:</u> char * <b>strrchr</b><i> (const char *<var>string</var>, int <var>c</var>)</i></dt>
<dd><p>The function <code>strrchr</code> is like <code>strchr</code>, except that it searches
backwards from the end of the string <var>string</var> (instead of forwards
from the front).
</p>
<p>For example,
</p><table><tr><td>&nbsp;</td><td><pre class="smallexample">strrchr (&quot;hello, world&quot;, 'l')
    &rArr; &quot;ld&quot;
</pre></td></tr></table>
</dd></dl>

<dl>
<dt><a name="index-wcsrchr"></a><u>Function:</u> wchar_t * <b>wcsrchr</b><i> (const wchar_t *<var>wstring</var>, wchar_t <var>c</var>)</i></dt>
<dd><p>The function <code>wcsrchr</code> is like <code>wcschr</code>, except that it searches
backwards from the end of the string <var>wstring</var> (instead of forwards
from the front).
</p></dd></dl>

<dl>
<dt><a name="index-strstr"></a><u>Function:</u> char * <b>strstr</b><i> (const char *<var>haystack</var>, const char *<var>needle</var>)</i></dt>
<dd><p>This is like <code>strchr</code>, except that it searches <var>haystack</var> for a
substring <var>needle</var> rather than just a single character.  It
returns a pointer into the string <var>haystack</var> that is the first
character of the substring, or a null pointer if no match was found.  If
<var>needle</var> is an empty string, the function returns <var>haystack</var>.
</p>
<p>For example,
</p><table><tr><td>&nbsp;</td><td><pre class="smallexample">strstr (&quot;hello, world&quot;, &quot;l&quot;)
    &rArr; &quot;llo, world&quot;
strstr (&quot;hello, world&quot;, &quot;wo&quot;)
    &rArr; &quot;world&quot;
</pre></td></tr></table>
</dd></dl>

<dl>
<dt><a name="index-wcsstr"></a><u>Function:</u> wchar_t * <b>wcsstr</b><i> (const wchar_t *<var>haystack</var>, const wchar_t *<var>needle</var>)</i></dt>
<dd><p>This is like <code>wcschr</code>, except that it searches <var>haystack</var> for a
substring <var>needle</var> rather than just a single wide character.  It
returns a pointer into the string <var>haystack</var> that is the first wide
character of the substring, or a null pointer if no match was found.  If
<var>needle</var> is an empty string, the function returns <var>haystack</var>.
</p></dd></dl>

<dl>
<dt><a name="index-wcswcs"></a><u>Function:</u> wchar_t * <b>wcswcs</b><i> (const wchar_t *<var>haystack</var>, const wchar_t *<var>needle</var>)</i></dt>
<dd><p><code>wcswcs</code> is an deprecated alias for <code>wcsstr</code>.  This is the
name originally used in the X/Open Portability Guide before the
Amendment 1 to ISO C90 was published.
</p></dd></dl>


<dl>
<dt><a name="index-strcasestr"></a><u>Function:</u> char * <b>strcasestr</b><i> (const char *<var>haystack</var>, const char *<var>needle</var>)</i></dt>
<dd><p>This is like <code>strstr</code>, except that it ignores case in searching for
the substring.   Like <code>strcasecmp</code>, it is locale dependent how
uppercase and lowercase characters are related.
</p>

<p>For example,
</p><table><tr><td>&nbsp;</td><td><pre class="smallexample">strcasestr (&quot;hello, world&quot;, &quot;L&quot;)
    &rArr; &quot;llo, world&quot;
strcasestr (&quot;hello, World&quot;, &quot;wo&quot;)
    &rArr; &quot;World&quot;
</pre></td></tr></table>
</dd></dl>


<dl>
<dt><a name="index-memmem"></a><u>Function:</u> void * <b>memmem</b><i> (const void *<var>haystack</var>, size_t <var>haystack-len</var>,<br>const void *<var>needle</var>, size_t <var>needle-len</var>)</i></dt>
<dd><p>This is like <code>strstr</code>, but <var>needle</var> and <var>haystack</var> are byte
arrays rather than null-terminated strings.  <var>needle-len</var> is the
length of <var>needle</var> and <var>haystack-len</var> is the length of
<var>haystack</var>.
</p>
<p>This function is a GNU extension.
</p></dd></dl>

<dl>
<dt><a name="index-strspn"></a><u>Function:</u> size_t <b>strspn</b><i> (const char *<var>string</var>, const char *<var>skipset</var>)</i></dt>
<dd><p>The <code>strspn</code> (&ldquo;string span&rdquo;) function returns the length of the
initial substring of <var>string</var> that consists entirely of characters that
are members of the set specified by the string <var>skipset</var>.  The order
of the characters in <var>skipset</var> is not important.
</p>
<p>For example,
</p><table><tr><td>&nbsp;</td><td><pre class="smallexample">strspn (&quot;hello, world&quot;, &quot;abcdefghijklmnopqrstuvwxyz&quot;)
    &rArr; 5
</pre></td></tr></table>

<p>Note that &ldquo;character&rdquo; is here used in the sense of byte.  In a string
using a multibyte character encoding (abstract) character consisting of
more than one byte are not treated as an entity.  Each byte is treated
separately.  The function is not locale-dependent.
</p></dd></dl>

<dl>
<dt><a name="index-wcsspn"></a><u>Function:</u> size_t <b>wcsspn</b><i> (const wchar_t *<var>wstring</var>, const wchar_t *<var>skipset</var>)</i></dt>
<dd><p>The <code>wcsspn</code> (&ldquo;wide character string span&rdquo;) function returns the
length of the initial substring of <var>wstring</var> that consists entirely
of wide characters that are members of the set specified by the string
<var>skipset</var>.  The order of the wide characters in <var>skipset</var> is not
important.
</p></dd></dl>

<dl>
<dt><a name="index-strcspn"></a><u>Function:</u> size_t <b>strcspn</b><i> (const char *<var>string</var>, const char *<var>stopset</var>)</i></dt>
<dd><p>The <code>strcspn</code> (&ldquo;string complement span&rdquo;) function returns the length
of the initial substring of <var>string</var> that consists entirely of characters
that are <em>not</em> members of the set specified by the string <var>stopset</var>.
(In other words, it returns the offset of the first character in <var>string</var>
that is a member of the set <var>stopset</var>.)
</p>
<p>For example,
</p><table><tr><td>&nbsp;</td><td><pre class="smallexample">strcspn (&quot;hello, world&quot;, &quot; \t\n,.;!?&quot;)
    &rArr; 5
</pre></td></tr></table>

<p>Note that &ldquo;character&rdquo; is here used in the sense of byte.  In a string
using a multibyte character encoding (abstract) character consisting of
more than one byte are not treated as an entity.  Each byte is treated
separately.  The function is not locale-dependent.
</p></dd></dl>

<dl>
<dt><a name="index-wcscspn"></a><u>Function:</u> size_t <b>wcscspn</b><i> (const wchar_t *<var>wstring</var>, const wchar_t *<var>stopset</var>)</i></dt>
<dd><p>The <code>wcscspn</code> (&ldquo;wide character string complement span&rdquo;) function
returns the length of the initial substring of <var>wstring</var> that
consists entirely of wide characters that are <em>not</em> members of the
set specified by the string <var>stopset</var>.  (In other words, it returns
the offset of the first character in <var>string</var> that is a member of
the set <var>stopset</var>.)
</p></dd></dl>

<dl>
<dt><a name="index-strpbrk"></a><u>Function:</u> char * <b>strpbrk</b><i> (const char *<var>string</var>, const char *<var>stopset</var>)</i></dt>
<dd><p>The <code>strpbrk</code> (&ldquo;string pointer break&rdquo;) function is related to
<code>strcspn</code>, except that it returns a pointer to the first character
in <var>string</var> that is a member of the set <var>stopset</var> instead of the
length of the initial substring.  It returns a null pointer if no such
character from <var>stopset</var> is found.
</p>
<p>For example,
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">strpbrk (&quot;hello, world&quot;, &quot; \t\n,.;!?&quot;)
    &rArr; &quot;, world&quot;
</pre></td></tr></table>

<p>Note that &ldquo;character&rdquo; is here used in the sense of byte.  In a string
using a multibyte character encoding (abstract) character consisting of
more than one byte are not treated as an entity.  Each byte is treated
separately.  The function is not locale-dependent.
</p></dd></dl>

<dl>
<dt><a name="index-wcspbrk"></a><u>Function:</u> wchar_t * <b>wcspbrk</b><i> (const wchar_t *<var>wstring</var>, const wchar_t *<var>stopset</var>)</i></dt>
<dd><p>The <code>wcspbrk</code> (&ldquo;wide character string pointer break&rdquo;) function is
related to <code>wcscspn</code>, except that it returns a pointer to the first
wide character in <var>wstring</var> that is a member of the set
<var>stopset</var> instead of the length of the initial substring.  It
returns a null pointer if no such character from <var>stopset</var> is found.
</p></dd></dl>


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<h3 class="subsection">5.7.1 Compatibility String Search Functions</h3>

<dl>
<dt><a name="index-index"></a><u>Function:</u> char * <b>index</b><i> (const char *<var>string</var>, int <var>c</var>)</i></dt>
<dd><p><code>index</code> is another name for <code>strchr</code>; they are exactly the same.
New code should always use <code>strchr</code> since this name is defined in
ISO C while <code>index</code> is a BSD invention which never was available
on System V derived systems.
</p></dd></dl>

<dl>
<dt><a name="index-rindex"></a><u>Function:</u> char * <b>rindex</b><i> (const char *<var>string</var>, int <var>c</var>)</i></dt>
<dd><p><code>rindex</code> is another name for <code>strrchr</code>; they are exactly the same.
New code should always use <code>strrchr</code> since this name is defined in
ISO C while <code>rindex</code> is a BSD invention which never was available
on System V derived systems.
</p></dd></dl>

<hr size="6">
<a name="Finding-Tokens-in-a-String"></a>
<table cellpadding="1" cellspacing="1" border="0">
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</tr></table>
<a name="Finding-Tokens-in-a-String-1"></a>
<h2 class="section">5.8 Finding Tokens in a String</h2>

<a name="index-tokenizing-strings"></a>
<a name="index-breaking-a-string-into-tokens"></a>
<a name="index-parsing-tokens-from-a-string"></a>
<p>It&rsquo;s fairly common for programs to have a need to do some simple kinds
of lexical analysis and parsing, such as splitting a command string up
into tokens.  You can do this with the <code>strtok</code> function, declared
in the header file &lsquo;<tt>string.h</tt>&rsquo;.
<a name="index-string_002eh-5"></a>
</p>
<dl>
<dt><a name="index-strtok"></a><u>Function:</u> char * <b>strtok</b><i> (char *restrict <var>newstring</var>, const char *restrict <var>delimiters</var>)</i></dt>
<dd><p>A string can be split into tokens by making a series of calls to the
function <code>strtok</code>.
</p>
<p>The string to be split up is passed as the <var>newstring</var> argument on
the first call only.  The <code>strtok</code> function uses this to set up
some internal state information.  Subsequent calls to get additional
tokens from the same string are indicated by passing a null pointer as
the <var>newstring</var> argument.  Calling <code>strtok</code> with another
non-null <var>newstring</var> argument reinitializes the state information.
It is guaranteed that no other library function ever calls <code>strtok</code>
behind your back (which would mess up this internal state information).
</p>
<p>The <var>delimiters</var> argument is a string that specifies a set of delimiters
that may surround the token being extracted.  All the initial characters
that are members of this set are discarded.  The first character that is
<em>not</em> a member of this set of delimiters marks the beginning of the
next token.  The end of the token is found by looking for the next
character that is a member of the delimiter set.  This character in the
original string <var>newstring</var> is overwritten by a null character, and the
pointer to the beginning of the token in <var>newstring</var> is returned.
</p>
<p>On the next call to <code>strtok</code>, the searching begins at the next
character beyond the one that marked the end of the previous token.
Note that the set of delimiters <var>delimiters</var> do not have to be the
same on every call in a series of calls to <code>strtok</code>.
</p>
<p>If the end of the string <var>newstring</var> is reached, or if the remainder of
string consists only of delimiter characters, <code>strtok</code> returns
a null pointer.
</p>
<p>Note that &ldquo;character&rdquo; is here used in the sense of byte.  In a string
using a multibyte character encoding (abstract) character consisting of
more than one byte are not treated as an entity.  Each byte is treated
separately.  The function is not locale-dependent.
</p></dd></dl>

<dl>
<dt><a name="index-wcstok"></a><u>Function:</u> wchar_t * <b>wcstok</b><i> (wchar_t *<var>newstring</var>, const char *<var>delimiters</var>)</i></dt>
<dd><p>A string can be split into tokens by making a series of calls to the
function <code>wcstok</code>.
</p>
<p>The string to be split up is passed as the <var>newstring</var> argument on
the first call only.  The <code>wcstok</code> function uses this to set up
some internal state information.  Subsequent calls to get additional
tokens from the same wide character string are indicated by passing a
null pointer as the <var>newstring</var> argument.  Calling <code>wcstok</code>
with another non-null <var>newstring</var> argument reinitializes the state
information.  It is guaranteed that no other library function ever calls
<code>wcstok</code> behind your back (which would mess up this internal state
information).
</p>
<p>The <var>delimiters</var> argument is a wide character string that specifies
a set of delimiters that may surround the token being extracted.  All
the initial wide characters that are members of this set are discarded.
The first wide character that is <em>not</em> a member of this set of
delimiters marks the beginning of the next token.  The end of the token
is found by looking for the next wide character that is a member of the
delimiter set.  This wide character in the original wide character
string <var>newstring</var> is overwritten by a null wide character, and the
pointer to the beginning of the token in <var>newstring</var> is returned.
</p>
<p>On the next call to <code>wcstok</code>, the searching begins at the next
wide character beyond the one that marked the end of the previous token.
Note that the set of delimiters <var>delimiters</var> do not have to be the
same on every call in a series of calls to <code>wcstok</code>.
</p>
<p>If the end of the wide character string <var>newstring</var> is reached, or
if the remainder of string consists only of delimiter wide characters,
<code>wcstok</code> returns a null pointer.
</p>
<p>Note that &ldquo;character&rdquo; is here used in the sense of byte.  In a string
using a multibyte character encoding (abstract) character consisting of
more than one byte are not treated as an entity.  Each byte is treated
separately.  The function is not locale-dependent.
</p></dd></dl>

<p><strong>Warning:</strong> Since <code>strtok</code> and <code>wcstok</code> alter the string
they is parsing, you should always copy the string to a temporary buffer
before parsing it with <code>strtok</code>/<code>wcstok</code> (see section <a href="#Copying-and-Concatenation">Copying and Concatenation</a>).  If you allow <code>strtok</code> or <code>wcstok</code> to modify
a string that came from another part of your program, you are asking for
trouble; that string might be used for other purposes after
<code>strtok</code> or <code>wcstok</code> has modified it, and it would not have
the expected value.
</p>
<p>The string that you are operating on might even be a constant.  Then
when <code>strtok</code> or <code>wcstok</code> tries to modify it, your program
will get a fatal signal for writing in read-only memory.  See section <a href="libc_24.html#Program-Error-Signals">Program Error Signals</a>.  Even if the operation of <code>strtok</code> or <code>wcstok</code>
would not require a modification of the string (e.g., if there is
exactly one token) the string can (and in the GNU libc case will) be
modified.
</p>
<p>This is a special case of a general principle: if a part of a program
does not have as its purpose the modification of a certain data
structure, then it is error-prone to modify the data structure
temporarily.
</p>
<p>The functions <code>strtok</code> and <code>wcstok</code> are not reentrant.
See section <a href="libc_24.html#Nonreentrancy">Signal Handling and Nonreentrant Functions</a>, for a discussion of where and why reentrancy is
important.
</p>
<p>Here is a simple example showing the use of <code>strtok</code>.
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">#include &lt;string.h&gt;
#include &lt;stddef.h&gt;

&hellip;

const char string[] = &quot;words separated by spaces -- and, punctuation!&quot;;
const char delimiters[] = &quot; .,;:!-&quot;;
char *token, *cp;

&hellip;

cp = strdupa (string);                /* Make writable copy.  */
token = strtok (cp, delimiters);      /* token =&gt; &quot;words&quot; */
token = strtok (NULL, delimiters);    /* token =&gt; &quot;separated&quot; */
token = strtok (NULL, delimiters);    /* token =&gt; &quot;by&quot; */
token = strtok (NULL, delimiters);    /* token =&gt; &quot;spaces&quot; */
token = strtok (NULL, delimiters);    /* token =&gt; &quot;and&quot; */
token = strtok (NULL, delimiters);    /* token =&gt; &quot;punctuation&quot; */
token = strtok (NULL, delimiters);    /* token =&gt; NULL */
</pre></td></tr></table>

<p>The GNU C library contains two more functions for tokenizing a string
which overcome the limitation of non-reentrancy.  They are only
available for multibyte character strings.
</p>
<dl>
<dt><a name="index-strtok_005fr"></a><u>Function:</u> char * <b>strtok_r</b><i> (char *<var>newstring</var>, const char *<var>delimiters</var>, char **<var>save_ptr</var>)</i></dt>
<dd><p>Just like <code>strtok</code>, this function splits the string into several
tokens which can be accessed by successive calls to <code>strtok_r</code>.
The difference is that the information about the next token is stored in
the space pointed to by the third argument, <var>save_ptr</var>, which is a
pointer to a string pointer.  Calling <code>strtok_r</code> with a null
pointer for <var>newstring</var> and leaving <var>save_ptr</var> between the calls
unchanged does the job without hindering reentrancy.
</p>
<p>This function is defined in POSIX.1 and can be found on many systems
which support multi-threading.
</p></dd></dl>

<dl>
<dt><a name="index-strsep"></a><u>Function:</u> char * <b>strsep</b><i> (char **<var>string_ptr</var>, const char *<var>delimiter</var>)</i></dt>
<dd><p>This function has a similar functionality as <code>strtok_r</code> with the
<var>newstring</var> argument replaced by the <var>save_ptr</var> argument.  The
initialization of the moving pointer has to be done by the user.
Successive calls to <code>strsep</code> move the pointer along the tokens
separated by <var>delimiter</var>, returning the address of the next token
and updating <var>string_ptr</var> to point to the beginning of the next
token.
</p>
<p>One difference between <code>strsep</code> and <code>strtok_r</code> is that if the
input string contains more than one character from <var>delimiter</var> in a
row <code>strsep</code> returns an empty string for each pair of characters
from <var>delimiter</var>.  This means that a program normally should test
for <code>strsep</code> returning an empty string before processing it.
</p>
<p>This function was introduced in 4.3BSD and therefore is widely available.
</p></dd></dl>

<p>Here is how the above example looks like when <code>strsep</code> is used.
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">#include &lt;string.h&gt;
#include &lt;stddef.h&gt;

&hellip;

const char string[] = &quot;words separated by spaces -- and, punctuation!&quot;;
const char delimiters[] = &quot; .,;:!-&quot;;
char *running;
char *token;

&hellip;

running = strdupa (string);
token = strsep (&amp;running, delimiters);    /* token =&gt; &quot;words&quot; */
token = strsep (&amp;running, delimiters);    /* token =&gt; &quot;separated&quot; */
token = strsep (&amp;running, delimiters);    /* token =&gt; &quot;by&quot; */
token = strsep (&amp;running, delimiters);    /* token =&gt; &quot;spaces&quot; */
token = strsep (&amp;running, delimiters);    /* token =&gt; &quot;&quot; */
token = strsep (&amp;running, delimiters);    /* token =&gt; &quot;&quot; */
token = strsep (&amp;running, delimiters);    /* token =&gt; &quot;&quot; */
token = strsep (&amp;running, delimiters);    /* token =&gt; &quot;and&quot; */
token = strsep (&amp;running, delimiters);    /* token =&gt; &quot;&quot; */
token = strsep (&amp;running, delimiters);    /* token =&gt; &quot;punctuation&quot; */
token = strsep (&amp;running, delimiters);    /* token =&gt; &quot;&quot; */
token = strsep (&amp;running, delimiters);    /* token =&gt; NULL */
</pre></td></tr></table>

<dl>
<dt><a name="index-basename"></a><u>Function:</u> char * <b>basename</b><i> (const char *<var>filename</var>)</i></dt>
<dd><p>The GNU version of the <code>basename</code> function returns the last
component of the path in <var>filename</var>.  This function is the preferred
usage, since it does not modify the argument, <var>filename</var>, and
respects trailing slashes.  The prototype for <code>basename</code> can be
found in &lsquo;<tt>string.h</tt>&rsquo;.  Note, this function is overriden by the XPG
version, if &lsquo;<tt>libgen.h</tt>&rsquo; is included.
</p>
<p>Example of using GNU <code>basename</code>:
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">#include &lt;string.h&gt;

int
main (int argc, char *argv[])
{
  char *prog = basename (argv[0]);

  if (argc &lt; 2)
    {
      fprintf (stderr, &quot;Usage %s &lt;arg&gt;\n&quot;, prog);
      exit (1);
    }

  &hellip;
}
</pre></td></tr></table>

<p><strong>Portability Note:</strong> This function may produce different results
on different systems.
</p>
</dd></dl>

<dl>
<dt><a name="index-basename-1"></a><u>Function:</u> char * <b>basename</b><i> (char *<var>path</var>)</i></dt>
<dd><p>This is the standard XPG defined <code>basename</code>. It is similar in
spirit to the GNU version, but may modify the <var>path</var> by removing
trailing &rsquo;/&rsquo; characters.  If the <var>path</var> is made up entirely of &rsquo;/&rsquo;
characters, then &quot;/&quot; will be returned.  Also, if <var>path</var> is
<code>NULL</code> or an empty string, then &quot;.&quot; is returned.  The prototype for
the XPG version can be found in &lsquo;<tt>libgen.h</tt>&rsquo;.
</p>
<p>Example of using XPG <code>basename</code>:
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">#include &lt;libgen.h&gt;

int
main (int argc, char *argv[])
{
  char *prog;
  char *path = strdupa (argv[0]);

  prog = basename (path);

  if (argc &lt; 2)
    {
      fprintf (stderr, &quot;Usage %s &lt;arg&gt;\n&quot;, prog);
      exit (1);
    }

  &hellip;

}
</pre></td></tr></table>
</dd></dl>

<dl>
<dt><a name="index-dirname"></a><u>Function:</u> char * <b>dirname</b><i> (char *<var>path</var>)</i></dt>
<dd><p>The <code>dirname</code> function is the compliment to the XPG version of
<code>basename</code>.  It returns the parent directory of the file specified
by <var>path</var>.  If <var>path</var> is <code>NULL</code>, an empty string, or
contains no &rsquo;/&rsquo; characters, then &quot;.&quot; is returned.  The prototype for this
function can be found in &lsquo;<tt>libgen.h</tt>&rsquo;.
</p></dd></dl>

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<a name="strfry-1"></a>
<h2 class="section">5.9 strfry</h2>

<p>The function below addresses the perennial programming quandary: &ldquo;How do
I take good data in string form and painlessly turn it into garbage?&rdquo;
This is actually a fairly simple task for C programmers who do not use
the GNU C library string functions, but for programs based on the GNU C
library, the <code>strfry</code> function is the preferred method for
destroying string data.
</p>
<p>The prototype for this function is in &lsquo;<tt>string.h</tt>&rsquo;.
</p>
<dl>
<dt><a name="index-strfry"></a><u>Function:</u> char * <b>strfry</b><i> (char *<var>string</var>)</i></dt>
<dd>
<p><code>strfry</code> creates a pseudorandom anagram of a string, replacing the
input with the anagram in place.  For each position in the string,
<code>strfry</code> swaps it with a position in the string selected at random
(from a uniform distribution).  The two positions may be the same.
</p>
<p>The return value of <code>strfry</code> is always <var>string</var>.
</p>
<p><strong>Portability Note:</strong>  This function is unique to the GNU C library.
</p>
</dd></dl>


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<a name="Trivial-Encryption-1"></a>
<h2 class="section">5.10 Trivial Encryption</h2>
<a name="index-encryption"></a>


<p>The <code>memfrob</code> function converts an array of data to something
unrecognizable and back again.  It is not encryption in its usual sense
since it is easy for someone to convert the encrypted data back to clear
text.  The transformation is analogous to Usenet&rsquo;s &ldquo;Rot13&rdquo; encryption
method for obscuring offensive jokes from sensitive eyes and such.
Unlike Rot13, <code>memfrob</code> works on arbitrary binary data, not just
text.
<a name="index-Rot13"></a>
</p>
<p>For true encryption, See section <a href="libc_32.html#Cryptographic-Functions">DES Encryption and Password Handling</a>.
</p>
<p>This function is declared in &lsquo;<tt>string.h</tt>&rsquo;.
<a name="index-string_002eh-6"></a>
</p>
<dl>
<dt><a name="index-memfrob"></a><u>Function:</u> void * <b>memfrob</b><i> (void *<var>mem</var>, size_t <var>length</var>)</i></dt>
<dd>
<p><code>memfrob</code> transforms (frobnicates) each byte of the data structure
at <var>mem</var>, which is <var>length</var> bytes long, by bitwise exclusive
oring it with binary 00101010.  It does the transformation in place and
its return value is always <var>mem</var>.
</p>
<p>Note that <code>memfrob</code> a second time on the same data structure
returns it to its original state.
</p>
<p>This is a good function for hiding information from someone who doesn&rsquo;t
want to see it or doesn&rsquo;t want to see it very much.  To really prevent
people from retrieving the information, use stronger encryption such as
that described in See section <a href="libc_32.html#Cryptographic-Functions">DES Encryption and Password Handling</a>.
</p>
<p><strong>Portability Note:</strong>  This function is unique to the GNU C library.
</p>
</dd></dl>

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<a name="Encode-Binary-Data-1"></a>
<h2 class="section">5.11 Encode Binary Data</h2>

<p>To store or transfer binary data in environments which only support text
one has to encode the binary data by mapping the input bytes to
characters in the range allowed for storing or transfering.  SVID
systems (and nowadays XPG compliant systems) provide minimal support for
this task.
</p>
<dl>
<dt><a name="index-l64a"></a><u>Function:</u> char * <b>l64a</b><i> (long int <var>n</var>)</i></dt>
<dd><p>This function encodes a 32-bit input value using characters from the
basic character set.  It returns a pointer to a 7 character buffer which
contains an encoded version of <var>n</var>.  To encode a series of bytes the
user must copy the returned string to a destination buffer.  It returns
the empty string if <var>n</var> is zero, which is somewhat bizarre but
mandated by the standard.<br>
<strong>Warning:</strong> Since a static buffer is used this function should not
be used in multi-threaded programs.  There is no thread-safe alternative
to this function in the C library.<br>
<strong>Compatibility Note:</strong> The XPG standard states that the return
value of <code>l64a</code> is undefined if <var>n</var> is negative.  In the GNU
implementation, <code>l64a</code> treats its argument as unsigned, so it will
return a sensible encoding for any nonzero <var>n</var>; however, portable
programs should not rely on this.
</p>
<p>To encode a large buffer <code>l64a</code> must be called in a loop, once for
each 32-bit word of the buffer.  For example, one could do something
like this:
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">char *
encode (const void *buf, size_t len)
{
  /* <span class="roman">We know in advance how long the buffer has to be.</span> */
  unsigned char *in = (unsigned char *) buf;
  char *out = malloc (6 + ((len + 3) / 4) * 6 + 1);
  char *cp = out, *p;

  /* <span class="roman">Encode the length.</span> */
  /* <span class="roman">Using `htonl' is necessary so that the data can be</span>
     <span class="roman">decoded even on machines with different byte order.</span>
     <span class="roman">`l64a' can return a string shorter than 6 bytes, so </span>
     <span class="roman">we pad it with encoding of 0 (</span>'.'<span class="roman">) at the end by </span>
     <span class="roman">hand.</span> */

  p = stpcpy (cp, l64a (htonl (len)));
  cp = mempcpy (p, &quot;......&quot;, 6 - (p - cp));

  while (len &gt; 3)
    {
      unsigned long int n = *in++;
      n = (n &lt;&lt; 8) | *in++;
      n = (n &lt;&lt; 8) | *in++;
      n = (n &lt;&lt; 8) | *in++;
      len -= 4;
      p = stpcpy (cp, l64a (htonl (n)));
      cp = mempcpy (p, &quot;......&quot;, 6 - (p - cp));
    }
  if (len &gt; 0)
    {
      unsigned long int n = *in++;
      if (--len &gt; 0)
        {
          n = (n &lt;&lt; 8) | *in++;
          if (--len &gt; 0)
            n = (n &lt;&lt; 8) | *in;
        }
      cp = stpcpy (cp, l64a (htonl (n)));
    }
  *cp = '\0';
  return out;
}
</pre></td></tr></table>

<p>It is strange that the library does not provide the complete
functionality needed but so be it.
</p>
</dd></dl>

<p>To decode data produced with <code>l64a</code> the following function should be
used.
</p>
<dl>
<dt><a name="index-a64l"></a><u>Function:</u> long int <b>a64l</b><i> (const char *<var>string</var>)</i></dt>
<dd><p>The parameter <var>string</var> should contain a string which was produced by
a call to <code>l64a</code>.  The function processes at least 6 characters of
this string, and decodes the characters it finds according to the table
below.  It stops decoding when it finds a character not in the table,
rather like <code>atoi</code>; if you have a buffer which has been broken into
lines, you must be careful to skip over the end-of-line characters.
</p>
<p>The decoded number is returned as a <code>long int</code> value.
</p></dd></dl>

<p>The <code>l64a</code> and <code>a64l</code> functions use a base 64 encoding, in
which each character of an encoded string represents six bits of an
input word.  These symbols are used for the base 64 digits:
</p>
<table>
<tr><td></td><td>0</td><td>1</td><td>2</td><td>3</td><td>4</td><td>5</td><td>6</td><td>7</td></tr>
<tr><td>0</td><td><code>.</code></td><td><code>/</code></td><td><code>0</code></td><td><code>1</code></td><td><code>2</code></td><td><code>3</code></td><td><code>4</code></td><td><code>5</code></td></tr>
<tr><td>8</td><td><code>6</code></td><td><code>7</code></td><td><code>8</code></td><td><code>9</code></td><td><code>A</code></td><td><code>B</code></td><td><code>C</code></td><td><code>D</code></td></tr>
<tr><td>16</td><td><code>E</code></td><td><code>F</code></td><td><code>G</code></td><td><code>H</code></td><td><code>I</code></td><td><code>J</code></td><td><code>K</code></td><td><code>L</code></td></tr>
<tr><td>24</td><td><code>M</code></td><td><code>N</code></td><td><code>O</code></td><td><code>P</code></td><td><code>Q</code></td><td><code>R</code></td><td><code>S</code></td><td><code>T</code></td></tr>
<tr><td>32</td><td><code>U</code></td><td><code>V</code></td><td><code>W</code></td><td><code>X</code></td><td><code>Y</code></td><td><code>Z</code></td><td><code>a</code></td><td><code>b</code></td></tr>
<tr><td>40</td><td><code>c</code></td><td><code>d</code></td><td><code>e</code></td><td><code>f</code></td><td><code>g</code></td><td><code>h</code></td><td><code>i</code></td><td><code>j</code></td></tr>
<tr><td>48</td><td><code>k</code></td><td><code>l</code></td><td><code>m</code></td><td><code>n</code></td><td><code>o</code></td><td><code>p</code></td><td><code>q</code></td><td><code>r</code></td></tr>
<tr><td>56</td><td><code>s</code></td><td><code>t</code></td><td><code>u</code></td><td><code>v</code></td><td><code>w</code></td><td><code>x</code></td><td><code>y</code></td><td><code>z</code></td></tr>
</table>

<p>This encoding scheme is not standard.  There are some other encoding
methods which are much more widely used (UU encoding, MIME encoding).
Generally, it is better to use one of these encodings.
</p>
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<a name="Argz-and-Envz-Vectors-1"></a>
<h2 class="section">5.12 Argz and Envz Vectors</h2>

<a name="index-argz-vectors-_0028string-vectors_0029"></a>
<a name="index-string-vectors_002c-null_002dcharacter-separated"></a>
<a name="index-argument-vectors_002c-null_002dcharacter-separated"></a>
<p><em>argz vectors</em> are vectors of strings in a contiguous block of
memory, each element separated from its neighbors by null-characters
(<code>'\0'</code>).
</p>
<a name="index-envz-vectors-_0028environment-vectors_0029"></a>
<a name="index-environment-vectors_002c-null_002dcharacter-separated"></a>
<p><em>Envz vectors</em> are an extension of argz vectors where each element is a
name-value pair, separated by a <code>'='</code> character (as in a Unix
environment).
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top"><a href="#Argz-Functions">5.12.1 Argz Functions</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">              Operations on argz vectors.
</td></tr>
<tr><td align="left" valign="top"><a href="#Envz-Functions">5.12.2 Envz Functions</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">              Additional operations on environment vectors.
</td></tr>
</table>

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<a name="Argz-Functions-1"></a>
<h3 class="subsection">5.12.1 Argz Functions</h3>

<p>Each argz vector is represented by a pointer to the first element, of
type <code>char *</code>, and a size, of type <code>size_t</code>, both of which can
be initialized to <code>0</code> to represent an empty argz vector.  All argz
functions accept either a pointer and a size argument, or pointers to
them, if they will be modified.
</p>
<p>The argz functions use <code>malloc</code>/<code>realloc</code> to allocate/grow
argz vectors, and so any argz vector creating using these functions may
be freed by using <code>free</code>; conversely, any argz function that may
grow a string expects that string to have been allocated using
<code>malloc</code> (those argz functions that only examine their arguments or
modify them in place will work on any sort of memory).
See section <a href="libc_3.html#Unconstrained-Allocation">Unconstrained Allocation</a>.
</p>
<p>All argz functions that do memory allocation have a return type of
<code>error_t</code>, and return <code>0</code> for success, and <code>ENOMEM</code> if an
allocation error occurs.
</p>
<a name="index-argz_002eh"></a>
<p>These functions are declared in the standard include file &lsquo;<tt>argz.h</tt>&rsquo;.
</p>
<dl>
<dt><a name="index-argz_005fcreate"></a><u>Function:</u> error_t <b>argz_create</b><i> (char *const <var>argv</var>[], char **<var>argz</var>, size_t *<var>argz_len</var>)</i></dt>
<dd><p>The <code>argz_create</code> function converts the Unix-style argument vector
<var>argv</var> (a vector of pointers to normal C strings, terminated by
<code>(char *)0</code>; see section <a href="libc_25.html#Program-Arguments">Program Arguments</a>) into an argz vector with
the same elements, which is returned in <var>argz</var> and <var>argz_len</var>.
</p></dd></dl>

<dl>
<dt><a name="index-argz_005fcreate_005fsep"></a><u>Function:</u> error_t <b>argz_create_sep</b><i> (const char *<var>string</var>, int <var>sep</var>, char **<var>argz</var>, size_t *<var>argz_len</var>)</i></dt>
<dd><p>The <code>argz_create_sep</code> function converts the null-terminated string
<var>string</var> into an argz vector (returned in <var>argz</var> and
<var>argz_len</var>) by splitting it into elements at every occurrence of the
character <var>sep</var>.
</p></dd></dl>

<dl>
<dt><a name="index-argz_005fcount"></a><u>Function:</u> size_t <b>argz_count</b><i> (const char *<var>argz</var>, size_t <var>arg_len</var>)</i></dt>
<dd><p>Returns the number of elements in the argz vector <var>argz</var> and
<var>argz_len</var>.
</p></dd></dl>

<dl>
<dt><a name="index-argz_005fextract"></a><u>Function:</u> void <b>argz_extract</b><i> (char *<var>argz</var>, size_t <var>argz_len</var>, char **<var>argv</var>)</i></dt>
<dd><p>The <code>argz_extract</code> function converts the argz vector <var>argz</var> and
<var>argz_len</var> into a Unix-style argument vector stored in <var>argv</var>,
by putting pointers to every element in <var>argz</var> into successive
positions in <var>argv</var>, followed by a terminator of <code>0</code>.
<var>Argv</var> must be pre-allocated with enough space to hold all the
elements in <var>argz</var> plus the terminating <code>(char *)0</code>
(<code>(argz_count (<var>argz</var>, <var>argz_len</var>) + 1) * sizeof (char *)</code>
bytes should be enough).  Note that the string pointers stored into
<var>argv</var> point into <var>argz</var>&mdash;they are not copies&mdash;and so
<var>argz</var> must be copied if it will be changed while <var>argv</var> is
still active.  This function is useful for passing the elements in
<var>argz</var> to an exec function (see section <a href="libc_26.html#Executing-a-File">Executing a File</a>).
</p></dd></dl>

<dl>
<dt><a name="index-argz_005fstringify"></a><u>Function:</u> void <b>argz_stringify</b><i> (char *<var>argz</var>, size_t <var>len</var>, int <var>sep</var>)</i></dt>
<dd><p>The <code>argz_stringify</code> converts <var>argz</var> into a normal string with
the elements separated by the character <var>sep</var>, by replacing each
<code>'\0'</code> inside <var>argz</var> (except the last one, which terminates the
string) with <var>sep</var>.  This is handy for printing <var>argz</var> in a
readable manner.
</p></dd></dl>

<dl>
<dt><a name="index-argz_005fadd"></a><u>Function:</u> error_t <b>argz_add</b><i> (char **<var>argz</var>, size_t *<var>argz_len</var>, const char *<var>str</var>)</i></dt>
<dd><p>The <code>argz_add</code> function adds the string <var>str</var> to the end of the
argz vector <code>*<var>argz</var></code>, and updates <code>*<var>argz</var></code> and
<code>*<var>argz_len</var></code> accordingly.
</p></dd></dl>

<dl>
<dt><a name="index-argz_005fadd_005fsep"></a><u>Function:</u> error_t <b>argz_add_sep</b><i> (char **<var>argz</var>, size_t *<var>argz_len</var>, const char *<var>str</var>, int <var>delim</var>)</i></dt>
<dd><p>The <code>argz_add_sep</code> function is similar to <code>argz_add</code>, but
<var>str</var> is split into separate elements in the result at occurrences of
the character <var>delim</var>.  This is useful, for instance, for
adding the components of a Unix search path to an argz vector, by using
a value of <code>':'</code> for <var>delim</var>.
</p></dd></dl>

<dl>
<dt><a name="index-argz_005fappend"></a><u>Function:</u> error_t <b>argz_append</b><i> (char **<var>argz</var>, size_t *<var>argz_len</var>, const char *<var>buf</var>, size_t <var>buf_len</var>)</i></dt>
<dd><p>The <code>argz_append</code> function appends <var>buf_len</var> bytes starting at
<var>buf</var> to the argz vector <code>*<var>argz</var></code>, reallocating
<code>*<var>argz</var></code> to accommodate it, and adding <var>buf_len</var> to
<code>*<var>argz_len</var></code>.
</p></dd></dl>

<dl>
<dt><a name="index-argz_005fdelete"></a><u>Function:</u> void <b>argz_delete</b><i> (char **<var>argz</var>, size_t *<var>argz_len</var>, char *<var>entry</var>)</i></dt>
<dd><p>If <var>entry</var> points to the beginning of one of the elements in the
argz vector <code>*<var>argz</var></code>, the <code>argz_delete</code> function will
remove this entry and reallocate <code>*<var>argz</var></code>, modifying
<code>*<var>argz</var></code> and <code>*<var>argz_len</var></code> accordingly.  Note that as
destructive argz functions usually reallocate their argz argument,
pointers into argz vectors such as <var>entry</var> will then become invalid.
</p></dd></dl>

<dl>
<dt><a name="index-argz_005finsert"></a><u>Function:</u> error_t <b>argz_insert</b><i> (char **<var>argz</var>, size_t *<var>argz_len</var>, char *<var>before</var>, const char *<var>entry</var>)</i></dt>
<dd><p>The <code>argz_insert</code> function inserts the string <var>entry</var> into the
argz vector <code>*<var>argz</var></code> at a point just before the existing
element pointed to by <var>before</var>, reallocating <code>*<var>argz</var></code> and
updating <code>*<var>argz</var></code> and <code>*<var>argz_len</var></code>.  If <var>before</var>
is <code>0</code>, <var>entry</var> is added to the end instead (as if by
<code>argz_add</code>).  Since the first element is in fact the same as
<code>*<var>argz</var></code>, passing in <code>*<var>argz</var></code> as the value of
<var>before</var> will result in <var>entry</var> being inserted at the beginning.
</p></dd></dl>

<dl>
<dt><a name="index-argz_005fnext"></a><u>Function:</u> char * <b>argz_next</b><i> (char *<var>argz</var>, size_t <var>argz_len</var>, const char *<var>entry</var>)</i></dt>
<dd><p>The <code>argz_next</code> function provides a convenient way of iterating
over the elements in the argz vector <var>argz</var>.  It returns a pointer
to the next element in <var>argz</var> after the element <var>entry</var>, or
<code>0</code> if there are no elements following <var>entry</var>.  If <var>entry</var>
is <code>0</code>, the first element of <var>argz</var> is returned.
</p>
<p>This behavior suggests two styles of iteration:
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">    char *entry = 0;
    while ((entry = argz_next (<var>argz</var>, <var>argz_len</var>, entry)))
      <var>action</var>;
</pre></td></tr></table>

<p>(the double parentheses are necessary to make some C compilers shut up
about what they consider a questionable <code>while</code>-test) and:
</p>
<table><tr><td>&nbsp;</td><td><pre class="smallexample">    char *entry;
    for (entry = <var>argz</var>;
         entry;
         entry = argz_next (<var>argz</var>, <var>argz_len</var>, entry))
      <var>action</var>;
</pre></td></tr></table>

<p>Note that the latter depends on <var>argz</var> having a value of <code>0</code> if
it is empty (rather than a pointer to an empty block of memory); this
invariant is maintained for argz vectors created by the functions here.
</p></dd></dl>

<dl>
<dt><a name="index-argz_005freplace"></a><u>Function:</u> error_t <b>argz_replace</b><i> (char **<var>argz</var>, size_t *<var>argz_len</var>, const char *<var>str</var>, const char *<var>with</var>, unsigned *<var>replace_count</var>)</i></dt>
<dd><p>Replace any occurrences of the string <var>str</var> in <var>argz</var> with
<var>with</var>, reallocating <var>argz</var> as necessary.  If
<var>replace_count</var> is non-zero, <code>*<var>replace_count</var></code> will be
incremented by number of replacements performed.
</p></dd></dl>

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<a name="Envz-Functions-1"></a>
<h3 class="subsection">5.12.2 Envz Functions</h3>

<p>Envz vectors are just argz vectors with additional constraints on the form
of each element; as such, argz functions can also be used on them, where it
makes sense.
</p>
<p>Each element in an envz vector is a name-value pair, separated by a <code>'='</code>
character; if multiple <code>'='</code> characters are present in an element, those
after the first are considered part of the value, and treated like all other
non-<code>'\0'</code> characters.
</p>
<p>If <em>no</em> <code>'='</code> characters are present in an element, that element is
considered the name of a &ldquo;null&rdquo; entry, as distinct from an entry with an
empty value: <code>envz_get</code> will return <code>0</code> if given the name of null
entry, whereas an entry with an empty value would result in a value of
<code>&quot;&quot;</code>; <code>envz_entry</code> will still find such entries, however.  Null
entries can be removed with <code>envz_strip</code> function.
</p>
<p>As with argz functions, envz functions that may allocate memory (and thus
fail) have a return type of <code>error_t</code>, and return either <code>0</code> or
<code>ENOMEM</code>.
</p>
<a name="index-envz_002eh"></a>
<p>These functions are declared in the standard include file &lsquo;<tt>envz.h</tt>&rsquo;.
</p>
<dl>
<dt><a name="index-envz_005fentry"></a><u>Function:</u> char * <b>envz_entry</b><i> (const char *<var>envz</var>, size_t <var>envz_len</var>, const char *<var>name</var>)</i></dt>
<dd><p>The <code>envz_entry</code> function finds the entry in <var>envz</var> with the name
<var>name</var>, and returns a pointer to the whole entry&mdash;that is, the argz
element which begins with <var>name</var> followed by a <code>'='</code> character.  If
there is no entry with that name, <code>0</code> is returned.
</p></dd></dl>

<dl>
<dt><a name="index-envz_005fget"></a><u>Function:</u> char * <b>envz_get</b><i> (const char *<var>envz</var>, size_t <var>envz_len</var>, const char *<var>name</var>)</i></dt>
<dd><p>The <code>envz_get</code> function finds the entry in <var>envz</var> with the name
<var>name</var> (like <code>envz_entry</code>), and returns a pointer to the value
portion of that entry (following the <code>'='</code>).  If there is no entry with
that name (or only a null entry), <code>0</code> is returned.
</p></dd></dl>

<dl>
<dt><a name="index-envz_005fadd"></a><u>Function:</u> error_t <b>envz_add</b><i> (char **<var>envz</var>, size_t *<var>envz_len</var>, const char *<var>name</var>, const char *<var>value</var>)</i></dt>
<dd><p>The <code>envz_add</code> function adds an entry to <code>*<var>envz</var></code>
(updating <code>*<var>envz</var></code> and <code>*<var>envz_len</var></code>) with the name
<var>name</var>, and value <var>value</var>.  If an entry with the same name
already exists in <var>envz</var>, it is removed first.  If <var>value</var> is
<code>0</code>, then the new entry will the special null type of entry
(mentioned above).
</p></dd></dl>

<dl>
<dt><a name="index-envz_005fmerge"></a><u>Function:</u> error_t <b>envz_merge</b><i> (char **<var>envz</var>, size_t *<var>envz_len</var>, const char *<var>envz2</var>, size_t <var>envz2_len</var>, int <var>override</var>)</i></dt>
<dd><p>The <code>envz_merge</code> function adds each entry in <var>envz2</var> to <var>envz</var>,
as if with <code>envz_add</code>, updating <code>*<var>envz</var></code> and
<code>*<var>envz_len</var></code>.  If <var>override</var> is true, then values in <var>envz2</var>
will supersede those with the same name in <var>envz</var>, otherwise not.
</p>
<p>Null entries are treated just like other entries in this respect, so a null
entry in <var>envz</var> can prevent an entry of the same name in <var>envz2</var> from
being added to <var>envz</var>, if <var>override</var> is false.
</p></dd></dl>

<dl>
<dt><a name="index-envz_005fstrip"></a><u>Function:</u> void <b>envz_strip</b><i> (char **<var>envz</var>, size_t *<var>envz_len</var>)</i></dt>
<dd><p>The <code>envz_strip</code> function removes any null entries from <var>envz</var>,
updating <code>*<var>envz</var></code> and <code>*<var>envz_len</var></code>.
</p></dd></dl>
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