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<a name="strerror_005fr_002d_002d_002dconvert-error-number-to-string-and-copy-to-buffer"></a>
<h3 class="section">5.28 <code>strerror_r</code>&mdash;convert error number to string and copy to buffer</h3>
<a name="index-strerror_005fr"></a>
<p><strong>Synopsis</strong>
</p><div class="example">
<pre class="example">#include &lt;string.h&gt;
#ifdef _GNU_SOURCE
char *strerror_r(int <var>errnum</var>, char *<var>buffer</var>, size_t <var>n</var>);
#else
int strerror_r(int <var>errnum</var>, char *<var>buffer</var>, size_t <var>n</var>);
#endif

</pre></div>
<p><strong>Description</strong><br>
<code>strerror_r</code> converts the error number <var>errnum</var> into a
string and copies the result into the supplied <var>buffer</var> for
a length up to <var>n</var>, including the NUL terminator. The value of
<var>errnum</var> is usually a copy of <code>errno</code>.  If <code>errnum</code> is not a known
error number, the result is the empty string.
</p>
<p>See <code>strerror</code> for how strings are mapped to <code>errnum</code>.
</p>
<br>
<p><strong>Returns</strong><br>
There are two variants: the GNU version always returns a NUL-terminated
string, which is <var>buffer</var> if all went well, but which is another
pointer if <var>n</var> was too small (leaving <var>buffer</var> untouched).  If the
return is not <var>buffer</var>, your application must not modify that string.
The POSIX version returns 0 on success, <var>EINVAL</var> if <code>errnum</code> was not
recognized, and <var>ERANGE</var> if <var>n</var> was too small.  The variant chosen
depends on macros that you define before inclusion of <code>string.h</code>.
</p>
<br>
<p><strong>Portability</strong><br>
<code>strerror_r</code> with a <var>char *</var> result is a GNU extension.
<code>strerror_r</code> with an <var>int</var> result is required by POSIX 2001.
This function is compliant only if <code>_user_strerror</code> is not provided,
or if it is thread-safe and uses separate storage according to whether
the second argument of that function is non-zero.  For more details
on <code>_user_strerror</code>, see the <code>strerror</code> documentation.
</p>
<p>POSIX states that the contents of <var>buf</var> are unspecified on error,
although this implementation guarantees a NUL-terminated string for
all except <var>n</var> of 0.
</p>
<p>POSIX recommends that unknown <var>errnum</var> result in a message including
that value, however it is not a requirement and this implementation
provides only an empty string (unless you provide <code>_user_strerror</code>).
POSIX also recommends that unknown <var>errnum</var> fail with EINVAL even
when providing such a message, however it is not a requirement and
this implementation will return success if <code>_user_strerror</code> provided
a non-empty alternate string without assigning into its third argument.
</p>
<p><code>strerror_r</code> requires no supporting OS subroutines.
</p>

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