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<a name="open_005fmemstream_002c-open_005fwmemstream_002d_002d_002dopen-a-write-stream-around-an-arbitrary_002dlength-string"></a>
<h3 class="section">4.43 <code>open_memstream</code>, <code>open_wmemstream</code>&mdash;open a write stream around an arbitrary-length string</h3>
<a name="index-open_005fmemstream"></a>
<a name="index-open_005fwmemstream"></a>
<p><strong>Synopsis</strong>
</p><div class="example">
<pre class="example">#include &lt;stdio.h&gt;
FILE *open_memstream(char **restrict <var>buf</var>,
    size_t *restrict <var>size</var>);

#include &lt;wchar.h&gt;
FILE *open_wmemstream(wchar_t **restrict <var>buf</var>,
    size_t *restrict <var>size</var>);

</pre></div>
<p><strong>Description</strong><br>
<code>open_memstream</code> creates a seekable, byte-oriented <code>FILE</code> stream that
wraps an arbitrary-length buffer, created as if by <code>malloc</code>.  The current
contents of *<var>buf</var> are ignored; this implementation uses *<var>size</var>
as a hint of the maximum size expected, but does not fail if the hint
was wrong.  The parameters <var>buf</var> and <var>size</var> are later stored
through following any call to <code>fflush</code> or <code>fclose</code>, set to the
current address and usable size of the allocated string; although
after fflush, the pointer is only valid until another stream operation
that results in a write.  Behavior is undefined if the user alters
either *<var>buf</var> or *<var>size</var> prior to <code>fclose</code>.
</p>
<p><code>open_wmemstream</code> is like <code>open_memstream</code> just with the associated
stream being wide-oriented.  The size set in <var>size</var> in subsequent
operations is the number of wide characters.
</p>
<p>The stream is write-only, since the user can directly read *<var>buf</var>
after a flush; see <code>fmemopen</code> for a way to wrap a string with a
readable stream.  The user is responsible for calling <code>free</code> on
the final *<var>buf</var> after <code>fclose</code>.
</p>
<p>Any time the stream is flushed, a NUL byte is written at the current
position (but is not counted in the buffer length), so that the string
is always NUL-terminated after at most *<var>size</var> bytes (or wide characters
in case of <code>open_wmemstream</code>).  However, data previously written beyond
the current stream offset is not lost, and the NUL value written during a
flush is restored to its previous value when seeking elsewhere in the string.
</p>
<br>
<p><strong>Returns</strong><br>
The return value is an open FILE pointer on success.  On error,
<code>NULL</code> is returned, and <code>errno</code> will be set to EINVAL if <var>buf</var>
or <var>size</var> is NULL, ENOMEM if memory could not be allocated, or
EMFILE if too many streams are already open.
</p>
<br>
<p><strong>Portability</strong><br>
POSIX.1-2008
</p>
<p>Supporting OS subroutines required: <code>sbrk</code>.
</p>
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