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<a name="fmemopen_002d_002d_002dopen-a-stream-around-a-fixed_002dlength-string"></a>
<h3 class="section">4.16 <code>fmemopen</code>&mdash;open a stream around a fixed-length string</h3>
<a name="index-fmemopen"></a>
<p><strong>Synopsis</strong>
</p><div class="example">
<pre class="example">#include &lt;stdio.h&gt;
FILE *fmemopen(void *restrict <var>buf</var>, size_t <var>size</var>,
    const char *restrict <var>mode</var>);

</pre></div>
<p><strong>Description</strong><br>
<code>fmemopen</code> creates a seekable <code>FILE</code> stream that wraps a
fixed-length buffer of <var>size</var> bytes starting at <var>buf</var>.  The stream
is opened with <var>mode</var> treated as in <code>fopen</code>, where append mode
starts writing at the first NUL byte.  If <var>buf</var> is NULL, then
<var>size</var> bytes are automatically provided as if by <code>malloc</code>, with
the initial size of 0, and <var>mode</var> must contain <code>+</code> so that data
can be read after it is written.
</p>
<p>The stream maintains a current position, which moves according to
bytes read or written, and which can be one past the end of the array.
The stream also maintains a current file size, which is never greater
than <var>size</var>.  If <var>mode</var> starts with <code>r</code>, the position starts at
<code>0</code>, and file size starts at <var>size</var> if <var>buf</var> was provided.  If
<var>mode</var> starts with <code>w</code>, the position and file size start at <code>0</code>,
and if <var>buf</var> was provided, the first byte is set to NUL.  If
<var>mode</var> starts with <code>a</code>, the position and file size start at the
location of the first NUL byte, or else <var>size</var> if <var>buf</var> was
provided.
</p>
<p>When reading, NUL bytes have no significance, and reads cannot exceed
the current file size.  When writing, the file size can increase up to
<var>size</var> as needed, and NUL bytes may be embedded in the stream (see
<code>open_memstream</code> for an alternative that automatically enlarges the
buffer).  When the stream is flushed or closed after a write that
changed the file size, a NUL byte is written at the current position
if there is still room; if the stream is not also open for reading, a
NUL byte is additionally written at the last byte of <var>buf</var> when the
stream has exceeded <var>size</var>, so that a write-only <var>buf</var> is always
NUL-terminated when the stream is flushed or closed (and the initial
<var>size</var> should take this into account).  It is not possible to seek
outside the bounds of <var>size</var>.  A NUL byte 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>size</var>
is zero or <var>mode</var> is invalid, ENOMEM if <var>buf</var> was NULL and memory
could not be allocated, or EMFILE if too many streams are already
open.
</p>
<br>
<p><strong>Portability</strong><br>
This function is being added to POSIX 200x, but is not in POSIX 2001.
</p>
<p>Supporting OS subroutines required: <code>sbrk</code>.
</p>
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