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 <h3 class="section">4.16 <code>fmemopen</code>&mdash;open a stream around a fixed-length string</h3>

 <p><a name="index-fmemopen-177"></a><strong>Synopsis</strong>
 <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>
    <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>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>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>
 <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>
 <strong>Portability</strong><br>
 This function is being added to POSIX 200x, but is not in POSIX 2001.

    <p>Supporting OS subroutines required: <code>sbrk</code>.

    <p><br>

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	<h3 class="section">4.16 <code>fmemopen</code>—open a stream around a fixed-length string</h3>

	<p><a name="index-fmemopen-177"></a><strong>Synopsis</strong>
	<pre class="example"> #include <stdio.h>
	FILE fmemopen(void restrict <var>buf</var>, size_t <var>size</var>,
	const char *restrict <var>mode</var>);

	</pre>
	<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>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>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>
	<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>
	<strong>Portability</strong><br>
	This function is being added to POSIX 200x, but is not in POSIX 2001.

	<p>Supporting OS subroutines required: <code>sbrk</code>.

	<p><br>

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