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<a name="funopen_002c-fropen_002c-fwopen_002d_002d_002dopen-a-stream-with-custom-callbacks"></a>
<h3 class="section">4.30 <code>funopen</code>, <code>fropen</code>, <code>fwopen</code>&mdash;open a stream with custom callbacks</h3>
<a name="index-funopen"></a>
<a name="index-fropen"></a>
<a name="index-fwopen"></a>
<p><strong>Synopsis</strong>
</p><div class="example">
<pre class="example">#include &lt;stdio.h&gt;
FILE *funopen(const void *<var>cookie</var>,
    int (*<var>readfn</var>) (void *cookie, char *buf, int n),
    int (*<var>writefn</var>) (void *cookie, const char *buf, int n),
    fpos_t (*<var>seekfn</var>) (void *cookie, fpos_t off, int whence),
    int (*<var>closefn</var>) (void *cookie));
FILE *fropen(const void *<var>cookie</var>,
    int (*<var>readfn</var>) (void *cookie, char *buf, int n));
FILE *fwopen(const void *<var>cookie</var>,
    int (*<var>writefn</var>) (void *cookie, const char *buf, int n));

</pre></div>
<p><strong>Description</strong><br>
<code>funopen</code> creates a <code>FILE</code> stream where I/O is performed using
custom callbacks.  At least one of <var>readfn</var> and <var>writefn</var> must be
provided, which determines whether the stream behaves with mode &lt;&quot;r&quot;&gt;,
&lt;&quot;w&quot;&gt;, or &lt;&quot;r+&quot;&gt;.
</p>
<p><var>readfn</var> should return -1 on failure, or else the number of bytes
read (0 on EOF).  It is similar to <code>read</code>, except that &lt;int&gt; rather
than &lt;size_t&gt; bounds a transaction size, and <var>cookie</var> will be passed
as the first argument.  A NULL <var>readfn</var> makes attempts to read the
stream fail.
</p>
<p><var>writefn</var> should return -1 on failure, or else the number of bytes
written.  It is similar to <code>write</code>, except that &lt;int&gt; rather than
&lt;size_t&gt; bounds a transaction size, and <var>cookie</var> will be passed as
the first argument.  A NULL <var>writefn</var> makes attempts to write the
stream fail.
</p>
<p><var>seekfn</var> should return (fpos_t)-1 on failure, or else the current
file position.  It is similar to <code>lseek</code>, except that <var>cookie</var>
will be passed as the first argument.  A NULL <var>seekfn</var> makes the
stream behave similarly to a pipe in relation to stdio functions that
require positioning.  This implementation assumes fpos_t and off_t are
the same type.
</p>
<p><var>closefn</var> should return -1 on failure, or 0 on success.  It is
similar to <code>close</code>, except that <var>cookie</var> will be passed as the
first argument.  A NULL <var>closefn</var> merely flushes all data then lets
<code>fclose</code> succeed.  A failed close will still invalidate the stream.
</p>
<p>Read and write I/O functions are allowed to change the underlying
buffer on fully buffered or line buffered streams by calling
<code>setvbuf</code>.  They are also not required to completely fill or empty
the buffer.  They are not, however, allowed to change streams from
unbuffered to buffered or to change the state of the line buffering
flag.  They must also be prepared to have read or write calls occur on
buffers other than the one most recently specified.
</p>
<p>The functions <code>fropen</code> and <code>fwopen</code> are convenience macros around
<code>funopen</code> that only use the specified callback.
</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 a
function pointer is missing, ENOMEM if the stream cannot be created,
or EMFILE if too many streams are already open.
</p>
<br>
<p><strong>Portability</strong><br>
This function is a newlib extension, copying the prototype from BSD.
It is not portable.  See also the <code>fopencookie</code> interface from Linux.
</p>
<p>Supporting OS subroutines required: <code>sbrk</code>.
</p>
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