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<a name="setvbuf_002d_002d_002dspecify-file-or-stream-buffering"></a>
<h3 class="section">4.58 <code>setvbuf</code>&mdash;specify file or stream buffering</h3>
<a name="index-setvbuf"></a>
<p><strong>Synopsis</strong>
</p><div class="example">
<pre class="example">#include &lt;stdio.h&gt;
int setvbuf(FILE *<var>fp</var>, char *<var>buf</var>,
    int <var>mode</var>, size_t <var>size</var>);

</pre></div>
<p><strong>Description</strong><br>
Use <code>setvbuf</code> to specify what kind of buffering you want for the
file or stream identified by <var>fp</var>, by using one of the following
values (from <code>stdio.h</code>) as the <var>mode</var> argument:
</p>
<dl compact="compact">
<dt><code>_IONBF</code></dt>
<dd><p>Do not use a buffer: send output directly to the host system for the
file or stream identified by <var>fp</var>.
</p>
</dd>
<dt><code>_IOFBF</code></dt>
<dd><p>Use full output buffering: output will be passed on to the host system
only when the buffer is full, or when an input operation intervenes.
</p>
</dd>
<dt><code>_IOLBF</code></dt>
<dd><p>Use line buffering: pass on output to the host system at every
newline, as well as when the buffer is full, or when an input
operation intervenes.
</p></dd>
</dl>


<p>Use the <var>size</var> argument to specify how large a buffer you wish.  You
can supply the buffer itself, if you wish, by passing a pointer to a
suitable area of memory as <var>buf</var>.  Otherwise, you may pass <code>NULL</code>
as the <var>buf</var> argument, and <code>setvbuf</code> will allocate the buffer.
</p>
<br>
<p><strong>Warnings</strong><br>
You may only use <code>setvbuf</code> before performing any file operation other
than opening the file.
</p>
<p>If you supply a non-null <var>buf</var>, you must ensure that the associated
storage continues to be available until you close the stream
identified by <var>fp</var>.
</p>
<br>
<p><strong>Returns</strong><br>
A <code>0</code> result indicates success, <code>EOF</code> failure (invalid <var>mode</var> or
<var>size</var> can cause failure).
</p>
<br>
<p><strong>Portability</strong><br>
Both ANSI C and the System V Interface Definition (Issue 2) require
<code>setvbuf</code>. However, they differ on the meaning of a <code>NULL</code> buffer
pointer: the SVID issue 2 specification says that a <code>NULL</code> buffer
pointer requests unbuffered output.  For maximum portability, avoid
<code>NULL</code> buffer pointers.
</p>
<p>Both specifications describe the result on failure only as a
nonzero value.
</p>
<p>Supporting OS subroutines required: <code>close</code>, <code>fstat</code>, <code>isatty</code>,
<code>lseek</code>, <code>read</code>, <code>sbrk</code>, <code>write</code>.
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