public/fd_printf.html


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  <title>`dprintf`: print formatted text directly to a file descriptor — Luke Shumaker</title>
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<header><a href="/">Luke Shumaker</a> » <a href=/blog>blog</a> » fd_printf</header>
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<h1 id="dprintf-print-formatted-text-directly-to-a-file-descriptor"><code>dprintf</code>: print formatted text directly to a file descriptor</h1>
<p>This already existed as <code>dprintf(3)</code>. I now feel stupid for having Implemented <code>fd_printf</code>.</p>
<p>The original post is as follows:</p>
<hr />
<p>I wrote this while debugging some code, and thought it might be useful to others:</p>
<pre><code>#define _GNU_SOURCE     /* vasprintf() */
#include &lt;stdarg.h&gt;     /* va_start()/va_end() */
#include &lt;stdio.h&gt;      /* vasprintf() */
#include &lt;stdlib.h&gt;     /* free() */
#include &lt;unistd.h&gt;     /* write() */

int
fd_printf(int fd, const char *format, ...)
{
    va_list arg;
    int len;
    char *str;

    va_start(arg, format);
    len = vasprintf(&amp;str, format, arg);
    va_end(arg);

    write(fd, str, len);

    free(str);
    return len;
}</code></pre>
<p>It is a version of <code>printf</code> that prints to a file descriptor—where <code>fprintf</code> prints to a <code>FILE*</code> data structure.</p>
<p>The appeal of this is that <code>FILE*</code> I/O is buffered—which means mixing it with raw file descriptor I/O is going to produce weird results.</p>

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