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<h4 class="subsection">13.14.4 Getting and Setting File Status Flags</h4>

<p>The <code>fcntl</code> function can fetch or change file status flags.

<!-- fcntl.h -->
<!-- POSIX.1 -->
<div class="defun">
&mdash; Macro: int <b>F_GETFL</b><var><a name="index-F_005fGETFL-1362"></a></var><br>
<blockquote><p>This macro is used as the <var>command</var> argument to <code>fcntl</code>, to
read the file status flags for the open file with descriptor
<var>filedes</var>.

        <p>The normal return value from <code>fcntl</code> with this command is a
nonnegative number which can be interpreted as the bitwise OR of the
individual flags.  Since the file access modes are not single-bit values,
you can mask off other bits in the returned flags with <code>O_ACCMODE</code>
to compare them.

        <p>In case of an error, <code>fcntl</code> returns -1.  The following
<code>errno</code> error conditions are defined for this command:

          <dl>
<dt><code>EBADF</code><dd>The <var>filedes</var> argument is invalid. 
</dl>
        </p></blockquote></div>

<!-- fcntl.h -->
<!-- POSIX.1 -->
<div class="defun">
&mdash; Macro: int <b>F_SETFL</b><var><a name="index-F_005fSETFL-1363"></a></var><br>
<blockquote><p>This macro is used as the <var>command</var> argument to <code>fcntl</code>, to set
the file status flags for the open file corresponding to the
<var>filedes</var> argument.  This command requires a third <code>int</code>
argument to specify the new flags, so the call looks like this:

     <pre class="smallexample">          fcntl (<var>filedes</var>, F_SETFL, <var>new-flags</var>)
</pre>
        <p>You can't change the access mode for the file in this way; that is,
whether the file descriptor was opened for reading or writing.

        <p>The normal return value from <code>fcntl</code> with this command is an
unspecified value other than -1, which indicates an error.  The
error conditions are the same as for the <code>F_GETFL</code> command. 
</p></blockquote></div>

   <p>If you want to modify the file status flags, you should get the current
flags with <code>F_GETFL</code> and modify the value.  Don't assume that the
flags listed here are the only ones that are implemented; your program
may be run years from now and more flags may exist then.  For example,
here is a function to set or clear the flag <code>O_NONBLOCK</code> without
altering any other flags:

<pre class="smallexample">     /* <span class="roman">Set the </span><code>O_NONBLOCK</code><span class="roman"> flag of </span><var>desc</var><span class="roman"> if </span><var>value</var><span class="roman"> is nonzero,</span>
        <span class="roman">or clear the flag if </span><var>value</var><span class="roman"> is 0.</span>
        <span class="roman">Return 0 on success, or -1 on error with </span><code>errno</code><span class="roman"> set.</span> */
     
     int
     set_nonblock_flag (int desc, int value)
     {
       int oldflags = fcntl (desc, F_GETFL, 0);
       /* <span class="roman">If reading the flags failed, return error indication now.</span> */
       if (oldflags == -1)
         return -1;
       /* <span class="roman">Set just the flag we want to set.</span> */
       if (value != 0)
         oldflags |= O_NONBLOCK;
       else
         oldflags &amp;= ~O_NONBLOCK;
       /* <span class="roman">Store modified flag word in the descriptor.</span> */
       return fcntl (desc, F_SETFL, oldflags);
     }
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