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<h4 class="subsection">25.2.1 Using the <code>getopt</code> function</h4>

<p>Here are the details about how to call the <code>getopt</code> function.  To
use this facility, your program must include the header file
<samp><span class="file">unistd.h</span></samp>. 
<a name="index-unistd_002eh-3023"></a>
<!-- unistd.h -->
<!-- POSIX.2 -->

<div class="defun">
&mdash; Variable: int <b>opterr</b><var><a name="index-opterr-3024"></a></var><br>
<blockquote><p>If the value of this variable is nonzero, then <code>getopt</code> prints an
error message to the standard error stream if it encounters an unknown
option character or an option with a missing required argument.  This is
the default behavior.  If you set this variable to zero, <code>getopt</code>
does not print any messages, but it still returns the character <code>?</code>
to indicate an error. 
</p></blockquote></div>

<!-- unistd.h -->
<!-- POSIX.2 -->
<div class="defun">
&mdash; Variable: int <b>optopt</b><var><a name="index-optopt-3025"></a></var><br>
<blockquote><p>When <code>getopt</code> encounters an unknown option character or an option
with a missing required argument, it stores that option character in
this variable.  You can use this for providing your own diagnostic
messages. 
</p></blockquote></div>

<!-- unistd.h -->
<!-- POSIX.2 -->
<div class="defun">
&mdash; Variable: int <b>optind</b><var><a name="index-optind-3026"></a></var><br>
<blockquote><p>This variable is set by <code>getopt</code> to the index of the next element
of the <var>argv</var> array to be processed.  Once <code>getopt</code> has found
all of the option arguments, you can use this variable to determine
where the remaining non-option arguments begin.  The initial value of
this variable is <code>1</code>. 
</p></blockquote></div>

<!-- unistd.h -->
<!-- POSIX.2 -->
<div class="defun">
&mdash; Variable: char * <b>optarg</b><var><a name="index-optarg-3027"></a></var><br>
<blockquote><p>This variable is set by <code>getopt</code> to point at the value of the
option argument, for those options that accept arguments. 
</p></blockquote></div>

<!-- unistd.h -->
<!-- POSIX.2 -->
<div class="defun">
&mdash; Function: int <b>getopt</b> (<var>int argc, char **argv, const char *options</var>)<var><a name="index-getopt-3028"></a></var><br>
<blockquote><p>The <code>getopt</code> function gets the next option argument from the
argument list specified by the <var>argv</var> and <var>argc</var> arguments. 
Normally these values come directly from the arguments received by
<code>main</code>.

        <p>The <var>options</var> argument is a string that specifies the option
characters that are valid for this program.  An option character in this
string can be followed by a colon (&lsquo;<samp><span class="samp">:</span></samp>&rsquo;) to indicate that it takes a
required argument.  If an option character is followed by two colons
(&lsquo;<samp><span class="samp">::</span></samp>&rsquo;), its argument is optional; this is a GNU extension.

        <p><code>getopt</code> has three ways to deal with options that follow
non-options <var>argv</var> elements.  The special argument &lsquo;<samp><span class="samp">--</span></samp>&rsquo; forces
in all cases the end of option scanning.

          <ul>
<li>The default is to permute the contents of <var>argv</var> while scanning it
so that eventually all the non-options are at the end.  This allows
options to be given in any order, even with programs that were not
written to expect this.

          <li>If the <var>options</var> argument string begins with a hyphen (&lsquo;<samp><span class="samp">-</span></samp>&rsquo;), this
is treated specially.  It permits arguments that are not options to be
returned as if they were associated with option character &lsquo;<samp><span class="samp">\1</span></samp>&rsquo;.

          <li>POSIX demands the following behavior: The first non-option stops option
processing.  This mode is selected by either setting the environment
variable <code>POSIXLY_CORRECT</code> or beginning the <var>options</var> argument
string with a plus sign (&lsquo;<samp><span class="samp">+</span></samp>&rsquo;). 
</ul>

        <p>The <code>getopt</code> function returns the option character for the next
command line option.  When no more option arguments are available, it
returns <code>-1</code>.  There may still be more non-option arguments; you
must compare the external variable <code>optind</code> against the <var>argc</var>
parameter to check this.

        <p>If the option has an argument, <code>getopt</code> returns the argument by
storing it in the variable <var>optarg</var>.  You don't ordinarily need to
copy the <code>optarg</code> string, since it is a pointer into the original
<var>argv</var> array, not into a static area that might be overwritten.

        <p>If <code>getopt</code> finds an option character in <var>argv</var> that was not
included in <var>options</var>, or a missing option argument, it returns
&lsquo;<samp><span class="samp">?</span></samp>&rsquo; and sets the external variable <code>optopt</code> to the actual
option character.  If the first character of <var>options</var> is a colon
(&lsquo;<samp><span class="samp">:</span></samp>&rsquo;), then <code>getopt</code> returns &lsquo;<samp><span class="samp">:</span></samp>&rsquo; instead of &lsquo;<samp><span class="samp">?</span></samp>&rsquo; to
indicate a missing option argument.  In addition, if the external
variable <code>opterr</code> is nonzero (which is the default), <code>getopt</code>
prints an error message. 
</p></blockquote></div>

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