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| <h4 class="subsection">25.2.3 Parsing Long Options with <code>getopt_long</code></h4> |
| |
| <p>To accept GNU-style long options as well as single-character options, |
| use <code>getopt_long</code> instead of <code>getopt</code>. This function is |
| declared in <samp><span class="file">getopt.h</span></samp>, not <samp><span class="file">unistd.h</span></samp>. You should make every |
| program accept long options if it uses any options, for this takes |
| little extra work and helps beginners remember how to use the program. |
| |
| <!-- getopt.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Data Type: <b>struct option</b><var><a name="index-struct-option-3029"></a></var><br> |
| <blockquote><p>This structure describes a single long option name for the sake of |
| <code>getopt_long</code>. The argument <var>longopts</var> must be an array of |
| these structures, one for each long option. Terminate the array with an |
| element containing all zeros. |
| |
| <p>The <code>struct option</code> structure has these fields: |
| |
| <dl> |
| <dt><code>const char *name</code><dd>This field is the name of the option. It is a string. |
| |
| <br><dt><code>int has_arg</code><dd>This field says whether the option takes an argument. It is an integer, |
| and there are three legitimate values: <code>no_argument</code><!-- /@w -->, |
| <code>required_argument</code> and <code>optional_argument</code>. |
| |
| <br><dt><code>int *flag</code><dt><code>int val</code><dd>These fields control how to report or act on the option when it occurs. |
| |
| <p>If <code>flag</code> is a null pointer, then the <code>val</code> is a value which |
| identifies this option. Often these values are chosen to uniquely |
| identify particular long options. |
| |
| <p>If <code>flag</code> is not a null pointer, it should be the address of an |
| <code>int</code> variable which is the flag for this option. The value in |
| <code>val</code> is the value to store in the flag to indicate that the option |
| was seen. |
| </dl> |
| </p></blockquote></div> |
| |
| <!-- getopt.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>getopt_long</b> (<var>int argc, char *const *argv, const char *shortopts, const struct option *longopts, int *indexptr</var>)<var><a name="index-getopt_005flong-3030"></a></var><br> |
| <blockquote><p>Decode options from the vector <var>argv</var> (whose length is <var>argc</var>). |
| The argument <var>shortopts</var> describes the short options to accept, just as |
| it does in <code>getopt</code>. The argument <var>longopts</var> describes the long |
| options to accept (see above). |
| |
| <p>When <code>getopt_long</code> encounters a short option, it does the same |
| thing that <code>getopt</code> would do: it returns the character code for the |
| option, and stores the options argument (if it has one) in <code>optarg</code>. |
| |
| <p>When <code>getopt_long</code> encounters a long option, it takes actions based |
| on the <code>flag</code> and <code>val</code> fields of the definition of that |
| option. |
| |
| <p>If <code>flag</code> is a null pointer, then <code>getopt_long</code> returns the |
| contents of <code>val</code> to indicate which option it found. You should |
| arrange distinct values in the <code>val</code> field for options with |
| different meanings, so you can decode these values after |
| <code>getopt_long</code> returns. If the long option is equivalent to a short |
| option, you can use the short option's character code in <code>val</code>. |
| |
| <p>If <code>flag</code> is not a null pointer, that means this option should just |
| set a flag in the program. The flag is a variable of type <code>int</code> |
| that you define. Put the address of the flag in the <code>flag</code> field. |
| Put in the <code>val</code> field the value you would like this option to |
| store in the flag. In this case, <code>getopt_long</code> returns <code>0</code>. |
| |
| <p>For any long option, <code>getopt_long</code> tells you the index in the array |
| <var>longopts</var> of the options definition, by storing it into |
| <code>*</code><var>indexptr</var>. You can get the name of the option with |
| <var>longopts</var><code>[*</code><var>indexptr</var><code>].name</code>. So you can distinguish among |
| long options either by the values in their <code>val</code> fields or by their |
| indices. You can also distinguish in this way among long options that |
| set flags. |
| |
| <p>When a long option has an argument, <code>getopt_long</code> puts the argument |
| value in the variable <code>optarg</code> before returning. When the option |
| has no argument, the value in <code>optarg</code> is a null pointer. This is |
| how you can tell whether an optional argument was supplied. |
| |
| <p>When <code>getopt_long</code> has no more options to handle, it returns |
| <code>-1</code>, and leaves in the variable <code>optind</code> the index in |
| <var>argv</var> of the next remaining argument. |
| </p></blockquote></div> |
| |
| <p>Since long option names were used before the <code>getopt_long</code> |
| options was invented there are program interfaces which require programs |
| to recognize options like ‘<samp><span class="samp">-option value</span></samp>’<!-- /@w --> instead of |
| ‘<samp><span class="samp">--option value</span></samp>’<!-- /@w -->. To enable these programs to use the GNU |
| getopt functionality there is one more function available. |
| |
| <!-- getopt.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>getopt_long_only</b> (<var>int argc, char *const *argv, const char *shortopts, const struct option *longopts, int *indexptr</var>)<var><a name="index-getopt_005flong_005fonly-3031"></a></var><br> |
| <blockquote> |
| <p>The <code>getopt_long_only</code> function is equivalent to the |
| <code>getopt_long</code> function but it allows to specify the user of the |
| application to pass long options with only ‘<samp><span class="samp">-</span></samp>’ instead of |
| ‘<samp><span class="samp">--</span></samp>’. The ‘<samp><span class="samp">--</span></samp>’ prefix is still recognized but instead of |
| looking through the short options if a ‘<samp><span class="samp">-</span></samp>’ is seen it is first |
| tried whether this parameter names a long option. If not, it is parsed |
| as a short option. |
| |
| <p>Assuming <code>getopt_long_only</code> is used starting an application with |
| |
| <pre class="smallexample"> app -foo |
| </pre> |
| <p class="noindent">the <code>getopt_long_only</code> will first look for a long option named |
| ‘<samp><span class="samp">foo</span></samp>’. If this is not found, the short options ‘<samp><span class="samp">f</span></samp>’, ‘<samp><span class="samp">o</span></samp>’, |
| and again ‘<samp><span class="samp">o</span></samp>’ are recognized. |
| </p></blockquote></div> |
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