getopt(3) — Linux manual page


GETOPT(3)               Linux Programmer's Manual              GETOPT(3)

NAME         top

       getopt, getopt_long, getopt_long_only, optarg, optind, opterr,
       optopt - Parse command-line options

SYNOPSIS         top

       #include <unistd.h>

       int getopt(int argc, char *const argv[],
                  const char *optstring);

       extern char *optarg;
       extern int optind, opterr, optopt;

       #include <getopt.h>

       int getopt_long(int argc, char *const argv[],
                  const char *optstring,
                  const struct option *longopts, int *longindex);
       int getopt_long_only(int argc, char *const argv[],
                  const char *optstring,
                  const struct option *longopts, int *longindex);

   Feature Test Macro Requirements for glibc (see

           _POSIX_C_SOURCE >= 2 || _XOPEN_SOURCE

       getopt_long(), getopt_long_only():

DESCRIPTION         top

       The getopt() function parses the command-line arguments.  Its
       arguments argc and argv are the argument count and array as
       passed to the main() function on program invocation.  An element
       of argv that starts with '-' (and is not exactly "-" or "--") is
       an option element.  The characters of this element (aside from
       the initial '-') are option characters.  If getopt() is called
       repeatedly, it returns successively each of the option characters
       from each of the option elements.

       The variable optind is the index of the next element to be
       processed in argv.  The system initializes this value to 1.  The
       caller can reset it to 1 to restart scanning of the same argv, or
       when scanning a new argument vector.

       If getopt() finds another option character, it returns that
       character, updating the external variable optind and a static
       variable nextchar so that the next call to getopt() can resume
       the scan with the following option character or argv-element.

       If there are no more option characters, getopt() returns -1.
       Then optind is the index in argv of the first argv-element that
       is not an option.

       optstring is a string containing the legitimate option
       characters.  If such a character is followed by a colon, the
       option requires an argument, so getopt() places a pointer to the
       following text in the same argv-element, or the text of the
       following argv-element, in optarg.  Two colons mean an option
       takes an optional arg; if there is text in the current argv-
       element (i.e., in the same word as the option name itself, for
       example, "-oarg"), then it is returned in optarg, otherwise
       optarg is set to zero.  This is a GNU extension.  If optstring
       contains W followed by a semicolon, then -W foo is treated as the
       long option --foo.  (The -W option is reserved by POSIX.2 for
       implementation extensions.)  This behavior is a GNU extension,
       not available with libraries before glibc 2.

       By default, getopt() permutes the contents of argv as it scans,
       so that eventually all the nonoptions are at the end.  Two other
       scanning modes are also implemented.  If the first character of
       optstring is '+' or the environment variable POSIXLY_CORRECT is
       set, then option processing stops as soon as a nonoption argument
       is encountered.  If the first character of optstring is '-', then
       each nonoption argv-element is handled as if it were the argument
       of an option with character code 1.  (This is used by programs
       that were written to expect options and other argv-elements in
       any order and that care about the ordering of the two.)  The
       special argument "--" forces an end of option-scanning regardless
       of the scanning mode.

       While processing the option list, getopt() can detect two kinds
       of errors: (1) an option character that was not specified in
       optstring and (2) a missing option argument (i.e., an option at
       the end of the command line without an expected argument).  Such
       errors are handled and reported as follows:

       *  By default, getopt() prints an error message on standard
          error, places the erroneous option character in optopt, and
          returns '?' as the function result.

       *  If the caller has set the global variable opterr to zero, then
          getopt() does not print an error message.  The caller can
          determine that there was an error by testing whether the
          function return value is '?'.  (By default, opterr has a
          nonzero value.)

       *  If the first character (following any optional '+' or '-'
          described above) of optstring is a colon (':'), then getopt()
          likewise does not print an error message.  In addition, it
          returns ':' instead of '?' to indicate a missing option
          argument.  This allows the caller to distinguish the two
          different types of errors.

   getopt_long() and getopt_long_only()
       The getopt_long() function works like getopt() except that it
       also accepts long options, started with two dashes.  (If the
       program accepts only long options, then optstring should be
       specified as an empty string (""), not NULL.)  Long option names
       may be abbreviated if the abbreviation is unique or is an exact
       match for some defined option.  A long option may take a
       parameter, of the form --arg=param or --arg param.

       longopts is a pointer to the first element of an array of struct
       option declared in <getopt.h> as

           struct option {
               const char *name;
               int         has_arg;
               int        *flag;
               int         val;

       The meanings of the different fields are:

       name   is the name of the long option.

              is: no_argument (or 0) if the option does not take an
              argument; required_argument (or 1) if the option requires
              an argument; or optional_argument (or 2) if the option
              takes an optional argument.

       flag   specifies how results are returned for a long option.  If
              flag is NULL, then getopt_long() returns val.  (For
              example, the calling program may set val to the equivalent
              short option character.)  Otherwise, getopt_long() returns
              0, and flag points to a variable which is set to val if
              the option is found, but left unchanged if the option is
              not found.

       val    is the value to return, or to load into the variable
              pointed to by flag.

       The last element of the array has to be filled with zeros.

       If longindex is not NULL, it points to a variable which is set to
       the index of the long option relative to longopts.

       getopt_long_only() is like getopt_long(), but '-' as well as "--"
       can indicate a long option.  If an option that starts with '-'
       (not "--") doesn't match a long option, but does match a short
       option, it is parsed as a short option instead.

RETURN VALUE         top

       If an option was successfully found, then getopt() returns the
       option character.  If all command-line options have been parsed,
       then getopt() returns -1.  If getopt() encounters an option
       character that was not in optstring, then '?' is returned.  If
       getopt() encounters an option with a missing argument, then the
       return value depends on the first character in optstring: if it
       is ':', then ':' is returned; otherwise '?' is returned.

       getopt_long() and getopt_long_only() also return the option
       character when a short option is recognized.  For a long option,
       they return val if flag is NULL, and 0 otherwise.  Error and -1
       returns are the same as for getopt(), plus '?' for an ambiguous
       match or an extraneous parameter.

ENVIRONMENT         top

              If this is set, then option processing stops as soon as a
              nonoption argument is encountered.

              This variable was used by bash(1) 2.0 to communicate to
              glibc which arguments are the results of wildcard
              expansion and so should not be considered as options.
              This behavior was removed in bash(1) version 2.01, but the
              support remains in glibc.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface          Attribute     Value                      │
       │getopt(),          │ Thread safety │ MT-Unsafe race:getopt env  │
       │getopt_long(),     │               │                            │
       │getopt_long_only() │               │                            │

CONFORMING TO         top

              POSIX.1-2001, POSIX.1-2008, and POSIX.2, provided the
              environment variable POSIXLY_CORRECT is set.  Otherwise,
              the elements of argv aren't really const, because these
              functions permute them.  Nevertheless, const is used in
              the prototype to be compatible with other systems.

              The use of '+' and '-' in optstring is a GNU extension.

              On some older implementations, getopt() was declared in
              <stdio.h>.  SUSv1 permitted the declaration to appear in
              either <unistd.h> or <stdio.h>.  POSIX.1-1996 marked the
              use of <stdio.h> for this purpose as LEGACY.  POSIX.1-2001
              does not require the declaration to appear in <stdio.h>.

       getopt_long() and getopt_long_only():
              These functions are GNU extensions.

NOTES         top

       A program that scans multiple argument vectors, or rescans the
       same vector more than once, and wants to make use of GNU
       extensions such as '+' and '-' at the start of optstring, or
       changes the value of POSIXLY_CORRECT between scans, must
       reinitialize getopt() by resetting optind to 0, rather than the
       traditional value of 1.  (Resetting to 0 forces the invocation of
       an internal initialization routine that rechecks POSIXLY_CORRECT
       and checks for GNU extensions in optstring.)

EXAMPLES         top

       The following trivial example program uses getopt() to handle two
       program options: -n, with no associated value; and -t val, which
       expects an associated value.

       #include <unistd.h>
       #include <stdlib.h>
       #include <stdio.h>

       main(int argc, char *argv[])
           int flags, opt;
           int nsecs, tfnd;

           nsecs = 0;
           tfnd = 0;
           flags = 0;
           while ((opt = getopt(argc, argv, "nt:")) != -1) {
               switch (opt) {
               case 'n':
                   flags = 1;
               case 't':
                   nsecs = atoi(optarg);
                   tfnd = 1;
               default: /* '?' */
                   fprintf(stderr, "Usage: %s [-t nsecs] [-n] name\n",

           printf("flags=%d; tfnd=%d; nsecs=%d; optind=%d\n",
                   flags, tfnd, nsecs, optind);

           if (optind >= argc) {
               fprintf(stderr, "Expected argument after options\n");

           printf("name argument = %s\n", argv[optind]);

           /* Other code omitted */


       The following example program illustrates the use of
       getopt_long() with most of its features.

       #include <stdio.h>     /* for printf */
       #include <stdlib.h>    /* for exit */
       #include <getopt.h>

       main(int argc, char **argv)
           int c;
           int digit_optind = 0;

           while (1) {
               int this_option_optind = optind ? optind : 1;
               int option_index = 0;
               static struct option long_options[] = {
                   {"add",     required_argument, 0,  0 },
                   {"append",  no_argument,       0,  0 },
                   {"delete",  required_argument, 0,  0 },
                   {"verbose", no_argument,       0,  0 },
                   {"create",  required_argument, 0, 'c'},
                   {"file",    required_argument, 0,  0 },
                   {0,         0,                 0,  0 }

               c = getopt_long(argc, argv, "abc:d:012",
                        long_options, &option_index);
               if (c == -1)

               switch (c) {
               case 0:
                   printf("option %s", long_options[option_index].name);
                   if (optarg)
                       printf(" with arg %s", optarg);

               case '0':
               case '1':
               case '2':
                   if (digit_optind != 0 && digit_optind != this_option_optind)
                     printf("digits occur in two different argv-elements.\n");
                   digit_optind = this_option_optind;
                   printf("option %c\n", c);

               case 'a':
                   printf("option a\n");

               case 'b':
                   printf("option b\n");

               case 'c':
                   printf("option c with value '%s'\n", optarg);

               case 'd':
                   printf("option d with value '%s'\n", optarg);

               case '?':

                   printf("?? getopt returned character code 0%o ??\n", c);

           if (optind < argc) {
               printf("non-option ARGV-elements: ");
               while (optind < argc)
                   printf("%s ", argv[optind++]);


SEE ALSO         top

       getopt(1), getsubopt(3)

COLOPHON         top

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GNU                            2021-03-22                      GETOPT(3)

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