getopt_long(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 feature_test_macros(7)):

       getopt(): _POSIX_C_SOURCE >= 2 || _XOPEN_SOURCE
       getopt_long(), getopt_long_only(): _GNU_SOURCE

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

       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

       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

   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 argu‐
              ment; required_argument (or 1) if the option requires an argu‐
              ment; or optional_argument (or 2) if the option takes an op‐
              tional 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

ATTRIBUTES         top

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

       │Interface                Attribute     Value                     │
       │getopt(), getopt_long(), │ Thread safety │ MT-Unsafe race:getopt env │
       │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                              2020-06-09                        GETOPT(3)

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