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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ENVIRONMENT | ATTRIBUTES | VERSIONS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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getopt(3) Library Functions Manual getopt(3)
getopt, getopt_long, getopt_long_only, optarg, optind, opterr,
optopt - Parse command-line options
Standard C library (libc, -lc)
#include <unistd.h>
int getopt(int argc, char *argv[],
const char *optstring);
extern char *optarg;
extern int optind, opterr, optopt;
#include <getopt.h>
int getopt_long(int argc, char *argv[],
const char *optstring,
const struct option *longopts, int *longindex);
int getopt_long_only(int argc, char *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
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.
A legitimate option character is any visible one byte ascii(7)
character (for which isgraph(3) would return nonzero) that is not
'-', ':', or ';'. 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 '+' is not the first character of optstring,
it is treated as a normal option. If POSIXLY_CORRECT behaviour is
required in this case optstring will contain two '+' symbols. 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.
has_arg
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.
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.
POSIXLY_CORRECT
If this is set, then option processing stops as soon as a
nonoption argument is encountered.
For an explanation of the terms used in this section, see
attributes(7).
┌────────────────────┬───────────────┬───────────────────────────┐
│ Interface │ Attribute │ Value │
├────────────────────┼───────────────┼───────────────────────────┤
│ getopt(), │ Thread safety │ MT-Unsafe race:getopt env │
│ getopt_long(), │ │ │
│ getopt_long_only() │ │ │
└────────────────────┴───────────────┴───────────────────────────┘
POSIX specifies that the argv array argument should be const, but
these functions permute its elements unless the environment
variable POSIXLY_CORRECT is set. const is used in the actual
prototype to be compatible with other systems; however, this page
doesn't show the qualifier, to avoid confusing readers.
getopt()
POSIX.1-2008.
getopt_long()
getopt_long_only()
GNU.
The use of '+' and '-' in optstring is a GNU extension.
getopt()
POSIX.1-2001, and POSIX.2.
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>.
Very old versions of glibc were affected by a
_PID_GNU_nonoption_argv_flags_ environment variable
⟨https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=bf079e19f50d64aa5e05⟩.
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.)
Command-line arguments are parsed in strict order meaning that an
option requiring an argument will consume the next argument,
regardless of whether that argument is the correctly specified
option argument or simply the next option (in the scenario the
user mis-specifies the command line). For example, if optstring
is specified as "1n:" and the user specifies the command line
arguments incorrectly as prog -n -1, the -n option will be given
the optarg value "-1", and the -1 option will be considered to
have not been specified.
getopt()
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 <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int
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;
break;
case 't':
nsecs = atoi(optarg);
tfnd = 1;
break;
default: /* '?' */
fprintf(stderr, "Usage: %s [-t nsecs] [-n] name\n",
argv[0]);
exit(EXIT_FAILURE);
}
}
printf("flags=%d; tfnd=%d; nsecs=%d; optind=%d\n",
flags, tfnd, nsecs, optind);
if (optind >= argc) {
fprintf(stderr, "Expected argument after options\n");
exit(EXIT_FAILURE);
}
printf("name argument = %s\n", argv[optind]);
/* Other code omitted */
exit(EXIT_SUCCESS);
}
getopt_long()
The following example program illustrates the use of getopt_long()
with most of its features.
#include <getopt.h>
#include <stdio.h> /* for printf */
#include <stdlib.h> /* for exit */
int
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)
break;
switch (c) {
case 0:
printf("option %s", long_options[option_index].name);
if (optarg)
printf(" with arg %s", optarg);
printf("\n");
break;
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);
break;
case 'a':
printf("option a\n");
break;
case 'b':
printf("option b\n");
break;
case 'c':
printf("option c with value '%s'\n", optarg);
break;
case 'd':
printf("option d with value '%s'\n", optarg);
break;
case '?':
break;
default:
printf("?? getopt returned character code 0%o ??\n", c);
}
}
if (optind < argc) {
printf("non-option ARGV-elements: ");
while (optind < argc)
printf("%s ", argv[optind++]);
printf("\n");
}
exit(EXIT_SUCCESS);
}
getopt(1), getsubopt(3)
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user-space interface documentation) project. Information about
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⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
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⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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Linux man-pages 6.15 2025-05-17 getopt(3)
Pages that refer to this page: getopt(1), pipesz(1), execve(2), getsubopt(3), pmdagetoptions(3), pmgetoptions(3), rpmlua(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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