getopt(3) Library Functions Manual getopt(3)
NAME
getopt, getopt_long, getopt_long_only, optarg, optind, opterr, optopt -
Parse command-line options
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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
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 charac-
ters of this element (aside from the initial '-') are option charac-
ters. 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 argu-
ment 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 op-
targ. 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, oth-
erwise 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 li-
braries 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 con-
tain 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 er-
rors: (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 com-
mand 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 ac-
cepts 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 argu-
ment.
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 char-
acter.) 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
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 charac-
ter 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 parame-
ter.
ENVIRONMENT
POSIXLY_CORRECT
If this is set, then option processing stops as soon as a nonop-
tion argument is encountered.
_<PID>_GNU_nonoption_argv_flags_
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) 2.01, but the support remains in glibc.
ATTRIBUTES
For an explanation of the terms used in this section, see at-
tributes(7).
┌───────────────────┬───────────────┬──────────────────────────────────┐
│Interface │ Attribute │ Value │
├───────────────────┼───────────────┼──────────────────────────────────┤
│getopt(), │ Thread safety │ MT-Unsafe race:getopt env │
│getopt_long(), │ │ │
│getopt_long_only() │ │ │
└───────────────────┴───────────────┴──────────────────────────────────┘
STANDARDS
getopt():
POSIX.1-2001, POSIX.1-2008, and POSIX.2, provided the environ-
ment 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 com-
patible 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 re-
quire the declaration to appear in <stdio.h>.
getopt_long() and getopt_long_only():
These functions are GNU extensions.
NOTES
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 POSIX-
LY_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 op-
tion 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 com-
mand 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.
EXAMPLES
getopt()
The following trivial example program uses getopt() to handle two pro-
gram 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);
}
SEE ALSO
getopt(1), getsubopt(3)
Linux man-pages 6.03 2023-02-05 getopt(3)
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