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GETOPT_LONG(3) Library Functions Manual GETOPT_LONG(3)

getopt_long, getopt_long_onlyget long options from command line argument list

#include <getopt.h>

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

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

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

The () function is similar to getopt(3) but it accepts options in two forms: words and characters. The getopt_long() function provides a superset of the functionality of getopt(3). getopt_long() can be used in two ways. In the first way, every long option understood by the program has a corresponding short option, and the option structure is only used to translate from long options to short options. When used in this fashion, getopt_long() behaves identically to getopt(3). This is a good way to add long option processing to an existing program with the minimum of rewriting.

In the second mechanism, a long option sets a flag in the option structure passed, or will store a pointer to the command line argument in the option structure passed to it for options that take arguments. Additionally, the long option's argument may be specified as a single argument with an equal sign, e.g.

$ myprogram --myoption=somevalue

When a long option is processed, the call to () will return 0. For this reason, long option processing without shortcuts is not backwards compatible with getopt(3).

It is possible to combine these methods, providing for long options processing with short option equivalents for some options. Less frequently used options would be processed as long options only.

Abbreviated long option names are accepted when () processes long options if the abbreviation is unique. An exact match is always preferred for a defined long option.

By default, () permutes argv such that all option arguments are evaluated before any non-options arguments. If the first character of optstring is a plus sign (‘+’) or if the environment variable POSIXLY_CORRECT is set, then argv is processed in order and option processing stops as soon as the first non-option argument is encountered.

The () call requires an array to be initialized describing the long options. Each element of the array is a structure:

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

The name field should contain the option name without the leading double dash.

The has_arg field should be one of:

no argument to the option is expected.
an argument to the option is required.
an argument to the option may be presented.

If flag is not NULL, then the integer pointed to by it will be set to the value in the val field. If the flag field is NULL, then the val field will be returned. Setting flag to NULL and setting val to the corresponding short option will make this function act just like getopt(3).

If the longindex argument is not NULL, then the integer pointed to by it will be set to the index of the long option relative to longopts.

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

The () function behaves identically to getopt_long() with the exception that long options may start with ‘-’ in addition to ‘--’. If an option starting with ‘-’ does not match a long option but does match a single-character option, the single-character option is returned.

If the flag field in struct option is NULL, getopt_long() and getopt_long_only() return the value specified in the val field, which is usually just the corresponding short option. If flag is not NULL, these functions return 0 and store val in the location pointed to by flag. These functions return ‘:’ if there was a missing option argument, ‘?’ if the user specified an unknown or ambiguous option, and -1 when the argument list has been exhausted.

This section describes differences to the GNU implementation found in glibc-2.1.3:

If set, option processing stops when the first non-option is found and a leading ‘+’ in the optstring is ignored.

int bflag, ch, fd;
int daggerset;

/* options descriptor */
static struct option longopts[] = {
	{ "buffy",	no_argument,		NULL, 		'b' },
	{ "fluoride",	required_argument,	NULL, 	       	'f' },
	{ "daggerset",	no_argument,		&daggerset,	1 },
	{ NULL, 	0,			NULL, 		0 }

bflag = 0;
while ((ch = getopt_long(argc, argv, "bf:", longopts, NULL)) != -1)
	switch (ch) {
	case 'b':
		bflag = 1;
	case 'f':
		if ((fd = open(optarg, O_RDONLY)) == -1)
			err(1, "unable to open %s", optarg);
	case 0:
		if (daggerset)
			fprintf(stderr, "Buffy will use her dagger to "
			    "apply fluoride to dracula's teeth\n");
argc -= optind;
argv += optind;


The getopt_long() and getopt_long_only() functions first appeared in GNU libiberty. This implementation first appeared in OpenBSD 3.3.

The argv argument is not really const as its elements may be permuted (unless POSIXLY_CORRECT is set).

September 11, 2022 OpenBSD-current