getopt_long(3) – get long options from command line argument list

NAME

getopt_long, getopt_long_only — get long options from command line argument list

SYNOPSIS

#include <getopt.h>

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

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);

The getopt_long() 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 getopt_long() 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 getopt_long() processes long options if the abbreviation is unique. An exact match is always preferred for a defined long option.

By default, getopt_long() 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 getopt_long() 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: no argument to the option is expected.
required_argument: an argument to the option is required.
optional_argument: 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 getopt_long_only() 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.

RETURN VALUES

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.

IMPLEMENTATION DIFFERENCES

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

handling of ‘-’ within the option string (not the first character):

GNU

treats a ‘-’ on the command line as a non-argument.

OpenBSD

a ‘-’ within the option string matches a ‘-’ (single dash) on the command line. This functionality is provided for backward compatibility with programs, such as su(1), that use ‘-’ as an option flag. This practice is wrong, and should not be used in any current development.
handling of ‘::’ in the option string in the presence of POSIXLY_CORRECT:

Both

GNU and OpenBSD ignore POSIXLY_CORRECT here and take ‘::’ to mean the preceding option takes an optional argument.
return value in case of missing argument if first character (after ‘+’ or ‘-’) in the option string is not ‘:’:

GNU

returns ‘?’

OpenBSD

returns ‘:’ (since OpenBSD's getopt(3) does).
handling of ‘--a’ in getopt(3):

GNU

parses this as option ‘-’, option ‘a’.

OpenBSD

parses this as ‘--’, and returns -1 (ignoring the ‘a’) (because the original getopt() did.)
setting of optopt for long options with flag non-NULL:

GNU

sets optopt to val.

OpenBSD

sets optopt to 0 (since val would never be returned).
handling of ‘-W’ with ‘W;’ in the option string in getopt(3) (not getopt_long()):

GNU

causes a segmentation fault.

OpenBSD

no special handling is done; ‘W;’ is interpreted as two separate options, neither of which take an argument.
setting of optarg for long options without an argument that are invoked via ‘-W’ (with ‘W;’ in the option string):

GNU

sets optarg to the option name (the argument of ‘-W’).

OpenBSD

sets optarg to NULL (the argument of the long option).
handling of ‘-W’ with an argument that is not (a prefix to) a known long option (with ‘W;’ in the option string):

GNU

returns ‘-W’ with optarg set to the unknown option.

OpenBSD

treats this as an error (unknown option) and returns ‘?’ with optopt set to 0 and optarg set to NULL (as GNU's man page documents).
The error messages are different.
OpenBSD does not permute the argument vector at the same points in the calling sequence as GNU does. The aspects normally used by the caller (ordering after -1 is returned, value of optind relative to current positions) are the same, though. (We do fewer variable swaps.)

ENVIRONMENT

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

EXAMPLES

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;
		break;
	case 'f':
		if ((fd = open(optarg, O_RDONLY)) == -1)
			err(1, "unable to open %s", optarg);
		break;
	case 0:
		if (daggerset)
			fprintf(stderr, "Buffy will use her dagger to "
			    "apply fluoride to dracula's teeth\n");
		break;
	default:
		usage();
	}
argc -= optind;
argv += optind;

HISTORY

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

BUGS

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