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GETOPT(3) Library Functions Manual GETOPT(3)

getopt — get option character from command line argument list

Standard C Library (libc, -lc)

#include <unistd.h>

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

int
getopt(int argc, char * const argv[], const char *optstring);

The () function incrementally parses a command line argument list argv and returns the next known option character. An option character is known if it has been specified in the string of accepted option characters, optstring.

The option string optstring may contain the following elements: individual characters, and characters followed by a colon to indicate an option argument is to follow. If an individual character is followed by two colons, then the option argument is optional; optarg is set to the rest of the current argv word, or NULL if there were no more characters in the current word. This is a GNU extension. For example, an option string "x" recognizes an option “-x”, and an option string "x:" recognizes an option and argument “-x argument”. It does not matter to () if a following argument has leading white space.

On return from (), optarg points to an option argument, if it is anticipated, and the variable optind contains the index to the next argv argument for a subsequent call to getopt(). The variable optopt saves the last known option character returned by getopt().

The variables opterr and optind are both initialized to 1. The optind variable may be set to another value before a set of calls to () in order to skip over more or less argv entries.

In order to use () to evaluate multiple sets of arguments, or to evaluate a single set of arguments multiple times, the variable optreset must be set to 1 before the second and each additional set of calls to getopt(), and the variable optind must be reinitialized.

The () function returns -1 when the argument list is exhausted. The interpretation of options in the argument list may be cancelled by the option ‘--’ (double dash) which causes getopt() to signal the end of argument processing and return -1. When all options have been processed (i.e., up to the first non-option argument), getopt() returns -1.

The getopt() function returns the next known option character in optstring. If getopt() encounters a character not found in optstring or if it detects a missing option argument, it returns ‘?’ (question mark). If optstring has a leading ‘:’ then a missing option argument causes ‘:’ to be returned instead of ‘?’. In either case, the variable optopt is set to the character that caused the error. The getopt() function returns -1 when the argument list is exhausted.

#include <unistd.h>
int bflag, ch, fd;

bflag = 0;
while ((ch = getopt(argc, argv, "bf:")) != -1) {
	switch (ch) {
	case 'b':
		bflag = 1;
		break;
	case 'f':
		if ((fd = open(optarg, O_RDONLY, 0)) < 0) {
			(void)fprintf(stderr,
			    "myname: %s: %s\n", optarg, strerror(errno));
			exit(1);
		}
		break;
	case '?':
	default:
		usage();
	}
}
argc -= optind;
argv += optind;

If the getopt() function encounters a character not found in the string optstring or detects a missing option argument it writes an error message to the stderr and returns ‘?’. Setting opterr to a zero will disable these error messages. If optstring has a leading ‘:’ then a missing option argument causes a ‘:’ to be returned in addition to suppressing any error messages.

Option arguments are allowed to begin with “-”; this is reasonable but reduces the amount of error checking possible.

getopt(1), getopt_long(3), getsubopt(3)

The optreset variable was added to make it possible to call the getopt() function multiple times. This is an extension to the IEEE Std 1003.2 (“POSIX.2”) specification.

The getopt() function appeared in 4.3BSD.

The getopt() function was once specified to return EOF instead of -1. This was changed by IEEE Std 1003.2-1992 (“POSIX.2”) to decouple getopt() from <stdio.h>.

A single dash “-” may be specified as a character in optstring, however it should have an argument associated with it. This allows getopt() to be used with programs that expect “-” as an option flag. This practice is wrong, and should not be used in any current development. It is provided for backward compatibility only. Care should be taken not to use ‘-’ as the first character in optstring to avoid a semantic conflict with GNU getopt(), which assigns different meaning to an optstring that begins with a ‘-’. By default, a single dash causes getopt() to return -1.

It is also possible to handle digits as option letters. This allows getopt() to be used with programs that expect a number (“-3”) as an option. This practice is wrong, and should not be used in any current development. It is provided for backward compatibility only. The following code fragment works in most cases.

int ch;
long length;
char *p, *ep;

while ((ch = getopt(argc, argv, "0123456789")) != -1)
	switch (ch) {
	case '0': case '1': case '2': case '3': case '4':
	case '5': case '6': case '7': case '8': case '9':
		p = argv[optind - 1];
		if (p[0] == '-' && p[1] == ch && !p[2]) {
			length = ch - '0';
			ep = "";
		} else if (argv[optind] && argv[optind][1] == ch) {
			length = strtol((p = argv[optind] + 1),
			    &ep, 10);
			optind++;
			optreset = 1;
		} else
			usage();
		if (*ep != '\0')
			errx(EX_USAGE, "illegal number -- %s", p);
		break;
	}
June 5, 2014 macOS