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Manual page for GETOPT(3)

getopt - get option letter from argv

SYNOPSIS

int getopt(argc, argv, optstring)
int argc;
char **argv;
char *optstring;

extern char *optarg;
extern int optind;

DESCRIPTION

Getopt returns the next option letter in argv that matches a letter in optstring. Optstring is a string of recognized option letters; if a letter is followed by a colon, the option is expected to have an argument that may or may not be separated from it by white space. Optarg is set to point to the start of the option argument on return from getopt.

Getopt places in optind the argv index of the next argument to be processed. Because optind is external, it is normally initialized to zero automatically before the first call to getopt.

When all options have been processed (i.e., up to the first non-option argument), getopt returns EOF. The special option -- may be used to delimit the end of the options; EOF will be returned, and -- will be skipped.

DIAGNOSTICS

Getopt prints an error message on stderr and returns a question mark (?) when it encounters an option letter not included in optstring.

EXAMPLE

The following code fragment shows how one might process the arguments for a command that can take the mutually exclusive options a and b, and the options f and o, both of which require arguments:

main(argc, argv)
int argc;
char **argv;
{
	int c;
	extern int optind;
	extern char *optarg;
	.
	.
	.
	while ((c = getopt(argc, argv, "abf:o:")) != EOF)
		switch (c) {
		case `a':
			if (bflg)
				errflg++;
			else
				aflg++;
			break;
		case `b':
			if (aflg)
				errflg++;
			else
				bproc();
			break;
		case `f':
			ifile = optarg;
			break;
		case `o':
			ofile = optarg;
			break;
		case `?':
		default:
			errflg++;
			break;
		}
	if (errflg) {
		fprintf(stderr, "Usage: ...");
		exit(2);
	}
	for (; optind < argc; optind++) {
		.
		.
		.
	}
	.
	.
	.
}

HISTORY

Written by Henry Spencer, working from a Bell Labs manual page. Modified by Keith Bostic to behave more like the System V version.

BUGS

It is not obvious how `-' standing alone should be treated; this version treats it as a non-option argument, which is not always right.

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

Getopt is quite flexible but the obvious price must be paid: there is much it could do that it doesn't, like checking mutually exclusive options, checking type of option arguments, etc.