GETOPT(3)

NAME
getopt - get option letter from argv

SYNOPSIS
#include <stdlib.h>
extern char *optarg
extern int optind
extern int opterr
int
getopt(int argc, char * const *argv, const char *optstring);

DESCRIPTION
The getopt() function gets the next known option character from argv. 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 characters; letters and letters followed by a colon to indicate an option argument is to follow. It does not matter to getopt() if a following argument has leading white space.

On return from getopt(), 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 opterr and optind are both initialized to 1. In order to use getopt() to evaluate multiple sets of arguments, or to evaluate a single set of arguments multiple times, optind must be initialized to the number of argv entries to be skipped in each evaluation.

The getopt() function returns an EOF when the argument list is exhausted, or a non-recognized option is encountered. 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 an EOF. When all options have been processed (i.e., up to the first non-option argument), getopt() returns EOF.

DIAGNOSTICS
If the getopt() function encounters a character not found in the string optarg or detects a missing option argument it writes error message ‘’? to the stderr. Setting opterr to a zero will disable these error messages.

EXAMPLE
    extern char *optarg;
    extern int optind;
    int bflag, ch, fd;
    bflag = 0;
    while ((ch = getopt(argc, argv, "bf:")) != EOF)
    	switch(ch) {
    	case 'b':
    		bflag = 1;
    		break;
    	case 'f':
    		if ((fd = open(optarg, O_RDONLY, 0)) < 0) {
    			(void)fprintf(stderr,
    				"myname: unable to read file %s.\n", optarg);
    			exit(1) ;
    		}
    		break;
    	case '?':
    	default:
    		usage();
    }
    argc -= optind;
    argv += optind;

HISTORY
The getopt() function appeared 4.3BSD.

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

A single dash “-” may be specified as an character in optstring, however it should never 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. By default, a single dash causes getopt() to return EOF. This is, we believe, compatible with System V.

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 fairly well.

    int length;
    char *p;
    while ((c = getopt(argc, argv, "0123456789")) != EOF)
    	switch (c) {
    	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 = atoi(++p);
    		else
    			length = atoi(argv[optind] + 1);
    		break;
    	}
    }