getopt(3C)

getopt: getopt, optarg, optind, opterr, optopt -- get option letter from argument vector

Synopsis

   #include <unistd.h>
   
   int getopt (int argc, char *const *argv, const char *optstring);
   

   extern char *optarg;
   

   extern int optind, opterr, optopt;

Description

optarg- pointer to start of option argument

optind- index of the next option argument

opterr- disable getopt error message

optopt- value of character that caused getopt error

getopt returns the next option letter in argv that matches a letter in optstring. It supports all the rules of the command syntax standard [see intro(1)]. Since all new commands are intended to adhere to the command syntax standard, they should use getopts(1), getopt(3C), or getsubopt(3C) to parse positional parameters and check for options that are legal for that command.

optstring must contain the option letters that the command using getopt will recognize. If a letter is followed by a colon, the option is expected to have an argument, or group of arguments, which may 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. optind is external and is initialized to 1 before the first call to getopt. When all options have been processed (that is, up to the first non-option argument), getopt returns EOF. The special option ``--'' (two hyphens) may be used to delimit the end of the options; when it is encountered, EOF is returned and ``--'' is skipped. This is useful in delimiting non-option arguments that begin with ``-'' (hyphen).

Files

/usr/lib/locale/locale/LC_MESSAGES/uxlibc: language-specific message file [See LANG on environ(5).]

Errors

getopt prints an error message on the standard error and returns a ``?'' (question mark) when it encounters an option letter not included in optstring or no argument after an option that expects one. This error message may be disabled by setting opterr to 0. The message is printed in the standard error format. The value of the character that caused the error is in optopt.

The label defined by a call to setlabel(3C) will be used if available; otherwise the name of the utility (argv[0]) will be used.

Usage

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 option o, which requires an argument:

   #include <unistd.h>
   #include <stdio.h>
   
   main (int argc, char argv)
   {
   	int c;
   	extern char optarg;
   	extern int optind;
   	int aflg = 0;
   	int bflg = 0;
   	int errflg = 0;
   	char ofile = NULL;
   

   	while ((c = getopt(argc, argv, "abo:")) != EOF)
   		switch (c) {
   		case 'a':
   			if (bflg)
   				errflg++;
   			else
   				aflg++;
   			break;
   		case 'b':
   			if (aflg)
   				errflg++;
   			else
   				bflg++;
   			break;
   		case 'o':
   			ofile = optarg;
   			(void)printf("ofile = %s\n", ofile);
   			break;
   		case '?':
   			errflg++;
   		}
   	if (errflg) {
   		(void)fprintf(stderr,
   			"usage: cmd [-a|-b] [-o<file>] files...\n");
   		exit (2);
   	}
   	for ( ; optind < argc; optind++)
   		(void)printf("%s\n", argv[optind]);
   	return 0;
   }

References

getopts(1), getsubopt(3C), intro(1), pfmt(3C), setlabel(3C)

Notices

The library routine getopt does not fully check for mandatory arguments. That is, given an option string a:b and the input -a -b, getopt assumes that -b is the mandatory argument to the option -a and not that -a is missing a mandatory argument.

It is a violation of the command syntax standard [see intro(1)] for options with arguments to be grouped with other options, as in cmd -aboxxx file, where a and b are options, o is an option that requires an argument, and xxx is the argument to o. Although this syntax is permitted in the current implementation, it should not be used because it may not be supported in future releases. The correct syntax is cmd -ab -o xxx file.