GETOPT(3) | Library Functions Manual | GETOPT(3)

# NAME

`getopt` — get option character from command line argument list

# SYNOPSIS

`#include <unistd.h>`

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

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

# DESCRIPTION

The `getopt`() 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, characters followed by a colon, and characters followed by two colons. A character followed by a single colon indicates that an argument is to follow the option on the command line. Two colons indicates that the argument is optional - this is an extension not covered by POSIX. 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 `getopt`() if a following argument has leading whitespace; except in the case where the argument is optional, denoted with two colons, no leading whitespace is permitted.

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 variables opterr and optind are both initialized to 1. The optind variable may be set to another value larger than 0 before a set of calls to `getopt`() in order to skip over more or less argv entries. An optind value of 0 is reserved for compatibility with GNU `getopt`().

In order to use `getopt`() 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 `getopt`() 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.

# RETURN VALUES

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.

# EXAMPLES

The following code accepts the options `-b` and `-f` argument and adjusts argc and argv after option argument processing has completed.

    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)) == -1)
    			err(1, "%s", optarg);
    		break;
    	default:
    		usage();
    	}
    }
    argc -= optind;
    argv += optind;

# DIAGNOSTICS

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 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.

# SEE ALSO

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

# STANDARDS

The `getopt`() function implements a superset of the functionality specified by IEEE Std 1003.1 (“POSIX.1”).

The following extensions are supported:

- The optreset variable was added to make it possible to call the `getopt`() function multiple times.
- If the optind variable is set to 0, `getopt`() will behave as if the optreset variable has been set. This is for compatibility with GNU `getopt`(). New code should use optreset instead.
- If the first character of optstring is a plus sign (‘`+`’), it will be ignored. This is for compatibility with GNU `getopt`().
- If the first character of optstring is a dash (‘`-`’), non-options will be returned as arguments to the option character ‘`\1`’. This is for compatibility with GNU `getopt`().
- A single dash (‘`-`’) may be specified as a 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. Care should be taken not to use ‘`-`’ as the first character in optstring to avoid a semantic conflict with GNU `getopt`() semantics (see above). By default, a single dash causes `getopt`() to return -1.

Historic BSD versions of `getopt`() set optopt to the last option character processed. However, this conflicts with IEEE Std 1003.1 (“POSIX.1”) which stipulates that optopt be set to the last character that caused an error.

# HISTORY

The `getopt`() function first appeared in AT&T System III UNIX and was reimplemented for 4.3BSD.

# BUGS

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>`.

It is 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 and can handle mixed number and letter arguments.

    int aflag = 0, bflag = 0, ch, lastch = '\0';
    int length = -1, newarg = 1, prevoptind = 1;

    while ((ch = getopt(argc, argv, "0123456789ab")) != -1) {
    	switch (ch) {
    	case '0': case '1': case '2': case '3': case '4':
    	case '5': case '6': case '7': case '8': case '9':
    		if (newarg || !isdigit(lastch))
    			length = 0;
    		else if (length > INT_MAX / 10)
    			usage();
    		length = (length * 10) + (ch - '0');
    		break;
    	case 'a':
    		aflag = 1;
    		break;
    	case 'b':
    		bflag = 1;
    		break;
    	default:
    		usage();
    	}
    	lastch = ch;
    	newarg = optind != prevoptind;
    	prevoptind = optind;
    }

July 25, 2022 | Debian