GETOPT

Section: C Library Functions (3)
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BSD mandoc
BSD 4.3  

NAME

getopt - get option letter from argv  

SYNOPSIS

Fd #include <stdlib.h> Vt extern char *optarg Vt extern int optind Vt extern int opterr Ft int Fn getopt int argc char * const *argv const char *optstring  

DESCRIPTION

The Fn getopt function gets the next known option character from Fa argv . An option character is known if it has been specified in the string of accepted option characters, Fa optstring .

The option string Fa 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 Fn getopt if a following argument has leading white space.

On return from Fn getopt , optarg points to an option argument, if it is anticipated, and the variable optind contains the index to the next Fa argv argument for a subsequent call to Fn getopt .

The variable opterr and optind are both initialized to 1. In order to use Fn 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 Fn 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 Fn 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), Fn getopt returns EOF  

DIAGNOSTICS

If the Fn 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 Fn getopt function appeared BSD 4.3  

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 Fa optstring , however it should never have an argument associated with it. This allows Fn 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 Fn getopt to return EOF This is, we believe, compatible with System V.

It is also possible to handle digits as option letters. This allows Fn 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;
        }
}


 

Index

NAME
SYNOPSIS
DESCRIPTION
DIAGNOSTICS
EXAMPLE
HISTORY
BUGS

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Time: 06:42:59 GMT, May 19, 2025