getopt

Name

getopt - parses command line options

Library

libc.lib

Synopsis

#include <unistd.h>

extern char *optarg;

extern int optind;

extern int optopt;

extern int opterr;

extern int optreset;

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

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.

Detailed 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, and characters followed by a colon to indicate an option argument is to follow. 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 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 optopt saves the last known option character returned by getopt.

The variables opterr and optind are both initialized to 1. The optind variable may be set to another value before a set of calls to getopt in order to skip over more or less argv entries.

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.

Examples

#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>
 
int main()
{
        int argc = 3;
         
        char *argv[] =
         {
                 "getopt","-f","hi"
         };
        
        int bflag, ch, fd;
        bflag = 0;
         
        while ((ch = getopt(argc, argv, "bf:")) != -1) {
        
        switch (ch) {
        case 'b':
                bflag = 1;
                printf("option is 'b' \n");
                break;
        case 'f':
                printf("option is 'f' \n");
                if ((fd = open(optarg, O_RDONLY, 0)) != 0) {
                        (void)fprintf(stderr,
                           "myname: %s: %s\n", optarg, strerror(errno));                
                }                             
                break;
        case '?':
                printf("missing option!");
        default:
                printf("unknown option!");
        }
       
}
argc -= optind;
return 0;
}

Output

option is ’f’
myname: hi: No such file or directory

Security considerations

The getopt function can be misused by a malicious user to arbitrarily change the functionality of a program under execution using the buffer overflow attack.

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 the 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.

Bugs

The getopt function was once specified to return EOF instead of -1. This was changed by -p1003.2-92 to decouple getopt from

#include <stdio.h>

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, which assigns different meaning to an optstring that begins with a ‘-.’ By default, a single dash causes getopt to return -1.

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 in most cases.

int ch;
long length;
char *p, *ep;
while ((ch = getopt(argc, argv, "0123456789")) != -1)
        switch (ch) {
        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] == ’-’ Am]Am] p[1] == ch Am]Am] !p[2]) {
                        length = ch - ’0’;
                        ep = "";
                } else if (argv[optind] Am]Am] argv[optind][1] == ch) {
                        length = strtol((p = argv[optind] + 1),
                            Am]ep, 10);
                        optind++;
                        optreset = 1;
                } else
                        usage();
                if (*ep != ’\0’)
                        errx(EX_USAGE, "illegal number -- %s", p);
                break;
        }

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