The option string Fa 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 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 optopt saves the last known option character returned by Fn getopt .
The variable opterr and optind are both initialized to 1. The optind variable may be set to another value before a set of calls to Fn getopt in order to skip over more or less argv entries.
In order to use Fn 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 Fn getopt , and the variable optind must be reinitialized.
The Fn getopt function returns -1 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 returns -1. When all options have been processed (i.e., up to the first non-option argument), Fn getopt returns -1.
Option arguments are allowed to begin with ``- '' this is reasonable but reduces the amount of error checking possible.
extern char *optarg;
extern int optind;
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, 0)) < 0) {
(void)fprintf(stderr,
"myname: %s: %s\n", optarg, strerror(errno));
exit(1);
}
break;
case '?':
default:
usage();
}
argc -= optind;
argv += optind;
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 -1. 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 in most cases.
int length;
char *p;
while ((c = getopt(argc, argv, "0123456789")) != -1)
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;
}
}