getopt() - get option character from command line argument list
#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)
The getopt(3) 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(3) if a following
argument has leading white space.
On return from getopt(3), 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(3). The variable optopt saves the last known option character returned by getopt(3).
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(3) in order to skip over more or less argv entries.
In order to use getopt(3) 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(3), and the variable optind must be reinitialized.
The double dash argument (--) stops option processing and is
skipped, so the argument following -- is considered
the first positional argument.
A single dash argument (-) stops option processing but isn't
skipped; optind is set so that - is the first
positional argument.
The getopt(3) function returns the next option character specified on the command line, and handles the following special cases:
It returns a colon (:) if there is a missing argument and the first character of optstring was a colon.
It returns a question mark (?) if it encounters an option character not in the optstring or there is a missing argument and the first character of optstring was not a colon.
Otherwise, getopt(3) returns -1 when all command line options have been parsed.
The interpretation of options in the argument list may be
cancelled by the option -- (double dash) which causes
getopt(3) 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(3) returns -1.
If the getopt(3) function encounters a character not
found in the string optarg or detects a missing option
argument it writes an error message and returns ? to
the stderr. 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.
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 optstring, however it should never have an argument associated with it. This allows getopt(3) 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 getopt(3) to return -1.
It is also possible to handle digits as option letters. This allows getopt(3) 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;
}
}