NAME
getopt - get option letter from argv

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

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

DESCRIPTION
Getopt returns the next option letter in argv that matches a letter
in optstring. Optstring is a string of recognized option letters;
if a letter is followed by a colon, the option is expected to have
an argument that may or may not be separated from it by white
space. Optarg is set to point to the start of the option argument
on return from getopt.

Getopt places in optind the argv index of the next argument to be
processed. Because optind is external, it is normally initialized
to zero automatically before the first call to getopt.

When all options have been processed (i.e., up to the first non-
option argument), getopt returns EOF. The special option -- may be
used to delimit the end of the options; EOF will be returned, and
-- will be skipped.

DIAGNOSTICS
Getopt prints an error message on stderr and returns a question
mark (?) when it encounters an option letter not included in
optstring. Setting opterr to a zero will disable this error
message.

EXAMPLE
The following code fragment shows how one might process the
arguments for a command that can take the mutually exclusive
options a and b, and the options f and o, both of which require
arguments:

main(argc, argv)
int argc;
char **argv;
{
int c;
extern int optind;
extern char *optarg;
.
.
.
while ((c = getopt(argc, argv, "abf:o:")) != EOF)
switch (c) {
case ‘a’:
if (bflg)
errflg++;
else
aflg++;
break;
case ‘b’:
if (aflg)
errflg++;
else
bproc();
break;
case ‘f’:
ifile = optarg;
break;
case ‘o’:
ofile = optarg;
break;
case ‘?’:
default:
errflg++;
break;
}
if (errflg) {
fprintf(stderr, "Usage: ...");
exit(2);
}
for (; optind < argc; optind++) {
.
.
.
}
.
.
.
}

HISTORY
Written by Henry Spencer, working from a Bell Labs manual page.
Modified by Keith Bostic to behave more like the System V version.

BUGS
"-" may be specified as an option letter, however it should never
have an argument associated with it. This allows getopt to be used
with programs that think that "-" means standard input.

Option arguments are allowed to begin with "-"; this is
reasonable but reduces the amount of error checking possible.

Getopt is quite flexible but the obvious price must be paid: there
is much it could do that it doesn’t, like checking mutually
exclusive options, checking type of option arguments, etc.