NAME
getopt —
get option character from command line
argument list
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);
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 variable 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, 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 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),
getopt() returns -1.
DIAGNOSTICS
If the getopt() 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.
EXTENSIONS
The optreset variable was added to make it
possible to call the
getopt()
function multiple times. This is an extension to the IEEE
Std 1003.2 (“POSIX.2”) specification.
EXAMPLE
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;
HISTORY
The getopt() function appeared
4.3BSD.
BUGS
The getopt() function was once specified
to return EOF instead of -1. This was changed by
IEEE Std 1003.2-1992 (“POSIX.2”) to
decouple getopt() from
<stdio.h>.
A single dash “-”
may be specified as an 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. By default, a single dash
causes getopt() to return -1. This is, we believe,
compatible with System V.
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 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;
}
}