GETOPT(3) Linux Programmer's Manual GETOPT(3)
NAME
getopt, optarg, optind, opterr, optopt - Parse command-line options
SYNOPSIS
#include <unistd.h>
int getopt(int argc, char * const argv[],
const char *optstring);
extern char *optarg;
extern int optind, opterr, optopt;
#include <getopt.h>
DESCRIPTION
The getopt() function parses the command-line arguments. Its arguments
argc and argv are the argument count and array as passed to the main()
function on program invocation. An element of argv that starts with '-'
(and is not exactly "-" or "--") is an option element. The characters of
this element (aside from the initial '-') are option characters. If
getopt() is called repeatedly, it returns successively each of the option
characters from each of the option elements.
The variable optind is the index of the next element to be processed in
argv. The system initializes this value to 1. The caller can reset it
to 1 to restart scanning of the same argv, or when scanning a new
argument vector.
If getopt() finds another option character, it returns that character,
updating the external variable optind and a static variable nextchar so
that the next call to getopt() can resume the scan with the following
option character or argv-element.
If there are no more option characters, getopt() returns -1. Then optind
is the index in argv of the first argv-element that is not an option.
optstring is a string containing the legitimate option characters. If
such a character is followed by a colon, the option requires an argument,
so getopt() places a pointer to the following text in the same
argv-element, or the text of the following argv-element, in optarg. Two
colons mean an option takes an optional arg; if there is text in the
current argv-element (i.e., in the same word as the option name itself,
for example, "-oarg"), then it is returned in optarg, otherwise
optarg is set to zero. This is a GNU extension. If optstring contains W
followed by a semicolon, then -W foo is treated as the long option --foo.
(The -W option is reserved by POSIX.2 for implementation extensions.)
This behavior is a GNU extension, not available with libraries before
glibc 2.
By default, getopt() permutes the contents of argv as it scans, so that
eventually all the nonoptions are at the end. Two other modes are also
implemented. If the first character of optstring is '+' or the
environment variable POSIXLY_CORRECT is set, then option processing
stops as soon as a nonoption argument is encountered. If the first
character of optstring is '-', then each nonoption argv-element is
handled as if it were the argument of an option with character code 1.
(This is used by programs that were written to expect options and other
argv-elements in any order and that care about the ordering of the two.)
The special argument "--" forces an end of option-scanning regardless
of the scanning mode.
If getopt() does not recognize an option character, it prints an error
message to stderr, stores the character in optopt, and returns '?'. The
calling program may prevent the error message by setting opterr to 0.
If getopt() finds an option character in argv that was not included in
optstring, or if it detects a missing option argument, it returns '?' and
sets the external variable optopt to the actual option character. If the
first character (following any optional '+' or '-' described above) of
optstring is a colon (':'), then getopt() returns ':' instead of '?' to
indicate a missing option argument. If an error was detected, and the
first character of optstring is not a colon, and the external variable
opterr is nonzero (which is the default), getopt() prints an error
message.
RETURN VALUE
If an option was successfully found, then getopt() returns the option
character. If all command-line options have been parsed, then getopt()
returns -1. If getopt() encounters an option character that was not in
optstring, then '?' is returned. If getopt() encounters an option with a
missing argument, then the return value depends on the first character in
optstring: if it is ':', then ':' is returned; otherwise '?' is returned.
EXAMPLE
getopt()
The following trivial example program uses getopt() to handle two program
options: -n, with no associated value; and -t val, which expects an
associated value.
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
int main(int argc, char *argv[])
{
int flags, opt;
int nsecs, tfnd;
nsecs = 0;
tfnd = 0;
flags = 0;
while ((opt = getopt(argc, argv, "nt:")) != -1) {
switch (opt) {
case 'n':
flags = 1;
break;
case 't':
nsecs = strtol(optarg, NULL, 10);
tfnd = 1;
break;
default: /* '?' */
fprintf(stderr, "Usage: %s [-t nsecs] [-n] name\n",
argv[0]);
exit(EXIT_FAILURE);
}
}
printf("flags=%d; tfnd=%d; nsecs=%d; optind=%d\n",
flags, tfnd, nsecs, optind);
if (optind >= argc) {
fprintf(stderr, "Expected argument after options\n");
exit(EXIT_FAILURE);
}
printf("name argument = %s\n", argv[optind]);
/* Other code omitted */
exit(EXIT_SUCCESS);
}
SEE ALSO
getopt(1), getsubopt(3)
COLOPHON
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GNU 2015-08-08 GETOPT(3)