NAME
getopt, optarg, opterr, optind, optopt - command option parsing
SYNOPSIS
#include <unistd.h>
int getopt(int argc, char * const argv[], const char *optstring);
extern char *optarg;
extern int optind, opterr, optopt;
DESCRIPTION
The getopt() function is a command-line parser that shall follow
Utility Syntax Guidelines 3, 4, 5, 6, 7, 9, and 10 in the Base
Definitions volume of IEEE Std 1003.1-2001, Section 12.2, Utility
Syntax Guidelines.
The parameters argc and argv are the argument count and argument array
as passed to main() (see exec() ). The argument optstring is a string
of recognized option characters; if a character is followed by a colon,
the option takes an argument. All option characters allowed by Utility
Syntax Guideline 3 are allowed in optstring. The implementation may
accept other characters as an extension.
The variable optind is the index of the next element of the argv[]
vector to be processed. It shall be initialized to 1 by the system, and
getopt() shall update it when it finishes with each element of argv[].
When an element of argv[] contains multiple option characters, it is
unspecified how getopt() determines which options have already been
processed.
The getopt() function shall return the next option character (if one is
found) from argv that matches a character in optstring, if there is one
that matches. If the option takes an argument, getopt() shall set the
variable optarg to point to the option-argument as follows:
1. If the option was the last character in the string pointed to by an
element of argv, then optarg shall contain the next element of
argv, and optind shall be incremented by 2. If the resulting value
of optind is greater than argc, this indicates a missing option-
argument, and getopt() shall return an error indication.
2. Otherwise, optarg shall point to the string following the option
character in that element of argv, and optind shall be incremented
by 1.
If, when getopt() is called:
argv[optind] is a null pointer*
argv[optind] is not the character -
argv[optind] points to the string "-"
getopt() shall return -1 without changing optind. If:
argv[optind] points to the string "--"
getopt() shall return -1 after incrementing optind.
If getopt() encounters an option character that is not contained in
optstring, it shall return the question-mark ( ’?’ ) character. If it
detects a missing option-argument, it shall return the colon character
( ’:’ ) if the first character of optstring was a colon, or a question-
mark character ( ’?’ ) otherwise. In either case, getopt() shall set
the variable optopt to the option character that caused the error. If
the application has not set the variable opterr to 0 and the first
character of optstring is not a colon, getopt() shall also print a
diagnostic message to stderr in the format specified for the getopts
utility.
The getopt() function need not be reentrant. A function that is not
required to be reentrant is not required to be thread-safe.
RETURN VALUE
The getopt() function shall return the next option character specified
on the command line.
A colon ( ’:’ ) shall be returned if getopt() detects a missing
argument and the first character of optstring was a colon ( ’:’ ).
A question mark ( ’?’ ) shall be returned if getopt() encounters an
option character not in optstring or detects a missing argument and the
first character of optstring was not a colon ( ’:’ ).
Otherwise, getopt() shall return -1 when all command line options are
parsed.
ERRORS
No errors are defined.
The following sections are informative.
EXAMPLES
Parsing Command Line Options
The following code fragment shows how you might process the arguments
for a utility that can take the mutually-exclusive options a and b and
the options f and o, both of which require arguments:
#include <unistd.h>
int
main(int argc, char *argv[ ])
{
int c;
int bflg, aflg, errflg;
char *ifile;
char *ofile;
extern char *optarg;
extern int optind, optopt;
. . .
while ((c = getopt(argc, argv, ":abf:o:")) != -1) {
switch(c) {
case ’a’:
if (bflg)
errflg++;
else
aflg++;
break;
case ’b’:
if (aflg)
errflg++;
else {
bflg++;
bproc();
}
break;
case ’f’:
ifile = optarg;
break;
case ’o’:
ofile = optarg;
break;
case ’:’: /* -f or -o without operand */
fprintf(stderr,
"Option -%c requires an operand\n", optopt);
errflg++;
break;
case ’?’:
fprintf(stderr,
"Unrecognized option: -%c\n", optopt);
errflg++;
}
}
if (errflg) {
fprintf(stderr, "usage: . . . ");
exit(2);
}
for ( ; optind < argc; optind++) {
if (access(argv[optind], R_OK)) {
. . .
}
This code accepts any of the following as equivalent:
cmd -ao arg path path
cmd -a -o arg path path
cmd -o arg -a path path
cmd -a -o arg -- path path
cmd -a -oarg path path
cmd -aoarg path path
Checking Options and Arguments
The following example parses a set of command line options and prints
messages to standard output for each option and argument that it
encounters.
#include <unistd.h>
#include <stdio.h>
...
int c;
char *filename;
extern char *optarg;
extern int optind, optopt, opterr;
...
while ((c = getopt(argc, argv, ":abf:")) != -1) {
switch(c) {
case ’a’:
printf("a is set\n");
break;
case ’b’:
printf("b is set\n");
break;
case ’f’:
filename = optarg;
printf("filename is %s\n", filename);
break;
case ’:’:
printf("-%c without filename\n", optopt);
break;
case ’?’:
printf("unknown arg %c\n", optopt);
break;
}
}
Selecting Options from the Command Line
The following example selects the type of database routines the user
wants to use based on the Options argument.
#include <unistd.h>
#include <string.h>
...
char *Options = "hdbtl";
...
int dbtype, i;
char c;
char *st;
...
dbtype = 0;
while ((c = getopt(argc, argv, Options)) != -1) {
if ((st = strchr(Options, c)) != NULL) {
dbtype = st - Options;
break;
}
}
APPLICATION USAGE
The getopt() function is only required to support option characters
included in Utility Syntax Guideline 3. Many historical implementations
of getopt() support other characters as options. This is an allowed
extension, but applications that use extensions are not maximally
portable. Note that support for multi-byte option characters is only
possible when such characters can be represented as type int.
RATIONALE
The optopt variable represents historical practice and allows the
application to obtain the identity of the invalid option.
The description has been written to make it clear that getopt(), like
the getopts utility, deals with option-arguments whether separated from
the option by <blank>s or not. Note that the requirements on getopt()
and getopts are more stringent than the Utility Syntax Guidelines.
The getopt() function shall return -1, rather than EOF, so that
<stdio.h> is not required.
The special significance of a colon as the first character of optstring
makes getopt() consistent with the getopts utility. It allows an
application to make a distinction between a missing argument and an
incorrect option letter without having to examine the option letter. It
is true that a missing argument can only be detected in one case, but
that is a case that has to be considered.
FUTURE DIRECTIONS
None.
SEE ALSO
exec() , the Base Definitions volume of IEEE Std 1003.1-2001,
<unistd.h>, the Shell and Utilities volume of IEEE Std 1003.1-2001
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .