GETOPT(3P) POSIX Programmer's Manual GETOPT(3P)
PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
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 opterr, optind, optopt;
DESCRIPTION
The getopt() function is a command-line parser that shall follow Util-
ity Syntax Guidelines 3, 4, 5, 6, 7, 9, and 10 in the Base Definitions
volume of POSIX.1-2008, 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 implementa-
tion may accept other characters as an extension.
The variable optind is the index of the next element of the argv[] vec-
tor 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[].
If the application sets optind to zero before calling getopt(), the
behavior is unspecified. 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> charac-
ter (':') 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 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 argu-
ment 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
If the application has not set the variable opterr to 0, the first
character of optstring is not a <colon>, and a write error occurs while
getopt() is printing a diagnostic message to stderr, then the error
indicator for stderr shall be set; but getopt() shall still succeed and
the value of errno after getopt() is unspecified.
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 <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int
main(int argc, char *argv[ ])
{
int c;
int bflg = 0, aflg = 0, errflg = 0;
char *ifile;
char *ofile;
. . .
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++;
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
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>
...
const char *Options = "hdbtl";
...
int dbtype, 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 por-
table. Note that support for multi-byte option characters is only pos-
sible when such characters can be represented as type int.
While ferror(stderr) may be used to detect failures to write a diagnos-
tic to stderr when getopt() returns '?', the value of errno is unspeci-
fied in such a condition. Applications desiring more control over han-
dling write failures should set opterr to 0 and independently perform
output to stderr, rather than relying on getopt() to do the output.
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> characters or not. Note that the requirements on
getopt() and getopts are more stringent than the Utility Syntax Guide-
lines.
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 opt-
string 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 POSIX.1-2008, Section 12.2, Utility Syn-
tax Guidelines, <unistd.h>
The Shell and Utilities volume of POSIX.1-2008, getopts
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.ker-
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 GETOPT(3P)
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