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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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GETOPT(3P) POSIX Programmer's Manual GETOPT(3P)
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.
getopt, optarg, opterr, optind, optopt — command option parsing
#include <unistd.h>
int getopt(int argc, char * const argv[], const char *optstring);
extern char *optarg;
extern int opterr, optind, optopt;
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 POSIX.1‐2017, 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[]. 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> 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,
unless the stderr stream has wide orientation, in which case the
behavior is undefined.
The getopt() function need not be thread-safe.
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.
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.
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;
}
}
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.
Applications which use wide-character output functions with stderr
should ensure that any calls to getopt() do not write to stderr,
either by setting opterr to 0 or by ensuring the first character
of optstring is always a <colon>.
While ferror(stderr) may be used to detect failures to write a
diagnostic to stderr when getopt() returns '?', the value of errno
is unspecified in such a condition. Applications desiring more
control over handling write failures should set opterr to 0 and
independently perform output to stderr, rather than relying on
getopt() to do the output.
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 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.
None.
exec(1p)
The Base Definitions volume of POSIX.1‐2017, Section 12.2, Utility
Syntax Guidelines, unistd.h(0p)
The Shell and Utilities volume of POSIX.1‐2017, getopts(1p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 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 .
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.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 GETOPT(3P)
Pages that refer to this page: stdio.h(0p), unistd.h(0p), getopts(1p), getsubopt(3p), optarg(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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