getopt - Man Page

command option parsing

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.

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 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:

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.
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.

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

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 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.

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 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.

Copyright

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 .

Referenced By

getopts(1p), getsubopt(3p), stdio.h(0p), unistd.h(0p).

2017 IEEE/The Open Group POSIX Programmer's Manual