--- name: "getopt" section: "3" platform: "linux" package: "man-pages" version: "6.19-1" release: "Arch Linux 2026-11-01" platforms: ["linux", "freebsd", "macos"] url: "https://new.carta.tech/getopt/3" --- # getopt(3) Parse command-line options ## NAME getopt, optarg, optind, opterr, optopt - Parse command-line options ## LIBRARY Standard C library (*libc*, *-lc*) ## SYNOPSIS ``` #include ``` **int getopt(int** *argc***, char \****argv***\[\],** **const char \****optstring***);** **extern char \****optarg***;** **extern int** *optind***,** *opterr***,** *optopt***;** Feature Test Macro Requirements for glibc (see [**feature\_test\_macros**(7)](/feature_test_macros/7.md)): **getopt**(): ``` _POSIX_C_SOURCE >= 2 || _XOPEN_SOURCE ``` ## 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. A legitimate option character is any visible one byte [**ascii**(7)](/ascii/7.md) character (for which [**isgraph**(3)](/isgraph/3.md) would return nonzero) that is not '-', ':', or ';'. 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 scanning 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 '+' is not the first character of *optstring*, it is treated as a normal option. If **POSIXLY\_CORRECT** behavior is required in this case *optstring* will contain two '+' symbols. 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. While processing the option list, **getopt**() can detect two kinds of errors: (1) an option character that was not specified in *optstring* and (2) a missing option argument (i.e., an option at the end of the command line without an expected argument). Such errors are handled and reported as follows: - • By default, **getopt**() prints an error message on standard error, places the erroneous option character in *optopt*, and returns '?' as the function result. - • If the caller has set the global variable *opterr* to zero, then **getopt**() does not print an error message. The caller can determine that there was an error by testing whether the function return value is '?'. (By default, *opterr* has a nonzero value.) - • If the first character (following any optional '+' or '-' described above) of *optstring* is a colon (':'), then **getopt**() likewise does not print an error message. In addition, it returns ':' instead of '?' to indicate a missing option argument. This allows the caller to distinguish the two different types of errors. ## 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. ## ENVIRONMENT - **POSIXLY\_CORRECT** If this is set, then option processing stops as soon as a nonoption argument is encountered. ## ATTRIBUTES For an explanation of the terms used in this section, see [**attributes**(7)](/attributes/7.md). | | | | | --- | --- | --- | | **Interface** | **Attribute** | **Value** | | **getopt**() | Thread safety | MT-Unsafe race:getopt env | ## VERSIONS POSIX specifies that the *argv* array argument should be *const*, but these functions permute its elements unless the environment variable **POSIXLY\_CORRECT** is set. *const* is used in the actual prototype to be compatible with other systems; however, this page doesn't show the qualifier, to avoid confusing readers. ## STANDARDS - **getopt**() POSIX.1-2008. - The use of '+' and '-' in *optstring* is a GNU extension. ## HISTORY - **getopt**() POSIX.1-2001, and POSIX.2. On some older implementations, **getopt**() was declared in *\*. SUSv1 permitted the declaration to appear in either *\* or *\*. POSIX.1-1996 marked the use of *\* for this purpose as LEGACY. POSIX.1-2001 does not require the declaration to appear in *\*. Very old versions of glibc were affected by a **\_***PID***\_GNU\_nonoption\_argv\_flags\_** environment variable ⟨https:\\://\\:sourceware.org/\\:git/\\:?p=glibc.git;a=commitdiff;h=bf079e19f50d64aa5e05⟩. ## NOTES A program that scans multiple argument vectors, or rescans the same vector more than once, and wants to make use of GNU extensions such as '+' and '-' at the start of *optstring*, or changes the value of **POSIXLY\_CORRECT** between scans, must reinitialize **getopt**() by resetting *optind* to 0, rather than the traditional value of 1. (Resetting to 0 forces the invocation of an internal initialization routine that rechecks **POSIXLY\_CORRECT** and checks for GNU extensions in *optstring*.) Command-line arguments are parsed in strict order meaning that an option requiring an argument will consume the next argument, regardless of whether that argument is the correctly specified option argument or simply the next option (in the scenario the user mis-specifies the command line). For example, if *optstring* is specified as "1n:" and the user specifies the command line arguments incorrectly as *prog -n -1*, the *-n* option will be given the **optarg** value "-1", and the *-1* option will be considered to have not been specified. ## EXAMPLES ### 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 #include #include 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 = atoi(optarg); 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)](/getopt/1.md), [**getopt\_long**(3)](/getopt_long/3.md), [**getopt\_long\_only**(3)](/getopt_long_only/3.md), [**getsubopt**(3)](/getsubopt/3.md)