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. 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 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 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.

If getopt () does not recognize an option character, it prints an error message to stderr , stores the character in optopt , and returns '?'. The calling program may prevent the error message by setting opterr to 0.

If getopt () finds an option character in argv that was not included in optstring , or if it detects a missing option argument, it returns '?' and sets the external variable optopt to the actual option character. If the first character (following any optional '+' or '-' described above) of optstring is a colon (':'), then getopt () returns ':' instead of '?' to indicate a missing option argument. If an error was detected, and the first character of optstring is not a colon, and the external variable opterr is nonzero (which is the default), getopt () prints an error message.

getopt_long() and getopt_long_only()

longopts is a pointer to the first element of an array of struct option declared in < getopt.h > as

struct option {
    const char *name;
    int         has_arg;
    int        *flag;
    int         val;
The meanings of the different fields are:
is the name of the long option.
has_arg

If longindex is not NULL, it points to a variable which is set to the index of the long option relative to longopts .

getopt_long_only () is like getopt_long (), but '-' as well as "--" can indicate a long option. If an option that starts with '-' (not "--") doesn't match a long option, but does match a short option, it is parsed as a short option instead.

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.

getopt_long () and getopt_long_only () also return the option character when a short option is recognized. For a long option, they return val if flag is NULL, and 0 otherwise. Error and -1 returns are the same as for getopt (), plus '?' for an ambiguous match or an extraneous parameter.

Environment

POSIXLY_CORRECT