Optarg

Optarg is an external variable used by the getopt family of functions to hold the argument value of the currently processed option. When a command-line option that requires an argument is parsed, optarg is set to point to the corresponding argument string located in argv. For options that do not take an argument, optarg is not changed by the getopt functions.

The symbol is declared as extern char *optarg and is provided by the C library. It is typically declared in POSIX headers such as unistd.h, or in getopt-specific headers such as getopt.h depending on the implementation. Programs access optarg directly after a successful getopt or getopt_long call to obtain the option’s argument.

Usage with getopt: In a loop, getopt returns the option character; if that option requires an argument, optarg points to its value. The global optind indicates the index of the next argv element to be processed, helping the program detect remaining operands after option parsing. getopt_long, which processes long options, uses the same optarg variable for option arguments.

Optional arguments: Some GNU implementations support optional arguments for short options by using a double colon in the option string. In such cases optarg is set only when an argument is actually supplied; if no argument follows the option, optarg may be NULL. For options without arguments, optarg is not meaningful.

Notes: optarg is a pointer into the original argv array; it does not require manual allocation or freeing. Programs should not modify its contents, and should treat it as a C string terminated by a null character. If an option that requires an argument is missing one, getopt may return '?' and set optopt to the option character.