optstring

Optstring is a string used by the getopt family of functions in C libraries to specify which short options a program accepts and whether they require arguments. Each character in the string represents a valid option. A single colon immediately after a character indicates that the option requires an argument. A double colon after a character indicates that the option has an optional argument (this is a GNU extension and not in the original POSIX standard). A leading colon changes how errors are reported by getopt, causing it to return a different value for missing arguments.

In a typical usage, getopt parses the command line arguments in argv and, for each option found, returns the option character. If the option takes an argument, the option’s argument is placed in the global variable optarg. The index of the next element to be processed is stored in optind, and optopt may hold the option causing an error. When the parser has finished, getopt returns -1 to indicate completion. If an error occurs, the function may return '?' or ':', depending on whether the optstring starts with a colon.

Optstring is specific to short options. For programs that also accept long options, the long options are described separately using an array of struct option and the function getopt_long. The optstring still defines the short options (those that appear as single characters like -a, -b, -c), while long options are handled via the long-options table and has_arg values (no_argument, required_argument, optional_argument).

Usage of optstring is common in Unix-like systems and is part of standard behavior for many C programs, enabling compact specification of option characters and their argument requirements.