Option Parsing

struct getopt_state

Saved state across getopt() calls


struct getopt_state {
  int index;
  int opt;
  char *arg;


Index of the next unparsed argument of argv. If getopt() has parsed all of argv, then index will equal argc.
Option being parsed when an error occurs. opt is only valid when getopt() returns ? or :.
The argument to an option, NULL if there is none. arg is only valid when getopt() returns an option character.
void getopt_init_state(struct getopt_state * gs)

Initialize a struct getopt_state


struct getopt_state * gs
The state to initialize


This must be called before using gs with getopt().

int getopt(struct getopt_state * gs, int argc, char *const argv, const char * optstring)

Parse short command-line options


struct getopt_state * gs
Internal state and out-of-band return arguments. This must be initialized with getopt_init_context() beforehand.
int argc
Number of arguments, not including the NULL terminator
char *const argv
Argument list, terminated by NULL
const char * optstring
Option specification, as described below


getopt() parses short options. Short options are single characters. They may be followed by a required argument or an optional argument. Arguments to options may occur in the same argument as an option (like -larg), or in the following argument (like -l arg). An argument containing options begins with a -. If an option expects no arguments, then it may be immediately followed by another option (like ls -alR).

optstring is a list of accepted options. If an option is followed by : in optstring, then it expects a mandatory argument. If an option is followed by :: in optstring, it expects an optional argument. gs.arg points to the argument, if one is parsed.

getopt() stops parsing options when it encounters the first non-option argument, when it encounters the argument --, or when it runs out of arguments. For example, in ls -l foo -R, option parsing will stop when getopt() encounters foo, if l does not expect an argument. However, the whole list of arguments would be parsed if l expects an argument.

An example invocation of getopt() might look like:

char *argv[] = { "program", "-cbx", "-a", "foo", "bar", 0 };
int opt, argc = ARRAY_SIZE(argv) - 1;
struct getopt_state gs;

while ((opt = getopt(&gs, argc, argv, "a::b:c")) != -1)
    printf("opt = %c, index = %d, arg = \"%s\"\n", opt, gs.index, gs.arg);
printf("%d argument(s) left\n", argc - gs.index);

and would produce an output of:

opt = c, index = 1, arg = "<NULL>"
opt = b, index = 2, arg = "x"
opt = a, index = 4, arg = "foo"
1 argument(s) left

For further information, refer to the getopt(3) man page.


  • An option character if an option is found. gs.arg is set to the argument if there is one, otherwise it is set to NULL.
  • -1 if there are no more options, if a non-option argument is encountered, or if an -- argument is encountered.
  • '?' if we encounter an option not in optstring. gs.opt is set to the unknown option.
  • ':' if an argument is required, but no argument follows the option. gs.opt is set to the option missing its argument.

gs.index is always set to the index of the next unparsed argument in argv.

int getopt_silent(struct getopt_state * gs, int argc, char *const argv, const char * optstring)

Parse short command-line options silently


struct getopt_state * gs
int argc
Argument count
char *const argv
Argument list
const char * optstring
Option specification


Same as getopt(), except no error messages are printed.