##### 25.3.5.1 Special Keys for Argp Parser Functions

In addition to key values corresponding to user options, the key argument to argp parser functions may have a number of other special values. In the following example arg and state refer to parser function arguments. See Argp Parser Functions.

`ARGP_KEY_ARG`
  This is not an option at all, but rather a command line argument, whose value is pointed to by arg.

  When there are multiple parser functions in play due to argp parsers being combined, it's impossible to know which one will handle a specific argument. Each is called until one returns 0 or an error other than `ARGP_ERR_UNKNOWN`; if an argument is not handled, `argp_parse` immediately returns success, without parsing any more arguments.

  Once a parser function returns success for this key, that fact is recorded, and the `ARGP_KEY_NO_ARGS` case won't be used. However, if while processing the argument a parser function decrements the `next` field of its state argument, the option won't be considered processed; this is to allow you to actually modify the argument, perhaps into an option, and have it processed again.

`ARGP_KEY_ARGS`
  If a parser function returns `ARGP_ERR_UNKNOWN` for `ARGP_KEY_ARG`, it is immediately called again with the key `ARGP_KEY_ARGS`, which has a similar meaning, but is slightly more convenient for consuming all remaining arguments. arg is 0, and the tail of the argument vector may be found at state`->argv +` state`->next`. If success is returned for this key, and state`->next` is unchanged, all remaining arguments are considered to have been consumed. Otherwise, the amount by which state`->next` has been adjusted indicates how many were used. Here's an example that uses both, for different args:

                ...
                case ARGP_KEY_ARG:
                  if (state->arg_num == 0)
                    /* First argument */
                    first_arg = arg;
                  else
                    /* Let the next case parse it.  */
                    return ARGP_KEY_UNKNOWN;
                  break;
                case ARGP_KEY_ARGS:
                  remaining_args = state->argv + state->next;
                  num_remaining_args = state->argc - state->next;
                  break;

`ARGP_KEY_END`
  This indicates that there are no more command line arguments. Parser functions are called in a different order, children first. This allows each parser to clean up its state for the parent.
`ARGP_KEY_NO_ARGS`
  Because it's common to do some special processing if there aren't any non-option args, parser functions are called with this key if they didn't successfully process any non-option arguments. This is called just before `ARGP_KEY_END`, where more general validity checks on previously parsed arguments take place.
`ARGP_KEY_INIT`
  This is passed in before any parsing is done. Afterwards, the values of each element of the `child_input` field of state, if any, are copied to each child's state to be the initial value of the `input` when their parsers are called.
`ARGP_KEY_SUCCESS`
  Passed in when parsing has successfully been completed, even if arguments remain.
`ARGP_KEY_ERROR`
  Passed in if an error has occurred and parsing is terminated. In this case a call with a key of `ARGP_KEY_SUCCESS` is never made.
`ARGP_KEY_FINI`
  The final key ever seen by any parser, even after `ARGP_KEY_SUCCESS` and `ARGP_KEY_ERROR`. Any resources allocated by `ARGP_KEY_INIT` may be freed here. At times, certain resources allocated are to be returned to the caller after a successful parse. In that case, those particular resources can be freed in the `ARGP_KEY_ERROR` case.

In all cases, `ARGP_KEY_INIT` is the first key seen by parser functions, and `ARGP_KEY_FINI` the last, unless an error was returned by the parser for `ARGP_KEY_INIT`. Other keys can occur in one the following orders. opt refers to an arbitrary option key:

opt... `ARGP_KEY_NO_ARGS` `ARGP_KEY_END` `ARGP_KEY_SUCCESS`
  The arguments being parsed did not contain any non-option arguments.
( opt | `ARGP_KEY_ARG` )... `ARGP_KEY_END` `ARGP_KEY_SUCCESS`
  All non-option arguments were successfully handled by a parser function. There may be multiple parser functions if multiple argp parsers were combined.
( opt | `ARGP_KEY_ARG` )... `ARGP_KEY_SUCCESS`
  Some non-option argument went unrecognized.

  This occurs when every parser function returns `ARGP_KEY_UNKNOWN` for an argument, in which case parsing stops at that argument if arg_index is a null pointer. Otherwise an error occurs.

In all cases, if a non-null value for arg_index gets passed to `argp_parse`, the index of the first unparsed command-line argument is passed back in that value.

If an error occurs and is either detected by argp or because a parser function returned an error value, each parser is called with `ARGP_KEY_ERROR`. No further calls are made, except the final call with `ARGP_KEY_FINI`.