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#### 25.3.5 Argp Parser Functions

The function pointed to by the `parser` field in a `struct argp` (see Argp Parsers) defines what actions take place in response to each option or argument parsed. It is also used as a hook, allowing a parser to perform tasks at certain other points during parsing.

Argp parser functions have the following type signature:

         error_t parser (int key, char *arg, struct argp_state *state)

where the arguments are as follows:

key
  For each option that is parsed, parser is called with a value of key from that option's `key` field in the option vector. See Argp Option Vectors. parser is also called at other times with special reserved keys, such as `ARGP_KEY_ARG` for non-option arguments. See Argp Special Keys.
arg
  If key is an option, arg is its given value. This defaults to zero if no value is specified. Only options that have a non-zero `arg` field can ever have a value. These must always have a value unless the `OPTION_ARG_OPTIONAL` flag is specified. If the input being parsed specifies a value for an option that doesn't allow one, an error results before parser ever gets called.

  If key is `ARGP_KEY_ARG`, arg is a non-option argument. Other special keys always have a zero arg.

state
  state points to a `struct argp_state`, containing useful information about the current parsing state for use by parser. See Argp Parsing State.

When parser is called, it should perform whatever action is appropriate for key, and return `0` for success, `ARGP_ERR_UNKNOWN` if the value of key is not handled by this parser function, or a unix error code if a real error occurred. See Error Codes.

— Macro: int ARGP_ERR_UNKNOWN

> Argp parser functions should return `ARGP_ERR_UNKNOWN` for any key value they do not recognize, or for non-option arguments (key`== ARGP_KEY_ARG`) that they are not equipped to handle.

A typical parser function uses a switch statement on key:

         error_t
         parse_opt (int key, char *arg, struct argp_state *state)
         {
           switch (key)
             {
             case option_key:
               action
               break;
             ...
             default:
               return ARGP_ERR_UNKNOWN;
             }
           return 0;
         }