Argp Parser Functions

The function pointed to by the parser field in a struct argp (see section Specifying Argp Parsers) defines what actions take place in response to each option or argument that is parsed, and is also used as a hook, to allow a parser to do something 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 section Specifying Options in an Argp Parser). parser is also called at other times with special reserved keys, such as ARGP_KEY_ARG for non-option arguments. See section Special Keys for Argp Parser Functions.

arg

If key is an option, arg is the value given for it, or zero if no value was specified. Only options that have a non-zero arg field can ever have a value, and those must always have a value, unless the OPTION_ARG_OPTIONAL flag was 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 section Argp Parsing State.

When parser is called, it should perform whatever action is appropriate for key, and return either 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 section 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 do not wish 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;
}

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