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:
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.
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 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: