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 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 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.
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.
Argp parser functions should return
ARGP_ERR_UNKNOWNfor 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;
}