argp_parse function parses the arguments in argv, of
length argc, using the argp parser argp. See Argp Parsers.
A value of zero is the same as a
struct argpcontaining all
zeros. flags is a set of flag bits that modify the parsing
behavior. See Argp Flags. input is passed through to the argp
parser argp, and has meaning defined by argp. A typical
usage is to pass a pointer to a structure which is used for specifying
parameters to the parser and passing back the results.
ARGP_NO_HELP flags are included
in flags, calling
argp_parse may result in the program
exiting. This behavior is true if an error is detected, or when an
unknown option is encountered. See Program Termination.
If arg_index is non-null, the index of the first unparsed option
in argv is returned as a value.
The return value is zero for successful parsing, or an error code
(see Error Codes) if an error is detected. Different argp parsers
may return arbitrary error codes, but the standard error codes are:
ENOMEM if a memory allocation error occurred, or
an unknown option or option argument is encountered.