The first argument to the argp_parse function is a pointer to a
struct argp, which is known as an argp parser:
— Data Type: struct argp
This structure specifies how to parse a given set of options and
arguments, perhaps in conjunction with other argp parsers. It has the
following fields:
const struct argp_option *options
A pointer to a vector of argp_option structures specifying which
options this argp parser understands; it may be zero if there are no
options at all. See Argp Option Vectors.
argp_parser_t parser
A pointer to a function that defines actions for this parser; it is
called for each option parsed, and at other well-defined points in the
parsing process. A value of zero is the same as a pointer to a function
that always returns ARGP_ERR_UNKNOWN. See Argp Parser Functions.
const char *args_doc
If non-zero, a string describing what non-option arguments are called by
this parser. This is only used to print the `Usage:' message. If
it contains newlines, the strings separated by them are considered
alternative usage patterns and printed on separate lines. Lines after
the first are prefixed by ` or: ' instead of `Usage:'.
const char *doc
If non-zero, a string containing extra text to be printed before and
after the options in a long help message, with the two sections
separated by a vertical tab ('\v', '\013') character. By
convention, the documentation before the options is just a short string
explaining what the program does. Documentation printed after the
options describe behavior in more detail.
const struct argp_child *children
A pointer to a vector of argp_children structures. This pointer
specifies which additional argp parsers should be combined with this
one. See Argp Children.
If non-zero, a pointer to a function that filters the output of help
messages. See Argp Help Filtering.
const char *argp_domain
If non-zero, the strings used in the argp library are translated using
the domain described by this string. If zero, the current default domain
is used.
Of the above group, options, parser, args_doc, and
the doc fields are usually all that are needed. If an argp
parser is defined as an initialized C variable, only the fields used
need be specified in the initializer. The rest will default to zero due
to the way C structure initialization works. This design is exploited in
most argp structures; the most-used fields are grouped near the
beginning, the unused fields left unspecified.
