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
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.
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
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.