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Specifying Argp Parsers

The first argument to the argp_parse function is a pointer to a struct argp, which is known as an argp parser:

struct argp Data Type
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.
char *(*help_filter)(int key, const char *text, void *input)
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.