#### 25.3.3 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:

— 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.
> `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.