#### 25.3.4 Specifying Options in an Argp Parser

The `options` field in a `struct argp` points to a vector of `struct argp_option` structures, each of which specifies an option that the argp parser supports. Multiple entries may be used for a single option provided it has multiple names. This should be terminated by an entry with zero in all fields. Note that when using an initialized C array for options, writing `{ 0 }` is enough to achieve this.

— Data Type: struct argp_option

> This structure specifies a single option that an argp parser understands, as well as how to parse and document that option. It has the following fields:

> `const char *name`
>   The long name for this option, corresponding to the long option ‘`--name`’; this field may be zero if this option only has a short name. To specify multiple names for an option, additional entries may follow this one, with the `OPTION_ALIAS` flag set. See Argp Option Flags.
> `int key`
>   The integer key provided by the current option to the option parser. If key has a value that is a printable ascii character (i.e., `isascii (`key`)` is true), it also specifies a short option ‘`-char`’, where char is the ascii character with the code key.
> `const char *arg`
>   If non-zero, this is the name of an argument associated with this option, which must be provided (e.g., with the ‘`--name=value`’ or ‘`-char value`’ syntaxes), unless the `OPTION_ARG_OPTIONAL` flag (see Argp Option Flags) is set, in which case it may be provided.
> `int flags`
>   Flags associated with this option, some of which are referred to above. See Argp Option Flags.
> `const char *doc`
>   A documentation string for this option, for printing in help messages.

>   If both the `name` and `key` fields are zero, this string will be printed tabbed left from the normal option column, making it useful as a group header. This will be the first thing printed in its group. In this usage, it's conventional to end the string with a ‘`:`’ character.

> `int group`
>   Group identity for this option.

>   In a long help message, options are sorted alphabetically within each group, and the groups presented in the order 0, 1, 2, ..., n, −m, ..., −2, −1.

>   Every entry in an options array with this field 0 will inherit the group number of the previous entry, or zero if it's the first one. If it's a group header with `name` and `key` fields both zero, the previous entry + 1 is the default. Automagic options such as ‘`--help`’ are put into group −1.

>   Note that because of C structure initialization rules, this field often need not be specified, because 0 is the correct value.