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25.3.9 The argp_help Function

Normally programs using argp need not be written with particular printing argument-usage-type help messages in mind as the standard ‘--help’ option is handled automatically by argp. Typical error cases can be handled using argp_usage and argp_error. See Argp Helper Functions. However, if it's desirable to print a help message in some context other than parsing the program options, argp offers the argp_help interface.

— Function: void argp_help (const struct argp *argp, FILE *stream, unsigned flags, char *name)

This outputs a help message for the argp parser argp to stream. The type of messages printed will be determined by flags.

Any options such as ‘--help’ that are implemented automatically by argp itself will not be present in the help output; for this reason it is best to use argp_state_help if calling from within an argp parser function. See Argp Helper Functions.