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Argp provides a number of functions available to the user of argp (see Argp Parser Functions), mostly for producing error messages. These take as their first argument the state argument to the parser function. See Argp Parsing State.

— Function: void argp_usage (const struct argp_state *state)

Outputs the standard usage message for the argp parser referred to by state to state->err_stream and terminate the program with exit (argp_err_exit_status). See Argp Global Variables.

— Function: void argp_error (const struct argp_state *state, const char *fmt, ...)

Prints the printf format string fmt and following args, preceded by the program name and `:', and followed by a `Try ... --help' message, and terminates the program with an exit status of argp_err_exit_status. See Argp Global Variables.

— Function: void argp_failure (const struct argp_state *state, int status, int errnum, const char *fmt, ...)

Similar to the standard gnu error-reporting function error, this prints the program name and `:', the printf format string fmt, and the appropriate following args. If it is non-zero, the standard unix error text for errnum is printed. If status is non-zero, it terminates the program with that value as its exit status.

The difference between argp_failure and argp_error is that argp_error is for parsing errors, whereas argp_failure is for other problems that occur during parsing but don't reflect a syntactic problem with the input, such as illegal values for options, bad phase of the moon, etc.

— Function: void argp_state_help (const struct argp_state *state, FILE *stream, unsigned flags)

Outputs a help message for the argp parser referred to by state, to stream. The flags argument determines what sort of help message is produced. See Argp Help Flags.

Error output is sent to state->err_stream, and the program name printed is state->name.

The output or program termination behavior of these functions may be suppressed if the ARGP_NO_EXIT or ARGP_NO_ERRS flags are passed to argp_parse. See Argp Flags.

This behavior is useful if an argp parser is exported for use by other programs (e.g., by a library), and may be used in a context where it is not desirable to terminate the program in response to parsing errors. In argp parsers intended for such general use, and for the case where the program doesn't terminate, calls to any of these functions should be followed by code that returns the appropriate error code:

     if (bad argument syntax)
          argp_usage (state);
          return EINVAL;

If a parser function will only be used when ARGP_NO_EXIT is not set, the return may be omitted.