static PL_option_t mypred_options[] =
{ PL_OPTION("quoted",   OPT_BOOL),
  PL_OPTION("length",   OPT_SIZE),
  PL_OPTION("callback", OPT_TERM),
  PL_OPTIONS_END
};

static foreign_t
mypred(term_t a1, term_t options)
{ int    quoted   = FALSE;
  size_t length   = 10;
  term_t callback = 0;

  if ( !PL_scan_options(options, 0, "mypred_options", mypred_options,
                        &quoted, &length, &callback) )
    return FALSE;

  <implement mypred>
}

The only defined value for flags is currently OPT_ALL, which causes this function to raise a domain error if an option is passed that is not in specs. Default in SWI-Prolog is to silently ignore unknown options, unless the Prolog flag iso is true. The opttype argument defines the type (group) of the options, e.g., "write_option". Option types are defined by the ISO standard. SWI-Prolog only uses this if OPT_ALL is specified, to raise a domain_error of the indicated type if some option is unused. The type name is normally the name of the predicate followed by _option or the name of a representative of a group of predicates to which the options apply.

Defined option types and their corresponding pointer type are described below.

OPT_BOOL int

Convert the option value to a bool. This converts the values described by