mini-readline.c

typedef char *rl_compentry_func_t PARAMS((const char *, int));
READLINE_DLL_IMPEXP rl_compentry_func_t *rl_completion_entry_function;

READLINE_DLL_IMPEXP char**
rl_completion_matches (text, entry_function)
     const char *text;
     rl_compentry_func_t *entry_function;
{
    ....snip...

    while (string = (*entry_function)(text, matches)) /* Need NULL return */
    {
        ....
    }

    ....snip...
}

test-callback-readline.rb

module Readline
  extend FFI::Library
  ffi_lib 'readline'

  callback :compentry_function, [:pointer, :int], :pointer
  attach_function :completion_matches, :rl_completion_matches, [:string, :compentry_function], :pointer
end

Readline.completion_matches(text) do |str_p, stat|
  # return NULL
  # nil convert NULL pointer
  nil
end