mrb_mod_define_method_m(mrb_state *mrb, struct RClass *c)
{
  struct RProc *p;
  mrb_method_t m;
  mrb_sym mid;
  mrb_value proc = mrb_undef_value();
  mrb_value blk;

  mrb_get_args(mrb, "n|o&", &mid, &proc, &blk);
  switch (mrb_type(proc)) {
    case MRB_TT_PROC:
      blk = proc;
      break;
    case MRB_TT_UNDEF:
      /* ignored */
      break;
    default:
      mrb_raisef(mrb, E_TYPE_ERROR, "wrong argument type %T (expected Proc)", proc);
      break;
  }
  if (mrb_nil_p(blk)) {
    mrb_raise(mrb, E_ARGUMENT_ERROR, "no block given");
  }
  p = MRB_OBJ_ALLOC(mrb, MRB_TT_PROC, mrb->proc_class);
  mrb_proc_copy(mrb, p, mrb_proc_ptr(blk));
  p->flags |= MRB_PROC_STRICT;
  MRB_METHOD_FROM_PROC(m, p);
  mrb_define_method_raw(mrb, c, mid, m);
  mrb_method_added(mrb, c, mid);
  return mrb_symbol_value(mid);
}
A NULL pointer dereference occurs when the application dereferences a pointer that it expects to be valid, but is NULL, typically causing a crash or exit.