IO::generic_readable#readpartial

static VALUE
strio_sysread(int argc, VALUE *argv, VALUE self)
{
    VALUE val = rb_funcallv_kw(self, rb_intern("read"), argc, argv, RB_PASS_CALLED_KEYWORDS);
    if (NIL_P(val)) {
        rb_eof_error();
    }
    return val;
}