StringIO#getbyte

static VALUE
strio_getbyte(VALUE self)
{
    struct StringIO *ptr = readable(self);
    int c;
    if (NIL_P(ptr->string) || ptr->pos >= RSTRING_LEN(ptr->string)) {
        return Qnil;
    }
    c = RSTRING_PTR(ptr->string)[ptr->pos++];
    return CHR2FIX(c);
}