ungetbyte

static VALUE
strio_ungetbyte(VALUE self, VALUE c)
{
    struct StringIO *ptr = readable(StringIO(self));
    char buf[1], *cp = buf;
    long pos = ptr->pos, cl = 1;
    VALUE str = ptr->string;

    if (NIL_P(c)) return Qnil;
    if (FIXNUM_P(c)) {
        buf[0] = (char)FIX2INT(c);
    }
    else {
        SafeStringValue(c);
        cp = RSTRING_PTR(c);
        cl = RSTRING_LEN(c);
        if (cl == 0) return Qnil;
    }
    rb_str_modify(str);
    if (cl > pos) {
        char *s;
        long rest = RSTRING_LEN(str) - pos;
        rb_str_resize(str, rest + cl);
        s = RSTRING_PTR(str);
        memmove(s + cl, s + pos, rest);
        pos = 0;
    }
    else {
        pos -= cl;
    }
    memcpy(RSTRING_PTR(str) + pos, cp, cl);
    ptr->pos = pos;
    RB_GC_GUARD(c);
    return Qnil;
}