Re: [Qemu-devel] [PATCH 1/3] qemu-char: Introduce Memory driver

[Markus Armbruster]

Luiz Capitulino <address@hidden> writes:

> On Fri, 12 Nov 2010 16:04:39 +0100
> Markus Armbruster <address@hidden> wrote:
>
>> Luiz Capitulino <address@hidden> writes:
>> 
>> > On Fri, 12 Nov 2010 15:16:33 +0100
>> > Markus Armbruster <address@hidden> wrote:
>> >
>> >> Luiz Capitulino <address@hidden> writes:
>> >> 
>> >> > On Fri, 12 Nov 2010 11:21:57 +0100
>> >> > Markus Armbruster <address@hidden> wrote:
>> >> >
>> >> >> Luiz Capitulino <address@hidden> writes:
>> >> [...]
>> >> >> > +QString *qemu_chr_mem_to_qs(CharDriverState *chr)
>> >> >> > +{
>> >> >> > +    MemoryDriver *d = chr->opaque;
>> >> >> > +
>> >> >> > +    if (d->outbuf_size == 0) {
>> >> >> > +        return qstring_new();
>> >> >> > +    }
>> >> >> 
>> >> >> Why is this necessary?  Is qstring_from_substr() broken for empty
>> >> >> substrings?  If it is, it ought to be fixed!
>> >> >
>> >> > qstring_from_substr() takes a character range; outbuf_size stores a 
>> >> > size,
>> >> > not a string length. So we do:
>> >> >
>> >> >> > +    return qstring_from_substr((char *) d->outbuf, 0, 
>> >> >> > d->outbuf_size - 1);
>> >> >
>> >> > If outbuf_size is 0, we'll be passing a negative value down.
>> >> 
>> >> What's wrong with that?
>> >
>> > Although it's going to work with the current QString implementation, I 
>> > don't
>> > think it's it's a good idea to rely on a negative index.
>> 
>> How should I extract the substring of S beginning at index B with length
>> L?  If I cant't do this for any B, L with interval [B,B+L-1] fully
>> within [0,length(S)], then the API is flawed, and ought to be replaced.
>
> Not sure we're talking about the same problem, anymore. When you said:
>
>> >> What's wrong with that?
>
> What did you mean? Did you mean 'let's not decrement outbuf_size' or did
> you mean 'let's pass -1 anyway'?

Yes, what's wrong with qstring_from_substr(S, 0, -1)?

Its function comment is imprecise, it doesn't tell us whether the END-th
character is included in the substring or not.

The code, however, is clear enough: it *is* included.  And the unit test
checks that.

Therefore, qstring_from_substr("abc", 0, 0) returns the qstring "a".

> Both seem wrong to me: the substring [0,-1] should be invalid

Why?

How do you express "the empty substring starting at 0" then?

>                                                               and not
> decrementing outbuf_size is wrong, because it contains the buffer size and
> qstring_from_substr() will consume an additional char from the buffer (which
> should be '\0' today, but we shouldn't count on that).
>
>> 
>> > Maybe, we could have:
>> >
>> > return qstring_from_substr((char *) d->outbuf, 0,
>> >                             d->outbuf_size > 0 ? d->outbuf_size - 1 : 0);
>> >
>> > A bit harder to read, but makes the function smaller.
>> 
>> Err, doesn't qstring_from_substr(s, 0, 0) extract a substring of length
>> 1?
>
> Yeah, it's a bug. But that doesn't change my suggestion, can we do this way?
>
> This should fix the bug (not even compiled tested):
>
> diff --git a/qstring.c b/qstring.c
> index 4e2ba08..72a25de 100644
> --- a/qstring.c
> +++ b/qstring.c
> @@ -42,10 +42,10 @@ QString *qstring_from_substr(const char *str, int start, 
> int end)
>  
>      qstring = qemu_malloc(sizeof(*qstring));
>  
> -    qstring->length = end - start + 1;
> -    qstring->capacity = qstring->length;
> +    qstring->length = end - start;
> +    qstring->capacity = qstring->length + 1;
>  
> -    qstring->string = qemu_malloc(qstring->capacity + 1);
> +    qstring->string = qemu_malloc(qstring->capacity);
>      memcpy(qstring->string, str + start, qstring->length);
>      qstring->string[qstring->length] = 0;

I suspect this will fail your unit test.