Re: Safer vararg calls

[Yair Lenga <yair.lenga@gmail.com>]

Hi Jonathan, thanks for taking the time to review.

I agree with your comment about the attribute name (va_vector, va_type). My
best improvement is "va_sametype". Is it better ? ? May be "va_matchtype" ?
any other suggestions ?

For the case of the sentinel/va_sametype, I hope that the implementation
will recognize the combination of the two, and will allow NULL to be used
as-is, without having to cast it. Altough I believe that NULL pointers are
considered compatible with with any pointer. Not 100% sure about this.

I'm not sure I understand the question about mixing char * and const char
*. Probably I cause confusion with my rushed example, which should be:

      // join all parameters, return newly allocate string.
__attribute__ ((malloc(free), va_matchtype, sentinel)) char
*delimitedstr(char delim, const char *p1, ...);


On Tue, Jun 21, 2022 at 6:44 AM Jonathan Wakely <jwakely.gcc@gmail.com>
wrote:

> On Tue, 21 Jun 2022 at 11:17, Yair Lenga via Gcc <gcc@gcc.gnu.org> wrote:
> >
> > Hi,
> >
> > Looking for feedback on the adding new attribute to function calls that
> will help create safer vararg functions.
> >
> > Consider the case where a vararg function takes list of arguments of the
> same type. In my case, there are terminated with a sentinel of null.
> >
> > Char *result = delimitedstr(‘:’ “foo”, “bar”, “zoo”, NULL) ;
> >
> > The standard prototype
> > is char * delimitedstr(char delim, char *p1…) ;
> >
> > Which will currently allow many incorrect calls:
> >  delimitedstr(‘:’, “foo”, 5, 7.3, ‘a’) ;    // bad types + missing
> sentinel.
> >
> > The __attribute__((sentinel)) can force the last arg to be null.
> >
> > My proposal is to add new attribute ((va_vector)) that will add a check
> that all parameters in a vararg list match the typeof the last parameter.
> So that:
>
> "va_vector" is a bad name IMHO. It tells me nothing about what it
> means. Does it have something to do with SIMD vectors?
>
> >
> > __attribute__ ((va_typed)) delimitedstr(char delim, char *p1…) ;
>
> "va_typed" at least suggests something to do with types, but it
> doesn't tell me they have to be the same type.
>
> >
> > Will flag a call where any of the parameter after p1, is not a string.
>
> In your example NULL does not have the same type as the earlier
> arguments. You would have to write (char*)NULL to suppress a
> diagnostic.
>
> I also wonder how a mixture of char* and const char* arguments would
> be handled in your example.
>
>
> >
> > This can result in cleaner, safer code, without making the calling
> sequence more difficult, or modifying the behavior of the call.
> >
> > For Java developers, this is basically the same type checking provided
> by the as ‘datatype …’ (without the conversion into array).
> >
> > I am Looking for feedback, Pointers on how to implement, as I do not
> have experience with extending gcc.
> >
> > Yair
>