[cfe-dev] About void* arithmetic and dereferencing.

[Enea Zaffanella]

Christopher Jefferson wrote:
> On 22 Aug 2010, at 14:06, Enea Zaffanella wrote:
> 
>> Hello.
>>
>> I am a bit confused regarding clang behavior wrt the support of void* 
>> arithmetic and dereferencing extensions.
>> It looks as if clang is treating differently syntactic constructs that 
>> (afaict) should be regarded as semantically equivalent.
> 
> Any arithmetic on void* pointers is (as the clang warning points out) a gcc extension, which should probably be discouraged.
> 
> Implementing '&p[8]' or 'p[0]' requires you to de-reference a void* pointer, which is a further extension over just allowing arithmetic on void* pointers. 
> 
> Chris

Hello Chris.

I don't think that the observations above are providing a proper answer 
to my question (probably I wasn't clear enough). The point I was trying 
to stress is the current behavior of clang, which is flagging some of 
the uses of these extensions as _warnings_ and some other uses 
(syntactically different, but afaict semantically equivalent) as _errors_.

Since I guess the intention was to properly support such an extension, I 
think the latter should be reported as extension warnings too.

Regards,
Enea.


>> Consider the following program fragment:
>>
>> ===========================
>> void* bar(void);
>>
>> unsigned foo(void) {
>>   unsigned u = 0;
>>   void* p;
>>   p = bar();
>>   p += 8;
>>   p = &p[8]; /* Equivalent to previous line (?) */
>>
>>   u += sizeof(*p);
>>   u += sizeof(p[0]); /* Equivalent to previous line (?) */
>>   return u;
>> }
>> ===========================
>>
>> When compiled using gcc and -Wpointer-arith, we get:
>> ===========================
>> gcc -Wpointer-arith -c bug.c -o /dev/null
>> bug.c: In function ‘foo’:
>> bug.c:7: warning: pointer of type ‘void *’ used in arithmetic
>> bug.c:8: warning: pointer of type ‘void *’ used in arithmetic
>> bug.c:8: warning: dereferencing ‘void *’ pointer
>> bug.c:10: warning: invalid application of ‘sizeof’ to a void type
>> bug.c:11: warning: pointer of type ‘void *’ used in arithmetic
>> bug.c:11: warning: invalid application of ‘sizeof’ to a void type
>> ===========================
>>
>> If instead we use clang (r111514), some of these warnings (i.e., the 
>> ones on the lines ending in a comment) are mapped to errors:
>> ===========================
>> $ clang -Wpointer-arith -fsyntax-only bug.c
>> bug.c:7:5: warning: use of GNU void* extension [-Wpointer-arith]
>>   p += 8;
>>   ~ ^  ~
>> bug.c:8:9: error: subscript of pointer to incomplete type 'void'
>>   p = &p[8]; /* Equivalent to previous line (?) */
>>        ~^
>> bug.c:10:8: warning: invalid application of 'sizeof' to a void type
>>       [-Wpointer-arith]
>>   u += sizeof(*p);
>>        ^     ~~~~
>> bug.c:11:16: error: subscript of pointer to incomplete type 'void'
>>   u += sizeof(p[0]); /* Equivalent to previous line (?) */
>>               ~^
>> 2 warnings and 2 errors generated.
>> ===========================
>>
>> This behavior seems to be inconsistent wrt C99 6.5.2.1p2,
>> where it is said that E1[E2] is identical to (*((E1)+(E2))).
>>
>> Is this just something that has to be fixed, or are there other reason 
>> explaining the currently observed behavior?
>>
>> Regards,
>> Enea.
>>
>> _______________________________________________
>> cfe-dev mailing list
>> cfe-dev at cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>