Michael Haubenwallner writes:
> $ cat test.c
> /* 1*/ extern void allocate(int size, void** p);
> /* 2*/
> /* 3*/ typedef struct X {
> /* 4*/ char A;
> /* 5*/ } X;
> /* 6*/
> /* 7*/ X* test1(void)
> /* 8*/ {
> /* 9*/ X *x;
> /*10*/ allocate((int)sizeof(X), (void**)&x); /* current code */
> /*11*/ allocate((int)sizeof(X), &x); /* as older gcc warned here already */
> /*12*/ return x;
> /*13*/ }
> /*14*/
> /*15*/ X* test2(void)
> /*16*/ {
> /*17*/ union {
> /*18*/ X *x;
> /*19*/ void* v;
> /*20*/ } u;
> /*21*/ allocate((int)sizeof(X), &u.v);
> /*22*/ allocate((int)sizeof(X), (void**)&u);
> /*23*/ return u.x;
> /*24*/ }
> $ gcc -Wall -O2 -c test.c -Wstrict-aliasing=2
> test.c: In function 'test1':
> test.c:10: warning: dereferencing type-punned pointer will break strict-aliasing rules
> test.c:11: warning: passing argument 2 of 'allocate' from incompatible pointer type
> test.c: In function 'test2':
> test.c:22: warning: dereferencing type-punned pointer might break strict-aliasing rules
>
> Strange is that line 10 "will", but line 22 "might" break
> strict-aliasing rules.
I don't see what's strange about it. Assigning to u.v and then
converting the address of u to a void** is perfectly legal C. It's a
pointless thing to do, though, and rather than this you might as well
do Line 21.
> Do I really need the union here like in test2() and line 21 ?
There's no way for us to tell.
> Or is there another "correct" way to get rid of warning in line 10 ?
If you could tell us what you're really trying to do we might be able
to help. The code in Line 10 makes no sense at all: it has no
meaning, and can't possibly do anything useful.
The sensible way to do what you need is simply:
void *p;
allocate((int)sizeof(X), &p);
X *x = (x*)p;
Andrew.
