Type punning with void * without breaking the strict aliasing rule in C99

Question

I recently came across the strict aliasing rule, but I\'m having trouble understanding how to use void * to perform type punning without breaking the rule.

Answer 1

void * has nothing to do with type-punning. Its main purposes are:

1. To allow for generic allocation and freeing operations that don't care about the type of the object the caller is storing there (e.g. malloc and free).

2. To allow a caller to pass a pointer to an arbitrary type through a function that will pass it back via a callback, (e.g. qsort and pthread_create). In this case, the compiler cannot enforce type checking; it's your responsibility when writing the caller and callback to ensure that the callback accesses the object with the correct type.

Pointers to void are also used in a few places (like memcpy) that actually operate on an object as the overlaid unsigned char [] representation for the object. This could be seen as type-punning, but it's not an aliasing violation because char types are allowed to alias anything to access its representation. In this case, unsigned char * would also work, but void * has the advantage that pointers automatically convert to void *.

In your example, since the original type is int and not a union, there is no legal way to type-pun and access it as short. You could instead copy the value of x to a union, perform well-defined type-punning there, then copy it back. A good compiler should omit the copy entirely. Alternatively, you could break the write down into char writes and then it would be legal aliasing.