While answering another question, I thought of the following example:

void *p;
unsigned x = 17;

assert(sizeof(void*) >= sizeof(unsigned));
*(unsigned*)&p = 17;        // (1)
memcpy(&p, &x, sizeof(x));  // (2)

Line 1 breaks aliasing rules. Line 2, however, is OK wrt. aliasing rules. The question is: why? Does the compiler have special built-in knowledge about functions such as memcpy, or are there some other rules that make memcpy OK? Is there a way of implementing memcpy-like functions in standard C without breaking the aliasing rules?

c++ c strict-aliasing
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