// For general cases use macro below
void mem_swap_heap(void* a, void* b, size_t size)
{
   void* temp = malloc(size);
   assert(temp);
   
   memcpy(temp, a, size); // copy a into temp
   memcpy(a, b, size); // copy b into a
   memcpy(b, temp, size); // copy temp into b\
   
   free(temp);
}

// For general cases use macro below
void mem_swap_alloca(void* a, void* b, size_t size)
{
   void* temp = alloca(size);
   assert(temp);
   
   memcpy(temp, a, size); // copy a into temp
   memcpy(a, b, size); // copy b into a
   memcpy(b, temp, size); // copy temp into b
}

// I presume that static size won't cause a stack overflow
#define swap(a, b) (void)(assert(sizeof(a) == sizeof(b)), \
                         mem_swap_alloca(&a, &b, sizeof(a)))

int main()
{
   int a = 5;
   int b = 10;

   puts("Before:");
   printf("a = %d\n", a);
   printf("b = %d\n", b);

   swap(a, b);
   puts("After:");
   printf("a = %d\n", a);
   printf("b = %d\n", b);
}