指向结构的指针可以别名为无符号char数组吗？

Question

#include "stdio.h"  


/* array to store data receeived from CAN Bus */
unsigned char a[8] = {0xCD, 0xEF, 0x12, 0x34, 0x50, 0x00, 0x00, 0x00};

typedef struct {
    unsigned int a:12;
    unsigned int b:12;
    unsigned int c:12;
    unsigned int unused:28;
}test;

test *tptr;

int main(void)
{

    tptr = (test*)( (void*)&a); // is this line braking any aliasing rule

    if(tptr->a == 0xCDE)
    {
        printf("\n data received ok");

    }

    return 0;
}

I recently learned about problems due to pointer aliasing in C. I want to know if the above code is breaking any rules. Could it lead to problems?

I know that the bitfield's order is machine and implementation dependent. My question, however, is regarding pointer aliasing rules, which I want to understand more clearly

Answer 1

It does break strict aliasing. Some (saner) compilers do deduce that obviously tptr should be allowed to alias a , but this is not guaranteed by the standard. In fact, GCC will not consider tptr to be aliasing a .

Another problem is the alignment of a . The x86 is quite tolerant in that regard, but many other architectures, including the ARM, are not.

Consider (int)&a == 0xFFF02 and sizeof(int) == 4 on architecture that support fetching int s only from addresses that are themselves multiples of sizeof(int) . Such an unaligned access would result in a bus error, if you do *(int*)&a or even worse the program accessing the wrong location.

To stay clear of undefined behavior, remember that it is guaranteed that accesses through unsigned char* are possible without alignment or aliasing constraints, not the other way round. Thus use:

test buf;
can_recv((unsigned char*)&buf, sizeof buf);
if(tptr->a == 0xCDE)
{
    printf("\n data received ok");

}

Another portability problem, is that you expect that bitfield to be packed a certain way. The standard doesn't guarantee that however . If you want portable behavior, restrict yourself to the bitfield interface ( bitfield.b1 = 1 ) and don't modify the bitfied by other means.

If portability isn't a concern, make sure to set the appropriate compiler flags that guarantee the bitfield packing you're expecting.