Question

Does it violate strict aliasing rules to move items of any type around using uint32_t, then read them back? If so, does it also violate strict aliasing rules to memcpy from an array of uint32_ts to an array of any type, then read the elements back?

The following code sample demonstrates both cases:

#include <assert.h>
#include <stdio.h>
#include <stdint.h>
#include <string.h>

int main(void) {
    const char *strings[5] = {
        "zero", "one", "two", "three", "four"
    };
    uint32_t buffer[5];
    int i;

    assert(sizeof(const char*) == sizeof(uint32_t));

    memcpy(buffer, strings, sizeof(buffer));

    //twiddle with the buffer a bit
    buffer[0] = buffer[3];
    buffer[2] = buffer[4];
    buffer[3] = buffer[1];

    //Does this violate strict aliasing?
    const char **buffer_cc = (const char**)buffer;
    printf("Test 1:\n");
    for (i=0; i<5; i++)
        printf("\t%s ", buffer_cc[i]);
    printf("\n");

    //How about this?
    memcpy(strings, buffer, sizeof(strings));
    printf("Test 2:\n");
    for (i=0; i<5; i++)
        printf("\t%s ", strings[i]);
    printf("\n");

    return 0;
}

Please disregard my assumption of a 32-bit platform. Also, if the elements aren’t the same size as uint32_t, I know to pad them and copy the correct number of uint32_t’s. My question focuses on whether or not doing so violates strict aliasing.

Answer 1

The first loop does technically violate strict aliasing – it accesses uint32_t objects through an lvalue of type char *. It’s hard to see how any optimiser would cause you a problem in this specific case, though. If you altered it a little so you were doing something like:

printf("\t%s ", buffer_cc[0]);
buffer[0] = buffer[3];
printf("\t%s ", buffer_cc[0]);

You might see the same string printed twice – since the optimiser would be within its rights to only load buffer_cc[0] into a register once, because the second line is only modifying an object of type uint32_t.

The second loop, that memcpys them back, is fine.