A simple question about C...

[Michael Tuexen]

Dear all,

I'm trying to get a C program (packetdrill) running FreeBSD r254973
on a Raspberry Pi.
Since tests are failing, I wrote the little C program:

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

int
main(void)
{
        uint8_t byte[] = {0x00, 0x01, 0x02, 0x03,
                          0x04, 0x05, 0x06, 0x07,
                          0x08, 0x09, 0x0a, 0x0b};
        uint32_t i, n, *p;

        for (i = 0; i <= 8; i++) {
                p = (uint32_t *)(byte + i);
                memcpy(&n, byte + i, sizeof(uint32_t));
                printf("p = %p, *p = %08x, n = %08x.\n", (void *)p, *p, n);
        }
        return (0);
}

It produces the following output:

p = 0xbfffec48, *p = 03020100, n = 03020100.
p = 0xbfffec49, *p = 00030201, n = 04030201.
p = 0xbfffec4a, *p = 01000302, n = 05040302.
p = 0xbfffec4b, *p = 02010003, n = 06050403.
p = 0xbfffec4c, *p = 07060504, n = 07060504.
p = 0xbfffec4d, *p = 04070605, n = 08070605.
p = 0xbfffec4e, *p = 05040706, n = 09080706.
p = 0xbfffec4f, *p = 06050407, n = 0a090807.
p = 0xbfffec50, *p = 0b0a0908, n = 0b0a0908.

So everything is fine when reading the 4 byte long uint32_t on a 4-byte aligned
address. Results are strange (at least for me), when the address is not 4-byte
aligned. There is no difference between clang and gcc. Can I only dereference
properly aligned pointers? Is the above expected or is it a bug in the compiler?

Best regards
Michael