Spot the bug!

See if you can spot the bug in this code!

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    union {
        unsigned char raw[8];
        struct {
            char one;
            int two;
            char three;
            char four;
            char five;
        } formatted __attribute__((packed));
    } test;

    printf("one   : %d\n", (int)&test.formatted.one - (int)&test);
    printf("two   : %d\n", (int)&test.formatted.two - (int)&test);
    printf("three : %d\n", (int)&test.formatted.three - (int)&test);
    printf("four  : %d\n", (int)&test.formatted.four - (int)&test);
    printf("five  : %d\n", (int)&test.formatted.five - (int)&test);
    return 0;
}

$ gcc -Wall  -o packing packing.c
$ ./packing
one   : 0
two   : 4
three : 8
four  : 9
five  : 10

Here's the relevant bit from the gcc manual:

For an enum, struct or union type, you may specify attributes either between the enum, struct or union tag and the name of the type, or just past the closing curly brace of the definition. The former syntax is preferred.

By getting the packed attribute in the wrong place (if it's not clear, it should be before formatted), it is applied to the variable rather than the type. The compiler (both gcc and icc do this) has already laid out the structure, so misses the packing directive, and unfortunately doesn't warn (that may be a bug?).

I can tell you from experience this can be hard to track down!