Check Structures with 64-bit Long Data Types on x86-Based Platforms

You should carefully check structures that use 64-bit long types, such as uint64_t, on the x86 platforms. The alignment and size can differ between compilation in 32-bit mode versus a 64-bit mode. Consider the following example.

#include <studio>
#include &ltsys>

struct myTestStructure {
        uint32_t        my1stInteger;
        uint64_t        my2ndInteger;
};

main()
{
        struct myTestStructure a;

        printf("sizeof myTestStructure is: %d\n", sizeof(a));
        printf("offset to my2ndInteger is: %d\n", (uintptr_t)&a.bar - (uintptr_t)&a);
}

On a 32-bit system, this example displays the following results:

sizeof myTestStructure is: 12
offset to my2ndInteger is: 4

Conversely, on a 64-bit system, this example displays the following results:

sizeof myTestStructure is: 16
offset to my2ndInteger is: 8

Thus, the 32-bit application and the 64-bit application view the structure differently. As a result, trying to make the same structure work in both a 32-bit and 64-bit environment can cause problems. This situation occurs often, particularly in situations where structures are passed into and out of the kernel through ioctl() calls.