Why unit tests crashed on windows but worked on the microcontroller#

When writing cross-platform code, it’s easy to forget that pointers don’t always have the same size. This caused a segmentation fault in one of our unit tests.

What happened#

On the Aurix MCU, pointers are 32-bit. In the code a pointer (memory address) was stored in a uint32_t variable. That worked fine on the target.

But, when running the unit tests on the PC (Windows with GCC), pointers are 64-bit. Storing a 64-bit pointer into a 32-bit integer truncated the address. The compiler only issued a warning and still compiled.

Later, when the truncated value was cast back to a pointer and dereferenced, it pointed to invalid memory and crashed with a segmentation fault.

How to fix it and make the code portable#

  • 🚫 Never store a pointer in a fixed-width integer type (uint32_t, uint64_t, …).

  • ✅ Always use uintptr_t from <stdint.h> because it is guaranteed to be wide enough for a pointer on the current platform.

Wrong vs Right#

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

int main(void) {
    int value = 42;
    int *ptr = &value;

    // ❌ Wrong: pointer stored in fixed 32-bit type
    uint32_t wrong = (uint32_t)ptr;   // Works on 32-bit MCU, truncates on 64-bit host
    int *bad = (int *)wrong;          // may segfault
    printf("wrong addr = 0x%08" PRIx32 "\n", wrong);

    // ✅ Right: use uintptr_t (pointer-sized integer type)
    uintptr_t right = (uintptr_t)ptr;
    int *good = (int *)right;
    printf("right addr = 0x%" PRIxPTR ", *good = %d\n", right, *good);

    return 0;
}

Both PRIx32 and PRIxPTR come from <inttypes.h>. They expand to the correct printf format string for the given integer type:

  • PRIx32 - prints a 32-bit integer in hex.

  • PRIxPTR - prints a uintptr_t (pointer-sized integer) in hex.

This ensures portable and correct logging across platforms.

Final thought#

Using uintptr_t instead of uint32_t to store pointers makes the code portable when running unit tests on a 64-bit host.

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