array struct assignments

[PaoloC13]

When I copy a returned struct { T arr[N]; } into the caller, it skips array elements whose value came from memset or ={0}. Only elements with explicit individual assignments are written.

Code Select Expand

typedef struct { float m[16]; } Mat4;

    Mat4 broken(void) {
        Mat4 s;
        memset(&s, 0, sizeof(s));   // set the block
        s.m[0] = 1.0f;              // explicit
        return s;
    }

    // Caller:
    Mat4 result;
    memset(&result, 0xCC, sizeof(result));   // fill with known garbage
    result = broken();

  /* Expected per C standard: result.m[1] == 0.0f           */
  /* Observed:                result.m[1] == -1.07374e+08   */
  /*   (the 0xCC garbage — the copy never wrote that slot)  */
Test: pre-fill the destination with a recognisable non-zero sentinel (0xCC bytes works well) before the call.
If any zero-expected field shows the sentinel value after the call, the copy skipped it.

My fix: explicitly assign every element of the array.
Is this a bug or a normal behaviour?

[TimoVJL]

Think using static keyword

Code Select Expand

Mat4 broken(void) {
    static Mat4 s;
    memset(&s, 0, sizeof(s));   // set the block
    s.m[0] = 1.0f;              // explicit
    return s;
}


[Michele]

You should never return an automatic variable.
's' is allocated on the stack, and is overwritten by other data when function exits.
You can declare it 'static' as suggested or define it in the caller and pass it.

[PaoloC13]

Regret. I have to go back to the beginners forum.

[John Z]

Don't sweat it  many of us end up there every once in a while....


John Z