Legitimate Use of Variable Length Arrays

Post by Gibbon1 » Tue, Oct 29, 2019, 3:01 AM UTC

bdisnainc wrote: post #13

theamk wrote: post #12

Earlier quoted context omitted.

Um, no? That COPY_MAX would still need a different value between small MCU and full-blown PC. Not to mention that using stack allocation on low-memory device is extremely dangerous... at least with malloc you get a nice diagnostic, while alloca will just do random program damage.

It does not do random damage, you get a segmentation fault in most systems. In really small embedded you should not be allocating anything meaningful in the stack anyway.

Post by gnode » Tue, Oct 29, 2019, 3:10 PM UTC

crististm wrote: post #19

gnode wrote: post #11

Earlier quoted context omitted.

They're trying to allocate an n·n element array. If int and size_t are 32b (typical 32-bit architectures), and n == INT_MAX, then n·n·sizeof(float) can't be represented by a size_t. (Used · for multiplication, because HN turns stars into emphasis.)

That may be true but an array of n x n chars where n is 2^32 won't fit (well obviously) in RAM and size_t should be at least 64 bit to match (2^32 x 2^32 = 2^64). Regardless, even without these unobtainium big machines, the cardinality of the largest array for a platform (by any number of dimensions) must be representable in size_t. A too large of an array would not fit in RAM and you should get a compile/link/out of…

    int n = /* run-time value */
    size_t sz = (size_t)n;
    // Check n fits in size_t.
    if ((int)sz != n) 
    sz *= sz * sizeof(float);
    // Check against overflow.
    if (((sz / (size_t)n) / sizeof(float)) != (size_t)n) 
    float *matrix = malloc(sz);
    // Check allocation succeeded.
    if (matrix == NULL)