What is "int (*arr)[cols]" where "cols" is a variable, in C ?-CodePudding

I am reading this to consider about how to dynamically allocate memory for a two-dimensional array.

I notice that a variable value cols can be used as size to define int (*arr)[cols], as C language has variable-length arrays(VLA) feature, then I try modifying the code into C like:

#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>

void* allocate(size_t rows, size_t cols)
{
    int (*arr)[cols] = (int (*)[cols])malloc(rows *sizeof(*arr));
    memset(arr, 0, rows *sizeof(*arr));
    return arr;
}

int main() {
    size_t rows, cols;
    scanf("%zu %zu", &rows, &cols);

    int (*arr)[cols] = (int (*)[cols])allocate(rows, cols);
    for (int i = 0; i < rows; i  ) {
        for (int j = 0; j < cols; j  ) {
            printf("=", arr[i][j]);
        }
        printf("\n");
    }
}

compile with gcc 11.2 -std=c 11

To my surprise, this works well and compiler does not report any warning. AFAIK C has no VLA feature, I used to think this code should be forbidden. So why could this work?

CodePudding user response：

-std=c 11 doesn't mean "compile strictly according to C 11" but "enable C 11 features." Just as -std=gnu 11 (the default setting) means enable gnu 11 features, which is a superset of C 11.

To get strictly compliant behavior, you must use -std=c 11 -pedantic-errors. And then you get this:

error: ISO C forbids variable length array 'arr' [-Wvla]

See What compiler options are recommended for beginners learning C? for details. It was written for C but applies identically to g as well.