I am getting some weird pointer-related errors that I do not understand how can happen and how to solve it. Since I am new to C any good coding practice advice is also appreciated.

typedef struct matrix {
    int n;
    int m;
    int **data;
    int *inverses;
} Matrix;

Matrix m_new(int n, int m, int *inverses) {

    int **data = malloc(n);

    for (int k = 0; k < n; k  ) {
        int *row = malloc(m   1);
        *row   = k;
        *(data k) = row; 
        // print_arr(*data); - introduced for debugging purposes
    }

    Matrix mat = {
        .n = n,
        .m = m,
        .data = data,
        .inverses = inverses,
    };

    return mat;
}

void print_arr(int *arr) {
    int len = *(arr-1);
    printf("len: %d\n", len);
}

Here, I am going to use this matrix for Gaussian Elimination over the integers mod some number, that's why I have the inverses array, and I use int **, so that I can swap rows easily later. For arrays represented as int * I use the -1th entry to store the length of the array. I got some weird bugs, so I wrote a small function to get the length of an array (print_arr) and check the first array in data. I got the following(for n=m=5):

len: 0
len: 0
len: 0
len: 0
len: -1219680380

Which I find really weird, because it seems like that something is changing in the last iteration, but I don't know why.

CodePudding user response：

Your allocations are too small. You need to multiply the number of elements by the size of the elements to compute how much memory to allocate:

int **data = malloc(n * sizeof (int *));

int *row = malloc((m   1) * sizeof (int));

With your current code, you overflow your too-small buffers and cause undefined behavior.