Question

I have a 2D jagged array declared in my main() block. This is to be passed to a function to have memory allocated to it. The following is the most reduced case which compiles but crashes when it runs. Where am I going wrong?

#include <stdio.h>
#include <stdlib.h>

void alloc2d(double ***p);


int main () {

    double **data;

    alloc2d(&data);
    printf("Before assign to data\n");
    data[0][0] = 0.1;
    printf("After assign to data\n");
    free(data);
}


void alloc2d(double ***p) {

    int i, n, m;

    // Get some dynamically assigned sizes
    printf("Enter size: ");
    scanf("%d %d", &n, &m);    
    // Now allocate
    *p = malloc(n * sizeof(double*));
    for (i = 0; i < n; i++) {
        *p[i] = malloc(m * sizeof(double));
    }
    printf("End of alloc2d\n");
}

This reads the values but crashes when I enter low numbers (i.e. ‘1 1′) but crashes when I enter high numbers (i.e. ’10 10’).

Answer 1

You made a very simple syntax error

*p[i] = (double*)malloc(m * sizeof(double));

should really be

(*p)[i] = (double*)malloc(m * sizeof(double));

This is because in C, [] operator has higher precedence than *.
So when you type *p[i],
it is translated into **(p + i).

This means: you are asking the compiler to calculate the address by offsetting the address of p by i * sizeof(double**), which is clearly not what you actually want.

So, in order to force the compiler to dereference p first, simply surroud *p with brackets.