Question

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

void setZero (double **, int);

int main (void) {
        double *ptr = NULL;
        int i, size = 3;
        ptr = (double *)malloc(size * sizeof(double));
//*
        setZero(&ptr, size);
/*/
        // Sanity test
        for ( i = 0 ; i < size ; ++i ) {
                printf("index %d/%d\n", i, (size-1));
                ptr[i] = 0;  // NOT EXPLODING...
        }
//*/
        free(ptr);
        return 0;
}

void setZero (double **_ref_array, int _size) {
    int i;

    for ( i = 0 ; i < _size; ++i ) {
        printf("index %d/%d\n", i, (_size-1));
        *_ref_array[i] = 0;  // EXPLODING...
    }
}

1) Why is this not working?

2) What is a “Bus error 10”

P.S. I know better than to initialize an array this way, but this just happens to be a simple and clean example of an underlying concept that I’m not understanding…

Answer 1

The dereference is happening after the index. I.e.

This says “Get the double pointer at index ‘i’, then set the value 0 to the memory at the address within that pointer.”

*_ref_array[i] = 0; 

This says “Get the address of the array of doubles from _ref_array, than index off that address by i-doubles.

(*_ref_array)[i] = 0;