Question

I have an array a whose values I wish to modify through another function. This is the code that I have:

#include "stdlib.h" 
#include "stdio.h"

void myfunc(int* );

int main() {

    int *a, i;

    a = (int*) calloc(10,sizeof(int));
    myfunc(&a);

    for (i=0; i<10; i++) printf("%d\n", a[i]);

    return 0;
}

void myfunc(int* a) {

    int i;

    for (i=0; i<10; i++) *a[i] = i;
}

Obviously something is wrong with my syntax and I Was wondering if someone could lend me a hand 🙂

Thanks!

Answer 1

In the main function, you’ve got a pointer to an integer, a. Then you pass the address of this pointer to myfunc, but myfunc is expecting a pointer to an int, not a pointer to a pointer to an int. You need to change your call to myfunc to this:

myfunc(a);

But, you’ve also got a problem inside myfunc. You don’t need to dereference a[i] since that indexes into an array a. These are (effectively) the same thing:

a[i]

and

*(a+i)

(This isn’t 100%, I think, because in C pointers and arrays are not completely the same thing, so some expert C programmer can correct me)

You’re probably just get a bit confused by the collection of syntax around pointers and arrays:

&a
*a
a[i]
*(a+i)

The first one is the address of a, the second dereferences a as a pointer, the third is an index into an array and the fourth is dereferencing a as a pointer with an offset.

You would only want to do this *(a[i]) if you had an array of pointers, not a pointer to an int.