Question

Complete C newb here. Trying to learn/understand pointers by messing with simple code fragments.

#include <stdio.h>

void swap(int *px, int *py)
{
    int tmp;
    tmp = *px;
    *px = *py;
    *py = tmp;
}

main()
{
    int *a, *b;
    *a = 1;
    *b = 2;
    swap(&a,&b);
    printf("%d %d\n", *a, *b);
}

Why is this not valid? The code works when I remove the dereferencing operator * from main.

Conceptually, this seems like it should work. I initialize a and b as pointers which point to int 1 and int 2, respectively. I then send their addresses to swap(), which should switch what they point to.

Answer 1

There are a couple of problems. First, the pointers a and b are not pointing to valid memory. So the assignment of the integer values is undefined (possible crash). Secondly, the call to swap (assuming a and b are pointing to valid memory) should not include the address (it is currently sending the address of the pointer variable).

The following changes would make it work:

int a, b;
a = 1;
b = 2;
swap(&a,&b);
printf("%d %d\n", a, b);