Swapping pointers in C (char, int)

Question

I have been struggling to understand the different behaviour when swapping pointers in C. If I want to swap two int pointers, then I can do

void         


        

Answer 1

This example does not swap two int pointers. It swaps the value of the integers that pa and pb are pointing to. Here's an example of what's going on when you call this:

void Swap1 (int *pa, int *pb){
    int temp = *pa;
    *pa = *pb;
    *pb = temp;
}

int main()
{
    int a = 42;
    int b = 17;


    int *pa = &a;
    int *pb = &b;

    printf("--------Swap1---------\n");
    printf("a = %d\n b = %d\n", a, b);
    swap1(pa, pb);
    printf("a = %d\n = %d\n", a, a);
    printf("pb address =  %p\n", pa);
    printf("pa address =  %p\n", pb);
}

The output here is:

a = 42
b = 17
pa address =  0x7fffdf933228
pb address =  0x7fffdf93322c
--------Swap---------
pa = 17
pb = 42
a = 17
b = 42
pa address =  0x7fffdf933228
pb address =  0x7fffdf93322c

Note that the values swapped, but the pointer's addresses did not swap!

In order to swap addresses we need to do this:

void swap2 (int **pa, int **pb){
    int temp = *pa;
    *pa = *pb;
    *pb = temp;
}

and in main call the function like swap2(&pa, &pb);

Now the addresses are swapped, as well as the values for the pointers. a and b have the same values that the are initialized with The integers a and b did not swap because it swap2 swaps the addresses being being pointed to by the pointers!:

a = 42
b = 17
pa address =  0x7fffddaa9c98
pb address =  0x7fffddaa9c9c
--------Swap---------
pa = 17
pb = 42
a = 42
b = 17
pa address =  0x7fffddaa9c9c
pb address =  0x7fffddaa9c98

Since Strings in C are char pointers, and you want to swap Strings, you are really swapping a char pointer. As in the examples with an int, you need a double pointer to swap addresses.

The values of integers can be swapped even if the address isn't, but Strings are by definition a character pointer. You could swap one char with single pointers as the parameter, but a character pointer needs to be a double pointer in order to swap the strings.