strswap. Explanation needed.

Hi Everyone. I'm trying to understand pointers. I know a few things but this example
really confuses me. Could you explain me why strswap's parameters should be char **,
or how it swaps a and b although the don't have the same size?

Code:

#include <stdio.h>
#include <string.h>
void strswap(char ** k,char ** l);

int main()
{
  char *a="Hello world";
  char *b="Good bye";
  printf("Before strswap a=%s , b=%s\n",a,b);
  strswap(&a,&b);
  printf("After strswap a=%s , b=%s\n",a,b);

getchar();
return 0;
}

/**********************************/
void strswap(char** k,char** l)
{
char *temp;
temp=*k;
*k=*l;
*l=temp;
}

Or why couldn't be like this?

Code:

#include <stdio.h>
#include <string.h>
void strswap(char * k,char * l);

int main()
{
  char *a="Hello world";
  char *b="Good bye";
  printf("Before strswap a=%s , b=%s\n",a,b);
  strswap(a,b);
  printf("After strswap a=%s , b=%s\n",a,b);

getchar();
return 0;
}

/**********************************/
void strswap(char* k,char* l)
{
char temp;
temp=*k;
*k=*l;
*l=temp;
}

thank you.

Try it with ints
Compare
void swap ( int *a, int *b );
with
void swap( int a, int b );

It's all down to everything being passed by value, so if you have a T which needs swapping, you need a T* to be passed to swap.
And since you start off with char*, this leads to needing char** in the function.

I tried both of them in dev-c++. the first works fine while the other doesn't.
Still don't get it... :-(

Is k equal to &&a?

Let's say you want to swap the values of two Things. For every type Thing, the swap procedure would look like this;

Code:

void Swap(Thing *k, Thing *l)
{
    Thing temp;
    temp = *k;
    *k = *l;
    *l = temp;
}

A pointer to Thing needs to be passed to allow the values to be swapped.

C-strings are the particular case where Thing is a pointer to char, or char *. If you want to swap strings, you need to pass a pointer to Thing (i.e. a pointer to pointer to char, which is a char **).

Understand the difference between char (a single character) and char* (a pointer to a character, which is often interpreted as the first in a string of characters, terminated by a character with the value 0) and you will understand why the function is written as it is written.