CITS2002 Systems Programming  
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Passing pointers to functions, continued

Instead, we need to pass a 'reference' to the two integers to be interchanged.

We need to give the swap() function "access" to the variables a and b, so that swap() may modify those variables: 

#include <stdio.h>

void swap(int *ip, int *jp)
{
    int temp;

    temp  = *ip;    // swap's temp is now 3
    *ip   = *jp;    // main's variable a is now 5
    *jp   = temp;   // main's variable b is now 3
}

int main(int argc, char *argv[])
{
    int a=3, b=5;

    printf("before a=%i, b=%i\n", a, b);

    swap(&a, &b);   // pass pointers to our local variables  

    printf("after  a=%i, b=%i\n", a, b);

    return 0;
}

before a=3, b=5
after  a=5, b=3

Much better! Of note:

  • The function swap() is now dealing with the original variables, rather than new copies of their values.
  • A function may permit another function to modify its variables, by passing pointers to those variables.
  • The receiving function now modifies what those pointers point to.

 

CITS2002 Systems Programming, Lecture 12, p2, 29th August 2023.