Question

I am studying pointers and I encountered this program:

#include <stdio.h>

void swap(int *,int *);

int main()
{
    int a=10;
    int b=20;
    swap(&a,&b);
    printf("the value is %d and %d",a,b);
    return 0;
}

void swap(int *a,int*b)
{
    int t;
    t=*a;
    *a=*b;
    *b=t;
    printf("%d and%d\n",*a,*b);
}

Can any one tell me why this main function return the value reversed? The thing I understood till now is that the function call in C does not affect the main function and it’s values.

I also want to know how much the space a pointer variable occupied like integer have occupied the 2 bytes and the various application use and advantages of the pointer…

Answer 1

You are correct: a function in C cannot directly affect the values in the caller. But the values that are being given to swap() are not the values of a and b. Rather, they are pointers to those values–basically, the address in memory where a and b live. Since swap has those addresses, it can then write to those memory locations and effectively change a and b.

Good luck with pointers, by the way. It’s the hardest thing about C to get if you’re coming from a context that doesn’t have them. I came to C by way of Perl, and I broke my brain on pointers several times. But it’ll open up a whole new world once you understand them.