Question

Just playing with pointers, trying to pass the address of a pointer to a function, using:

void changer(int **ptr)
{
    if(**ptr==NULL)
    {   
        *ptr=(int *)malloc(sizeof(int));
        **ptr=2;
    }

}

int main()
{
    int *ptr;
    clrscr();
    changer(&ptr);
    printf("%d",*ptr);
    getch();
    return 0;
}

Though the program compiles, the results are not as expected!

Answer 1

You have an extra dereference of ptr in the NULL check, and you are not initializing the pointer. The program should look like this after the fix:

#include <stdio.h>
#include <stdlib.h>

void changer(int **ptr)
{
    if(*ptr==NULL) // Dereference once, not twice
    {
        *ptr=(int *)malloc(sizeof(int));
        **ptr=2;
    }
}

int main()
{
    int *ptr = 0; // Initialize the pointer
    clrscr();
    changer(&ptr);
    printf("%d",*ptr);
    getch();
    return 0;
}