Question

I’m trying to learn C, and I’m hung up a bit on Pointers to Pointers. I think I understand why you need them, but can’t quite get my head wrapped around what is happening.

For instance, the following code doesn’t seem to work as I would expect it to:

#include <stdio.h>

int newp(char **p) {
  char d = 'b';
  *p = &d;
  /*printf("**p = %c\n", **p);*/
  return 1;
}

int main() {
  char c = 'a';
  char *p = &c;
  int result;

  result = newp(&p);
  printf("result = %d\n", result);
  printf("*p = %c\n", *p);
  printf("c = %c\n", c);

  return 0;
}

The result I get is this:

result = 1
*p = 
c = a

*p prints as nothing. Instead, I would expect *p = b.

However, if I uncomment line 6 (the printf in the newp function), then I get this:

**p = b
result = 1
*p = b
c = a

What am I missing?

Answer 1

You are dealing with undefined behaviour. The variable d is local (resides on the stack) and is not available after the enclosing function (newp) returns.

When dereferencing p outside newp, the address on the stack &d may be overwritten by some other local variable or it may contain garbage.