Why use double indirection? or Why use pointers to pointers?

Question

When should a double indirection be used in C? Can anyone explain with a example?

What I know is that a double indirection is a pointer to a pointer. Why would I ne

Answer 1

Compare modifying value of variable versus modifying value of pointer:

#include 
#include 

void changeA(int (*a))
{
  (*a) = 10;
}

void changeP(int *(*P))
{
  (*P) = malloc(sizeof((*P)));
}

int main(void)
{
  int A = 0;

  printf("orig. A = %d\n", A);
  changeA(&A);
  printf("modi. A = %d\n", A);

  /*************************/

  int *P = NULL;

  printf("orig. P = %p\n", P);
  changeP(&P);
  printf("modi. P = %p\n", P);

  free(P);

  return EXIT_SUCCESS;
}

This helped me to avoid returning value of pointer when the pointer was modified by the called function (used in singly linked list).

OLD (bad):

int *func(int *P)
{
  ...
  return P;
}

int main(void)
{
  int *pointer;
  pointer = func(pointer);
  ...
}    

NEW (better):

void func(int **pointer)
{
  ...
}

int main(void)
{
  int *pointer;
  func(&pointer);
  ...
}