Thread: I don't understand this alias

Given the following:

Code:

#include <stdio.h>

int main(void) {
  char x = 1;
  char y = 2;

  char *head, **tailp, *new;

  head = &x;
  new = &y;

  tailp = &head;

  printf("THe value of **tailp: %d \n", **tailp);
  printf("The value of head: %x and &x: %x \n", head, &x);

  *tailp = NULL;

  printf("THe new value of head: %d \n", head);
  printf("THe value of *tailp is: %d \n", *tailp);

  return 0;
}

I don't see how *tailp is an alias for head when *tailp is NULL.

Thanks

tailp is a pointer to head (or it contains the address of head, &head). *tailp is therefore an alias of head (in C++ *tailp would be refered to as a reference to head). The operative bits are

Code:

   tailp = &head;
  *tailp = NULL;

The second line in this does not change tailp, but it does change the value at the address identified by tailp. Or, to put it another way, the second line is functionally equivalent to

Code:

   *(&head) = NULL;

or (simplified)

Code:

    head = NULL;

Okay,

I think I see this. For some reason, I thought when I went

*tailp = NULL;

it went somewhere else. Like the following


tailp ---->head
|
|------>NULL (when I go *tailp)

Originally Posted by grumpy

Code:

   tailp = &head;

can this be replaced by tailp=head?

Originally Posted by KidMan

can this be replaced by tailp=head?

Only if you want code that won't compile due to a mismatch of types.

head is a pointer to char (char *). tailp is a pointer to pointer to char (char **) meaning it can hold the address of a pointer to char. Implicit conversion of a char * to a char ** is not allowed.