14.10 Pointer Arithmetic

Adding an integer (positive or negative) to a pointer is valid in C. It assumes that the pointer points to an element in an array, and advances or retracts the pointer across as many array elements as the integer specifies. Here is an example, in which adding a positive integer advances the pointer to a later element in the same array.

void
incrementing_pointers ()
{
  int array[5] = { 45, 29, 104, -3, 123456 };
  int elt0, elt1, elt4;

  int *p = &array[0];
  /* Now p points at element 0.  Fetch it.  */
  elt0 = *p;

  ++p;
  /* Now p points at element 1.  Fetch it.  */
  elt1 = *p;

  p += 3;
  /* Now p points at element 4 (the last).  Fetch it.  */
  elt4 = *p;

  printf ("elt0 %d  elt1 %d  elt4 %d.\n",
          elt0, elt1, elt4);
  /* Prints elt0 45  elt1 29  elt4 123456.  */
}

Here’s an example where adding a negative integer retracts the pointer to an earlier element in the same array.

void
decrementing_pointers ()
{
  int array[5] = { 45, 29, 104, -3, 123456 };
  int elt0, elt3, elt4;

  int *p = &array[4];
  /* Now p points at element 4 (the last).  Fetch it.  */
  elt4 = *p;

  --p;
  /* Now p points at element 3.  Fetch it.  */
  elt3 = *p;

  p -= 3;
  /* Now p points at element 0.  Fetch it.  */
  elt0 = *p;

  printf ("elt0 %d  elt3 %d  elt4 %d.\n",
          elt0, elt3, elt4);
  /* Prints elt0 45  elt3 -3  elt4 123456.  */
}

If one pointer value was made by adding an integer to another pointer value, it should be possible to subtract the pointer values and recover that integer. That works too in C.

void
subtract_pointers ()
{
  int array[5] = { 45, 29, 104, -3, 123456 };
  int *p0, *p3, *p4;

  int *p = &array[4];
  /* Now p points at element 4 (the last).  Save the value.  */
  p4 = p;

  --p;
  /* Now p points at element 3.  Save the value.  */
  p3 = p;

  p -= 3;
  /* Now p points at element 0.  Save the value.  */
  p0 = p;

  printf ("%d, %d, %d, %d\n",
          p4 - p0, p0 - p0, p3 - p0, p0 - p3);
  /* Prints 4, 0, 3, -3.  */
}

The addition operation does not know where arrays begin or end in memory. All it does is add the integer (multiplied by target object size) to the numeric value of the pointer. When the initial pointer and the result point into the same array, the result is well-defined.

Warning: Only experts should do pointer arithmetic involving pointers into different memory objects.

The difference between two pointers has type int, or long if necessary (see Integer Types). The clean way to declare it is to use the typedef name ptrdiff_t defined in the file stddef.h.

C defines pointer subtraction to be consistent with pointer-integer addition, so that (p3 - p1) + p1 equals p3, as in ordinary algebra. Pointer subtraction works by subtracting p1’s numeric value from p3’s, and dividing by target object size. The two pointer arguments should point into the same array.

In standard C, addition and subtraction are not allowed on void *, since the target type’s size is not defined in that case. Likewise, they are not allowed on pointers to function types. However, these operations work in GNU C, and the “size of the target type” is taken as 1 byte.