CSCI 255 Pointers in C

Pointers and arrays

• C: Lvalues and Rvalues
• Java: Objects and primitive data types
• & — address-of operator
• * — indirection (or dereference) operator
• If P is an array of integers, then P is a constant pointer to an integer.
• Pointer arithmetic — The follow rules formally apply only when the pointers address lvalues within the same array
  • P[i] is *(P+i).
  • If P is a pointer to an integer, then P+i is a pointer to the i’th integer after the integer referenced by P.
  • If P is a pointer to an integer, then P-i is a pointer to the i’th integer before the integer referenced by P.
  • P++ increments p to point to the next integer in memory.
  • P-- decrements p to point to the previous integer in memory.
• NULL in C is similar to null in Java.
• Pascal had pointers: @P for &P and P^ for P* .

It only gets worse

Suppose both P and Q are pointers to integers.

• P+i and P-i and P<Q and P>Q and P++ and P-- and P-Q are formally defined only if the relevant pointers are elements of the same array
  • or the non-allocated location one past the end of the array
• P + Q is not allowed for pointers
• void * is the type of a generic pointer
• * (long *) (void *) &P[5] is weird but implies intent
• (long) (void *) &P[5] is (long) (void *) P + 5 * sizeof(*P)
• (long) (void *) &P[5] - (int) (void *) &P[3] is 2 * sizeof(*P)

Take a look at the following.

• qsort man page
• illustrative program that may be run at GeeksforGeeks

Trying it out

Take a look at Problem 16 from the Fall 2014 Final exam.