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c 4/5 40 XP

Pointers

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Quick Reference

Pointer operations:

Operation Syntax Meaning
Declare int *p; p is a pointer to int
Address-of p = &x; p now holds x’s address
Dereference *p Follow p to access the value
Write through *p = 42; Write 42 to where p points
NULL check if (p != NULL) Verify before dereferencing
Print address printf("%p", (void *)p) Show the hex address

The mental model:

  Variable     Address      Value
  ┌──────┐
  │  x   │  0x1000  →   [  42  ]
  └──────┘
  ┌──────┐
  │  p   │  0x1008  →   [0x1000]  ──→ points to x
  └──────┘

  *p reads/writes the value at 0x1000 (which is x)

Declaration vs. dereference:

Context * means Example
Declaration “pointer to” int *p; → p is a pointer to int
Expression (right of =) “read through” y = *p; → read what p points to
Expression (left of =) “write through” *p = 42; → write to where p points

Common Pitfalls

  • Uninitialized pointers — int *p; *p = 42; writes to a garbage address. Always initialize to &variable or NULL.
  • Dangling pointers — After free(p), p still holds the old address. Set p = NULL immediately.
  • Forgetting & in scanf — scanf("%d", x) passes the value of x, not its address. Use scanf("%d", &x).
  • Confusing * in declarations vs. expressions — int *p declares a pointer. *p in code dereferences it. Same symbol, different meaning.
  • == on pointers — Compares addresses, not values. p == q checks if they point to the same location, not if the pointed-to values are equal.

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