Pointers store memory addresses. They enable dynamic memory, efficient array passing, and direct hardware access — and cause most C bugs when misused.

Basics 

  int x = 42;
int *p = &x;       /* p holds address of x */
printf("value: %d\n", *p);
*p = 100;          /* modifies x */
printf("x: %d\n", x);

Pointer Arithmetic 

  int arr[] = {10, 20, 30, 40};
int *p = arr;      /* points to arr[0] */

printf("%d\n", *(p + 1));   /* 20 */
printf("%d\n", p[2]);       /* 30 — equivalent syntax */

/* Difference of pointers: number of elements between them */
int *end = arr + 4;
printf("%td\n", end - p);   /* 4 */

Pointers and Functions 

  void scale(int *arr, size_t n, int factor) {
    for (size_t i = 0; i < n; i++)
        arr[i] *= factor;
}

int main(void) {
    int data[] = {1, 2, 3};
    scale(data, 3, 10);  /* data becomes {10, 20, 30} */
}

Null and Void Pointers 

  int *p = NULL;
if (p != NULL)
    printf("%d\n", *p);

void *generic = malloc(64);
int *ip = (int *)generic;   /* cast before use */
free(generic);

const with Pointers 

  const int *p1;         /* data read-only, pointer movable */
int *const p2 = &x;    /* pointer fixed, data writable */
const int *const p3;   /* both fixed */

/* const char * = pointer to const char (common for strings) */
void print(const char *s) { puts(s); }

Common Pitfalls

  • Treating compiler warnings as optional rather than actionable feedback.
  • Skipping error checks on library and system calls.
  • Copy-pasting examples without adapting to your project’s conventions.

Best Practices

  • Enable strict compiler warnings and fix them before merging.
  • Write small, testable units with clear input/output contracts.
  • Document non-obvious invariants and preconditions.
  • Use version control and code review for every change.

Memory and Performance Notes

Pointer size matches address bus width (4 bytes on 32-bit, 8 on 64-bit). Dereferencing invalid pointers is undefined behavior.

Exercise

Implement void *memset_custom(void *s, int c, size_t n) that sets n bytes to value c without using string.h.

Hint: Cast void* to unsigned char* for byte-level access.

Summary

Apply these concepts in small programs before moving to larger projects. Combine with adjacent topics in the learning path for deeper mastery.

Real-World Application

These concepts appear in production codebases — from operating system kernels to embedded firmware. Study open-source projects that use this topic extensively to see idiomatic patterns at scale.

Debugging Checklist

  1. Reproduce the issue with the smallest possible input.
  2. Enable compiler warnings and sanitizers.
  3. Use a debugger to inspect state at the failure point.
  4. Verify assumptions about types, sizes, and return values.
  5. Compare working and broken code paths side by side.
  6. Write a regression test once the bug is fixed.

Further Reading

Consult the ISO C standard, Effective C by Robert C. Seacord, and your compiler documentation for platform-specific behavior.

Quick Reference

Review the code examples on this page before starting the exercise. Type them manually to build muscle memory.

Additional Examples

Consider how this topic applies in a larger project:

  // Break the problem into smaller functions
// Test each function independently
// Integrate incrementally

Working through variations of the examples above builds deeper understanding than reading alone.

Interview and Review Questions

  1. Explain the core concept of this topic in your own words.
  2. What happens when this code runs with edge-case input (empty, null, zero, max value)?
  3. How would you debug a bug related to this topic in production?
  4. What are the performance implications of the approach shown here?
  5. How does this feature compare to the equivalent in another language you know?