Double Pointers

Quick Reference

Double pointer patterns:

// Modify a caller's pointer
void alloc(int **pp) {
    *pp = calloc(10, sizeof(int));
}
int *arr = NULL;
alloc(&arr);  // arr is now allocated

// 2D array allocation
int **grid = calloc(rows, sizeof(int *));
for (int i = 0; i < rows; i++)
    grid[i] = calloc(cols, sizeof(int));

Indirection levels:

Common Pitfalls

• Modifying a copy – void f(int *p) { p = malloc(...); } changes a local copy. Use int ** to change the caller’s pointer.
• Forgetting to free rows – 2D arrays need each row freed before the row-pointer array.
• Dereferencing NULL – Always check that *pp is not NULL before accessing **pp.