C Memory Management Cheat Sheet

Covers dynamic memory allocation, deallocation, common pitfalls like leaks and dangling pointers, and debugging tools for safe C memory management.

2 PagesIntermediateMar 25, 2026

malloc, calloc, realloc, free

Core heap allocation functions from stdlib.h.

#include <stdlib.h>int *arr = malloc(10 * sizeof(int));   // Allocate 10 ints (uninitialized)if (arr == NULL) {                     // Always check for allocation failure    // handle error}int *zeroed = calloc(10, sizeof(int)); // Allocate 10 ints, zero-initializedarr = realloc(arr, 20 * sizeof(int));  // Resize to 20 ints (may move the block)free(arr);                             // Release memory back to the heaparr = NULL;                            // Avoid leaving a dangling pointer

Stack vs Heap

Where variables live and who is responsible for freeing them.

void stack_example(void) {    int x = 42;          // Stored on the stack, freed automatically on return    static int y = 0;    // Stored in the static/data segment, persists across calls}void heap_example(void) {    int *p = malloc(sizeof(int));  // Stored on the heap, lives until free()    *p = 42;    free(p);                       // Caller is responsible for freeing}

Common Pitfalls

Memory bugs that are easy to introduce and hard to debug.

Memory leak- Forgetting to free() heap memory before losing the last pointer to it
Dangling pointer- Using a pointer after the memory it points to has been freed
Double free- Calling free() twice on the same pointer; corrupts the heap allocator
Buffer overflow- Writing past the bounds of an allocated block, corrupting adjacent memory
Use-after-free- Reading or writing memory through a pointer after free() was called
Uninitialized read- Reading a malloc'd block before writing to it (malloc does not zero memory)
Alignment- malloc guarantees alignment suitable for storing any standard C type

Debugging with Valgrind

Detect leaks and invalid memory access at runtime.

bash

gcc -g -o app app.c                 # Compile with debug symbolsvalgrind --leak-check=full ./app    # Detect leaks and invalid accessvalgrind --track-origins=yes ./app  # Trace origin of uninitialized values

Pro Tip

Set pointers to NULL immediately after free() so accidental reuse crashes predictably instead of causing silent heap corruption that surfaces far from the actual bug.

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