Cheat Sheet

C Programming Quick Reference

Essential C syntax, memory management, and common patterns for systems programming.

Type	Size (bytes)	Range	Format
char	1	-128 to 127	%c
unsigned char	1	0 to 255	%c
short	2	-32,768 to 32,767	%hd
int	4	±2.1×10⁹	%d
long	8	±9.2×10¹⁸	%ld
long long	8	±9.2×10¹⁸	%lld
float	4	±3.4×10³⁸	%f
double	8	±1.7×10³⁰⁸	%lf
void*	8	—	%p
size_t	8	0 to 2⁶⁴−1	%zu

malloc — allocate uninitialized memory

int* arr = (int*)malloc(n * sizeof(int));
if (!arr) { perror("malloc"); exit(1); }

calloc — allocate zero-initialized memory

int* arr = (int*)calloc(n, sizeof(int));

realloc — resize existing allocation

int* tmp = (int*)realloc(arr, new_n * sizeof(int));
if (!tmp) { free(arr); return NULL; }
arr = tmp;

free — deallocate memory

free(arr);
arr = NULL; // prevent dangling pointer

•Buffer overflow: gets() is banned. Use fgets(buf, size, stdin).
•Dangling pointers: Set pointers to NULL after free().
•Integer overflow: Use bounds checking before arithmetic on untrusted input.
•Undefined behavior: Signed integer overflow is UB. Use unsigned for wraparound.
•String literals are read-only: char* s = "hello"; s[0] = 'H'; crashes.
•sizeof on pointers: sizeof(arr) on a function parameter gives pointer size, not array size.