C

Quick Reference

Critical Rules

• malloc returns void* — cast required in C++, optional in C but check for NULL
• free(ptr); ptr = NULL; — always null after free to prevent double-free
• sizeof(array) in function gives pointer size, not array size — pass length separately
• char str[5] = "hello"; — no room for null terminator, UB when used as string
• strcpy doesn't check bounds — use strncpy and manually null-terminate
• Signed overflow is UB — compiler can optimize assuming it never happens
• i++ + i++ is UB — no sequence point between modifications
• Returning pointer to local variable — dangling pointer, UB on use
• #define SQUARE(x) x*x — SQUARE(1+2) = 1+2*1+2 = 5, not 9
• memcpy with overlapping regions — use memmove instead
• Uninitialized variables — contain garbage, UB if used
• Array out of bounds — no runtime check, silent corruption or crash