B. Undefined-behavior and pitfalls cheat sheet

• Signed overflow is UB; unsigned wraps predictably — Section 3.3, 28.
• arr[i] is *(arr + i); there is no bounds check, ever — Section 6.1.
• Arrays decay to pointers on function-call; sizeof on a decayed pointer gives the pointer's size, not the array's — Section 6.1.
• Never return a pointer to a local (stack) variable — Section 5.1.
• strcpy/strcat/sprintf/gets have no bound; prefer snprintf/strlcpy/fgets — Section 7.2, 29.
• malloc's return value is not guaranteed non-NULL; always check — Section 10.1.
• Match every malloc/strdup/_new with exactly one free/_free; never twice, never after — Section 10.2, 10.3.
• free(NULL) is always safe; make your own _free functions mirror that — Section 10.3.
• Data races are UB, not just "unpredictable" — the compiler may assume they don't happen — Section 20.1, 28.
• Format-string/argument mismatches in printf/scanf are UB — Section 3.2, 22.1.
• va_arg's type must match what the caller actually passed; there is no runtime check — Section 22.1.