Undefined Behaviour & Memory Layout

Concept

A running C program's memory has segments: text (code), data (initialised globals), BSS (zeroed globals), heap (malloc, grows up) and stack (locals, grows down).

Undefined behaviour — out-of-bounds access, signed overflow, use-after-free, unsequenced modifications — lets the compiler do anything, and optimised builds often 'break' such code.

Compile with -Wall -Wextra -fsanitize=address,undefined and use gdb/Valgrind to catch problems early.

Cheat sheet

Code (C17)

#include <stdio.h>

int g_init = 5;   /* data */
int g_zero;       /* BSS  */

int *bad(void) {
    int local = 42;
    return &local;   /* UB: returns address of dead stack variable */
}

int main(void) {
    int a[3] = {1, 2, 3};
    /* a[3] = 4;  UB: out of bounds */
    printf("%d %d\n", g_init, g_zero);
    return 0;
}
/* gcc -Wall -Wextra -g -fsanitize=address,undefined app.c */

Remember

• Never return addresses of locals.
• Signed overflow is UB; unsigned wraps.
• Sanitizers catch bugs at run time.

Interview questions