Question 1

What is a function pointer in C and when is it useful?

Accepted Answer

A pointer that stores the address of a function, enabling dynamic dispatch and callbacks. int (*fp)(int, int) = &add; fp(2, 3) calls add(). Used in callbacks (e.g., qsort's comparator), plugin systems, state machines, and implementing vtables for OOP-like behavior in C.

Question 2

What is undefined behavior (UB) in C?

Accepted Answer

Code whose behavior the C standard does not define — the compiler may do anything: crash, produce wrong output, or appear to work. Common UB: signed integer overflow, dereferencing NULL, reading uninitialized memory, out-of-bounds access. The compiler may optimize assuming UB never occurs — leading to baffling bugs. Use -fsanitize=address,undefined to detect it.

Question 3

What does the 'volatile' keyword do in C?

Accepted Answer

Tells the compiler not to optimize accesses to this variable since it may change unexpectedly (e.g., hardware registers, signal handlers). Without volatile, the compiler may cache the variable in a register and skip re-reading it. With volatile, every read/write goes to memory. Used for memory-mapped I/O, shared ISR variables, and spinlocks.

Question 4

What does the '->' operator do in C?

Accepted Answer

Accesses a member of a struct or union through a pointer (shorthand for (*ptr).member). ptr->field is equivalent to (*ptr).field. It dereferences the pointer and accesses the member in one step. Used extensively when working with linked lists, trees, and any pointer-to-struct pattern.

Question 5

What is a race condition in concurrent C programs?

Accepted Answer

A bug where two threads access shared data concurrently and the outcome depends on the unpredictable order of execution. If two threads read-modify-write a shared counter without a lock, increments can be lost. Fix with mutexes (pthread_mutex), atomic operations (__atomic_*), or lock-free data structures.

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