Question 24

Let us analyze each function:

• Option A (exit): The standard library exit() flushes all open stdio streams, invokes functions registered with atexit(), and must terminate the process. Process termination can only be performed by the kernel; hence it must always invoke a system call (exit_group or exit). (Correct)
• Option B (malloc): The memory allocator maintains a heap memory pool in user space. If there is sufficient free space in the heap arena, malloc() satisfies the request directly in user space without requesting more memory from the kernel via brk() or mmap(). Hence, it does not always invoke a system call. (Incorrect)
• Option C (sleep): Suspending a process requires relinquishing the CPU and asking the operating system scheduler to wake the process after the requested duration. This requires a timer-based system call such as nanosleep() or clock_nanosleep(). Hence, it always invokes a system call. (Correct)
• Option D (strlen): Simply counts characters in a string in user-space memory until the null terminator is encountered. It never invokes a system call. (Incorrect)

Thus, options A and C are correct.