Which of the following standard C library functions will always invoke a system call when executed from a single-threaded process in a UNIX/Linux operating system?

Let us analyze the behavior of each library function in a UNIX/Linux environment: [cite: 1]

1. exit: After running registered exit handlers (atexit) and flushing open stdio streams, the exit() library function must terminate the calling process, which strictly requires invoking a kernel termination system call such as exit_group() or _exit(). Hence, it always invokes a system call.

2. malloc: The standard memory allocator manages a heap pool (arena) in user space. If a memory request can be satisfied from existing free chunks in the arena, no system call is made. A system call (brk() or mmap()) is only invoked when the user-space heap needs to be expanded.

3. sleep: To suspend the calling thread/process for a requested duration, user space cannot manage hardware timers independently; it must request the kernel to pause execution via system calls such as nanosleep() or clock_nanosleep(). Hence, it always invokes a system call.

4. strlen: Calculates the length of a string entirely in user-space memory by iterating through characters until the null terminator \0 is found. It never invokes a system call.

Therefore, the correct options are A and C [cite: 1].