GATE CS 2021 Set 1 — Question 24

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MSQ+1 / -0MediumSystem CallsOS Structure & System CallsOperating System

Operating System → OS Structure & System Calls → System Calls

Last updated 5 September 2026

Question

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?

Correct answer (A) exit; (C) sleep Solution A system call is a mechanism for a user-level process to request a service from the operating system's kernel. (A) exit: The exit() function terminates the process. This requires kernel intervention to deallocate resources (memory, file descriptors, process control block, etc.) and update system-wide data structures. Therefore, exit() must make a system call (e.g., exit or exit_group in Linux). (B) malloc: The malloc() function is a C library function for dynamic memory allocation. It manages a region of memory on the heap for the process. It requests large chunks of memory from the kernel using system calls like brk or mmap and then dispenses smaller pieces to the application from this pool. If a suitable free block is already available in its managed pool, malloc() can satisfy a request without making a system call. A system call is only made when the library's internal pool is exhausted and needs to be expanded. Thus, malloc() does not always invoke a system call. (C) sleep: The sleep() function suspends the process's execution for a specified duration. Process scheduling (putting a process to sleep and waking it up) is a fundamental kernel responsibility. The process must make a request to the kernel to be removed from the ready queue and be woken up later. This always requires a system call (e.g., nanosleep). (D) strlen: The strlen() function computes the length of a string. The string resides in the process's own address space. The function simply reads memory from the starting address until it finds a null terminator. This entire operation is performed in user space and does not require any kernel services. Thus, strlen() never invokes a system call. Therefore, exit and sleep are the functions that will always invoke a system call. Turn this into a strength. Explore AI-powered practice and doubt support with Success Tracker.

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Correct answer (A) exit; (C) sleep Solution A system call is a mechanism for a user-level process to request a service from the operating system's kernel. (A) exit: The exit() function terminates the process. This requires kernel intervention to deallocate resources (memory, file descriptors, process control block, etc.) and update system-wide data structures. Therefore, exit() must make a system call (e.g., exit or exit_group in Linux). (B) malloc: The malloc() function is a C library function for dynamic memory allocation. It manages a region of memory on the heap for the process. It requests large chunks of memory from the kernel using system calls like brk or mmap and then dispenses smaller pieces to the application from this pool. If a suitable free block is already available in its managed pool, malloc() can satisfy a request without making a system call. A system call is only made when the library's internal pool is exhausted and needs to be expanded. Thus, malloc() does not always invoke a system call. (C) sleep: The sleep() function suspends the process's execution for a specified duration. Process scheduling (putting a process to sleep and waking it up) is a fundamental kernel responsibility. The process must make a request to the kernel to be removed from the ready queue and be woken up later. This always requires a system call (e.g., nanosleep). (D) strlen: The strlen() function computes the length of a string. The string resides in the process's own address space. The function simply reads memory from the starting address until it finds a null terminator. This entire operation is performed in user space and does not require any kernel services. Thus, strlen() never invokes a system call. Therefore, exit and sleep are the functions that will always invoke a system call.

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GATE CS 2021 Set 1 — Question 24

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