Operating System Interview Questions

Question	Answer
What is the difference between a process and a thread?	A process is an independent program with its own memory space, whereas a thread is a smaller unit of a process, sharing the process's memory and resources. For example, a web browser (process) has multiple threads for loading different tabs simultaneously.
What is an Operating System kernel?	The kernel is the core component of an operating system that manages system resources and hardware communication. It acts as an intermediary between applications and hardware. For example, when you run a program like a web browser, the kernel handles memory allocation, process scheduling, and hardware resources, ensuring smooth execution. If the browser needs to use the CPU or read from disk, the kernel manages these resources on behalf of the browser.
What is the difference between a monolithic kernel and a microkernel?	A monolithic kernel is a single large kernel that handles multiple tasks, such as memory management and hardware interaction, in one unit. A microkernel, on the other hand, has a minimalistic approach, with only essential services running in kernel mode, while other services run in user mode. For example, Linux uses a monolithic kernel, meaning most of the operating system functionality is inside the kernel itself. In contrast, MINIX uses a microkernel where the kernel only handles low-level tasks, like IPC (inter-process communication), while other services run in user space.
What is a deadlock?	A deadlock occurs when two or more processes are blocked forever because each is waiting for the other to release resources. For example, Process A holds Resource X and waits for Resource Y, while Process B holds Resource Y and waits for Resource X. This leads to a deadlock, as neither process can proceed. The operating system needs to detect or prevent such situations to avoid system halts.
What is the Banker's algorithm in operating systems?	The Banker's algorithm is a resource allocation and deadlock avoidance algorithm that ensures the system remains in a safe state by allocating resources based on certain conditions. For example, suppose there are three processes and two resources (A and B). The Banker's algorithm checks if granting a resource request leaves the system in a safe state where all processes can complete without causing a deadlock. If it’s safe, the request is granted; otherwise, it’s denied.
What is virtual memory, and how does it work?	Virtual memory allows a computer to use hard drive space to simulate extra memory. This technique makes it appear that the computer has more RAM than it physically does. For example, if your computer has 4 GB of RAM and you open several applications that exceed this amount, the operating system moves some of the data from RAM to the swap space on the disk to free up memory. The program doesn't know the difference between real RAM and virtual memory, allowing it to run smoothly despite memory limits.
What is a context switch?	A context switch occurs when the CPU switches from executing one process or thread to another. It involves saving the state of the current process and loading the state of the next process. For example, in a multi-tasking system, when you switch between different applications like a web browser and a text editor, the operating system performs a context switch, saving the state of the browser process and restoring the state of the text editor process.
What is a semaphore?	A semaphore is a synchronization primitive used to control access to shared resources by multiple processes. There are two types: binary semaphores and counting semaphores. For example, a binary semaphore can be used to implement mutual exclusion in a critical section. If two processes attempt to access the same shared resource simultaneously, one is blocked until the semaphore is available, ensuring only one process accesses the resource at a time.
What is paging in operating systems?	Paging is a memory management technique that breaks memory into fixed-sized blocks called pages, allowing non-contiguous allocation of memory. For example, consider a system with 4 KB page size. If a program requires 12 KB of memory, the system will allocate 3 pages (each 4 KB) non-contiguously in memory. Paging eliminates fragmentation and allows the OS to efficiently use available memory by swapping pages between RAM and disk as needed.
What is a system call in operating systems?	A system call is a mechanism for programs to request services from the operating system. For example, the open() system call in Unix allows a program to request permission to open a file. Similarly, the fork() system call is used to create a new process. System calls provide an interface for programs to interact with hardware and OS resources securely.
What is a page fault?	A page fault occurs when a process accesses a page that is not currently loaded in memory. The operating system then loads the page from secondary storage (like a hard drive) into RAM. For example, if a program tries to access an array element but that page is not in memory, the OS will trigger a page fault, causing the required page to be loaded from the disk into RAM before the program can resume.
What is a context switch?	A context switch occurs when the CPU switches from executing one process to another. For example, when a high-priority process like a timer interrupt occurs, the CPU saves the state of the current process and loads the state of the new one.
What is virtual memory?	Virtual memory is a technique that allows the OS to use disk space as if it were RAM. For instance, when your computer runs out of RAM, it uses part of the hard disk as 'virtual memory,' allowing you to run more applications than your physical memory would otherwise allow.
What are system calls in an operating system?	System calls are functions that allow programs to interact with the OS. For example, the read() system call is used to read data from a file in Linux-based systems.
Explain the concept of deadlock in an operating system.	Deadlock occurs when two or more processes are blocked forever, each waiting for the other to release resources. For example, if Process A holds a printer and waits for a scanner, and Process B holds the scanner and waits for the printer, neither can proceed, causing a deadlock.
What are the four necessary conditions for deadlock to occur?	The four conditions are: Mutual Exclusion: Only one process can use a resource at a time. Hold and Wait: Processes holding resources are waiting for other resources. No Preemption: Resources cannot be forcibly removed from processes holding them. Circular Wait: A circular chain of processes exists, each waiting for a resource held by the next process.
What is a semaphore?	A semaphore is a variable used to control access to a shared resource in a concurrent system. For instance, a binary semaphore can be used to control access to a critical section in a multithreaded environment, allowing only one thread to execute it at a time.
What is paging in memory management?	Paging is a technique that divides memory into fixed-size blocks called pages. When a program runs, it is divided into pages that can be loaded into any available part of physical memory. For example, if a program is larger than the available RAM, it can be broken into pages and swapped in and out as needed.
What is thrashing?	Thrashing occurs when the system spends most of its time swapping data between the disk and RAM, causing poor performance. For example, when you run too many applications simultaneously, and your computer starts swapping data continuously, it can lead to thrashing.
What is a file system?	A file system is a method used by the OS to store and organize files on a storage device. For example, the NTFS file system in Windows allows the operating system to manage how data is stored, retrieved, and organized on the disk.
What is the difference between a monolithic kernel and a microkernel?	A monolithic kernel includes all the operating system services within one large block of code, while a microkernel includes only essential services. For example, Linux uses a monolithic kernel, whereas MINIX uses a microkernel design.
What is the role of the operating system scheduler?	The OS scheduler decides which process should run next on the CPU. For example, in round-robin scheduling, the scheduler allocates CPU time to each process in a cyclic order, allowing fair sharing of CPU time among all processes.
What is the difference between preemptive and non-preemptive scheduling?	Preemptive scheduling allows the OS to interrupt a running process and allocate the CPU to another process, while non-preemptive scheduling lets a process run to completion before switching. For instance, in preemptive scheduling, a higher-priority task might interrupt a running lower-priority task.
What is an interrupt?	An interrupt is a signal to the processor that an immediate action is required. For example, when you press a key on the keyboard, an interrupt is generated, and the OS processes it by interpreting the key pressed.
What is a file descriptor?	A file descriptor is an integer that represents an open file in the operating system. For example, when you open a file in a C program using open(), the returned integer is the file descriptor.
What is the difference between a process and a program?	A program is a collection of instructions, while a process is a program in execution. For example, when you run a Python script, the script is a program, and the execution of the script is a process.
What is a race condition?	A race condition occurs when two or more processes access shared data and try to change it at the same time. For example, if two threads try to increment the same counter variable simultaneously without proper synchronization, it might result in an incorrect value due to inconsistent access.
What are the types of file access methods?	The common types are: Sequential Access: Data is read or written in a linear sequence, like a tape drive. Direct Access: Data can be accessed randomly, as in the case of a hard disk. Indexed Access: A special index file maps data blocks for quicker access.
What is a memory leak?	A memory leak occurs when a program allocates memory but fails to release it after use, causing a gradual decrease in available memory. For example, in a C program, forgetting to free memory allocated with malloc() leads to a memory leak.
What is load balancing in the context of operating systems?	Load balancing involves distributing processes or tasks across multiple processors or machines to optimize resource utilization and prevent overload on any single resource. For example, in a cloud-based server farm, load balancing ensures that no server is overloaded while others remain idle.
What is a critical section?	A critical section is a part of the program where shared resources are accessed, and it must be protected to prevent data inconsistency. For example, when two threads attempt to update a shared counter, the critical section ensures only one thread accesses the counter at a time.
Explain the concept of multi-threading in operating systems.	Multi-threading allows multiple threads to execute concurrently within a single process. For example, a web server uses multi-threading to handle multiple client requests simultaneously, improving performance and responsiveness.
What is a zombie process?	A zombie process is a process that has completed execution but still has an entry in the process table, usually because its parent has not read its exit status. For example, when a child process finishes execution, it becomes a zombie until the parent process acknowledges its termination.
What is the difference between internal and external fragmentation?	Internal fragmentation occurs when memory allocated to a process is larger than the memory it actually needs, leaving gaps inside allocated memory. External fragmentation occurs when free memory is scattered in small blocks, making it difficult to allocate large contiguous blocks. For example, allocating several small memory blocks may leave fragmented space between them.
What is a memory-mapped file?	A memory-mapped file is a file that is mapped into the virtual memory address space of a process, allowing the file to be read and written as if it were part of the process's memory. For example, a large database file can be memory-mapped to improve access speed by directly accessing it in memory.
What are the different types of system calls?	There are three primary types of system calls: Process control: For managing processes (e.g., fork(), exec()) File management: For file operations (e.g., open(), read()) Device management: For device input/output (e.g., ioctl(), write())
What is a fork() system call?	The fork() system call creates a new process by duplicating the calling process. The new process is a child process, and the original process is the parent. For example, when a process calls fork(), it creates a copy of itself to perform tasks simultaneously.
What is the function of the operating system's I/O scheduler?	The I/O scheduler manages the sequence in which I/O requests are serviced. For example, the CFQ (Completely Fair Queuing) scheduler in Linux ensures that I/O requests from different processes are distributed fairly and efficiently.
What is the role of the disk scheduler?	The disk scheduler decides the order in which I/O requests for data on the disk are processed. For example, the Shortest Seek Time First (SSTF) algorithm minimizes seek time by selecting the disk request closest to the current head position.
What are soft and hard limits in resource management?	Soft limits are user-set thresholds that can be increased within the system's allowed range, whereas hard limits are the maximum values that cannot be exceeded. For example, a user may set a soft limit on memory usage, but the system imposes a hard limit on how much memory can be allocated.
What is the difference between a process control block (PCB) and a task control block (TCB)?	A Process Control Block (PCB) stores all information about a process, such as its state, program counter, registers, and memory management information. A Task Control Block (TCB) is used in multi-threaded systems to manage threads, including thread ID and state.
What is a user mode and kernel mode?	User mode is a restricted mode in which user applications execute, while kernel mode is where the operating system kernel runs and has full access to hardware resources. For example, when a program makes a system call, the CPU switches from user mode to kernel mode to execute the call.
What is the difference between FIFO and LRU page replacement algorithms?	FIFO (First In, First Out) replaces the oldest page in memory, while LRU (Least Recently Used) replaces the page that hasn't been used for the longest time. For example, in FIFO, the first page to enter memory will be the first one to be replaced, while LRU tracks page usage to replace the least accessed page.
What is a file system hierarchy?	A file system hierarchy is a tree-like structure where files are organized into directories. The root directory is the topmost directory, and all other files and directories are structured beneath it. For example, in Linux, the root directory is represented as /, with directories like /home, /usr, and /etc.
What are threads and how do they differ from processes?	Threads are smaller units of execution within a process. Unlike processes, threads share the same memory space and resources. For example, in a web browser, each tab might run in its own thread, sharing resources such as the browser window, but operating independently.
Explain the concept of demand paging.	Demand paging is a technique where pages of a process are only loaded into memory when they are accessed. For example, if a program needs to read data from a specific file, that part of the file is brought into memory only when it is required.
What is an inotify system in Linux?	Inotify is a Linux kernel subsystem that provides file system event monitoring. For example, it can notify a program when a file is modified or deleted, allowing real-time updates for file monitoring applications.
What is the purpose of the exec() system call?	The exec() system call replaces the current process's memory with a new program. For example, when a shell runs a command, it uses exec() to load the new program, replacing the shell's execution context.
What is the difference between swap space and virtual memory?	Swap space is a part of the disk that the OS uses as an extension of physical memory, while virtual memory is an abstraction that allows programs to use more memory than is physically available. Swap space is typically used to store less frequently used data to free up RAM.
What is a race condition in the context of a critical section?	A race condition occurs when multiple processes or threads access shared resources concurrently, and the final outcome depends on the order in which the operations occur. For example, if two threads increment the same counter without synchronization, the final value could be incorrect.