System Calls in OS: Different Types Explained

System calls in an operating system are predefined interfaces that enable user-space applications to request essential services from the OS kernel. They play a critical role in file handling, process creation, and network communication, ensuring secure and efficient interactions with hardware. 

Over time, system call implementations have evolved across various OS architectures, including Linux, Windows, and macOS, adapting to the complex computing needs of these platforms. Despite architectural differences, their core function remains the same, bridging user applications with kernel-level operations. 

This blog explores the types of system calls, their internal mechanisms, and their role in enabling low-level communication in modern operating systems.

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What is a System Call in OS?  

System Calls in programs can ask the operating system (OS) for services through a system call. Puting it more simply, it’s a means via which a program can communicate with the underlying system, for example, by gaining access to hardware resources or by using privileged commands. 

A program can request a service from the OS by executing a specified instruction, which sets off a transition to kernel mode and initiates a system call. After handling the request and carrying out the required actions, the OS sends the outcome back to the program.

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System calls is the connection through which the process communicates with the system calls. Computer operates two types of modes: User Mode and the Kernel Mode. The process runs in a user mode, and when the system call is performed, the mode changes to kernel mode. Once the system call is done, control is returned to the process in user mode. 

Let’s break down both of these modes' execution process:

1. User Mode: This is where regular applications like browsers, editors, and games run with limited privileges. Programs in user mode can’t directly access hardware or critical system components, which protects the OS from crashes or security threats. When tasks like file access or memory allocation are needed, these programs must make system calls to request the OS’s help. This isolation between user programs and the OS core ensures better stability and security.
   
   Example: Imagine you're using a photo editing app and decide to save your edited image. While you're interacting with the app in user mode, the moment you hit “Save,” the app sends a system call to the OS to write the file to disk. The actual file operation doesn't happen in user mode; it transitions to kernel mode briefly to handle the save request securely.
2. Kernel Mode: This is the privileged mode where the operating system’s core functions run with full access to hardware and memory. When a system call is triggered, the process switches to kernel mode to perform tasks like file handling, process control, or hardware communication. Since any error here can impact the entire system, access is tightly controlled. Only trusted, validated calls are allowed to run in this mode.
   
   Example: When you download a file in your browser, it sends a system call to ask the OS to save the file. The OS switches to kernel mode to allocate space and write the data to your drive. Once done, it returns control to the browser running in user mode.

Also Read: Difference Between Process and Program

Now that you have explored what system calls in OS and their modes, let’s look at some real-world examples of system calls in action below.

Example of System Calls in OS 

System calls have evolved into platform-specific implementations that optimize how operating systems handle hardware, security, and process control. Unix operating systems like Linux and macOS support hundreds of system calls. Linux has over 300, and FreeBSD exceeds 500, covering everything from file access to interprocess communication. 

Windows, on the other hand, splits its system calls between the core (ntdll) and graphical (win32k) subsystems, with nearly 2,000 distinct calls. This division allows Windows to manage system functions more precisely based on application needs.

Below is a table highlighting key system calls, roles, and real-world examples across major operating systems to give you a clearer picture of how they function.

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Now that you understand the types and examples of system calls, let’s look at how they are actually invoked by programs during execution.

How are System Calls Made? 

When computer software wants to access the kernel of the operating system, it makes a system call. The system call in OS employs an API to make the operating system’s services available to user programs. It is the only way to reach the kernel system. All applications or processes that require resources to run must use system calls, which act as a link between the operating system and user programs. 

Here are some examples of how a system call differs from a user function. 

• A system call function can generate and use kernel processes to perform asynchronous processing. 
• A system call has more authority than a regular subroutine. A system call with kernel-mode privilege runs in the kernel protection domain. 
• System calls are not allowed to use shared libraries or symbols that are not part of the kernel protection domain. 
• The code and data for system calls are kept in global kernel memory.  

To truly understand how system calls in OS functions, it is first important to grasp their importance. Let’s understand this in the section below. 

Importance of System Calls in OS 

1. open(): Opens a file and returns a file descriptor. Multiple processes can access the same file, depending on the file system's rules.
2. read(): Reads data from a file into a buffer.
Arguments:

• File descriptor
• Buffer to store data
• Number of bytes to read

 3.  write(): Writes data from a buffer to a file or device.
Arguments:

• File descriptor
• Buffer containing data
• Number of bytes to write

4. wait(): Used by a parent process to pause execution until a child process finishes. Once the child terminates, control returns to the parent.
5. fork(): Creates a new process (child) by duplicating the calling process (parent). Both run concurrently unless explicitly synchronized using wait().
6. exit(): Terminates a process or thread. It signals the OS to clean up and reclaim any allocated resources.

After understanding what system calls are and their importance, now it's time to explore different types to see how they support core operating system functions. 

Types of System Calls in OS

Understanding these system call types and their real-world applications provides insight into how operating systems facilitate various fundamental operations, ensuring efficient and secure interactions between software and hardware components. 

Let’s explore them one by one below:

1. Process Control

Process control system calls manage the creation, execution, and termination of processes. They are fundamental for multitasking and process management within an OS.

Common System Calls:

• fork() (Unix/Linux): Creates a new process by duplicating the calling process.
• exec() (Unix/Linux): Replaces the current process image with a new process image.
• CreateProcess() (Windows): Creates a new process and its primary thread.
• exit() (Unix/Linux) / ExitProcess() (Windows): Terminates a process.
• wait() (Unix/Linux) / WaitForSingleObject() (Windows): Waits for a process to change state.

For Example: When you open a new tab in a web browser, the browser may create a new process to handle that tab. This isolation ensures that if one tab crashes, it doesn't affect the others. The browser uses process control system calls like fork() and exec() (or their Windows equivalents) to manage these processes.

2. File Management

File management system calls handle operations related to file creation, deletion, reading, writing, and permission management. They are essential for data storage and retrieval.

Common System Calls:

• open() / CreateFile() (Windows): Opens or creates a file.
• read() / ReadFile() (Windows): Reads data from a file.
• write() / WriteFile() (Windows): Writes data to a file.
• close() / CloseHandle() (Windows): Closes an open file descriptor or handle.
• unlink() / DeleteFile() (Windows): Deletes a file.

For Example: When you save a document in a text editor, the application uses system calls like open() to create or open the file, write() to save the content, and close() to close the file after saving.

3. Device Management

Device management system calls allow programs to interact with hardware devices, facilitating operations like reading from or writing to devices and managing device attributes.

Common System Calls:

• ioctl() (Unix/Linux): Controls device parameters.
• read() / write(): Reads from or writes to a device.
• open() / close(): Opens or closes a device.

For Example: When you print a document, the operating system uses device management system calls to communicate with the printer, sending the document data and managing the printing process.

4. Information Maintenance

Information maintenance system calls retrieve and set system data and attributes, such as system time, process information, and system configuration.

Common System Calls:

• getpid() / GetCurrentProcessId() (Windows): Retrieves the process ID.
• gettimeofday() / GetSystemTime() (Windows): Retrieves the current time.
• uname() (Unix/Linux): Retrieves system information.

For Example: When a logging application records the time of an event, it uses system calls like gettimeofday() to obtain the current system time for accurate timestamping.

5. Communication

Communication system calls enable processes to exchange information, either within the same system or over a network, facilitating inter-process communication (IPC).

Common System Calls:

• pipe() (Unix/Linux): Creates a unidirectional data channel for IPC.
• socket(): Creates an endpoint for communication.
• send() / recv(): Sends or receives messages over a socket.
• shmget() / mmap(): Allocates shared memory segments. 

For Example: When you use a messaging app to send a message, the application uses communication system calls to transmit your message over the network to the recipient's device.

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Now that you’ve explored what system calls are and their various types, let’s understand how they actually work behind the scenes during program execution.

Working on System Calls in OS   

System calls serve as the gateway between user programs and the operating system’s core. When an application needs access to resources like files, memory, or hardware, it doesn’t interact with the system directly; instead, it issues a system call. This request follows a specific sequence involving mode switching, validation, and execution by the kernel. 

In this section, you’ll learn how a system call is triggered, processed, and completed, step by step.

1. Limited Power: Programs sometimes need resources they can’t access directly, like reading files, writing data, interacting with hardware, or requesting memory.
2. Making the Request: Special instructions called “system calls” exist within the program’s code to ask the operating system (OS) for help.
3. OS to the Rescue: When the OS encounters a system call, it recognizes the program’s need and temporarily pauses its execution. Control then shifts to the kernel, a central part of the OS responsible for core operations.
4. Kernel in Action: The kernel takes over, carrying out the requested operation. Imagine the kernel as a powerful assistant handling tasks like reading file content or allocating memory.
5. Back to Business: Once the task is complete, the kernel returns control to the program, allowing it to resume execution with the needed resources or information.

This process highlights how system calls safely bridge user programs with core OS functions, ensuring controlled access to system resources.

Now that you understand how system calls work, let’s explore the key advantages they offer in managing resources and ensuring efficient system performance.

Advantages of System Calls in OS

System calls in the OS provide controlled access to system resources, ensuring that applications can perform critical tasks. Below are some of the advantages of system calls to help you understand their crucial contribution to the functionality of the  OS. 

• Hardware Control: Forget direct hardware access hassles! System calls empower programs to interact with essential components like disk drives, printers, and network devices. 
• Memory Mastery: System calls grant programs the ability to request and release memory, ensuring efficient resource allocation. They also facilitate access to hardware devices mapped directly into memory for seamless data transfer. 
• Process Powerhouse: Need to create or terminate processes? System calls are your command centre. They also enable programs to communicate with each other, fostering collaboration within your system. 
• Security Sentinel: System calls act as gatekeepers, ensuring programs only access resources they’re authorized to. This safeguards your system from unauthorized modifications and risky operations. 
• Standardized Synergy: No matter the hardware or operating system version, system calls provide a consistent interface for programs to interact with the underlying environment. This promotes compatibility and smooths development across different platforms. 

Conclusion

System calls in an operating system act as the vital bridge between user applications and the kernel, enabling secure access to hardware and core system functions like file handling, memory management, and process control. They perform critical tasks behind the scenes, but mastering them requires solid systems programming skills and a deep understanding of OS architecture.

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