A System call is a programmatic way for a user application to request a service from the operating system's kernel. These calls are fundamental components of an operating system that provide an interface for user programs to request services from the kernel.

They enable user applications to perform operations that require higher privileges, such as accessing hardware, managing files, and creating or terminating processes. This mechanism is crucial for maintaining system security and stability while allowing applications to function effectively.

Characteristics of System Calls: -

1. User-Kernel Boundary: System calls provide a controlled gateway for user programs to interact with the kernel, ensuring that user programs cannot directly access sensitive system resources.
2. Mode Switching: When a system call is invoked, the CPU switches from user mode to kernel mode. After the system call is executed, the control returns to user mode.
3. Error Handling: System calls can return error codes to indicate the success or failure of the requested operation, allowing programs to handle issues appropriately.
4. Synchronous and Asynchronous Operations: Most system calls operate synchronously, meaning the calling process is blocked until the operation completes. However, some system calls can be asynchronous, allowing the process to continue executing while waiting for the operation to finish.

System Call Interface

The system call interface (SCI) is the set of functions and protocols that define how user applications interact with the operating system through system calls. It provides a consistent way for applications to request services from the kernel, abstracting the complexities of hardware interactions.

Features of the System Call Interface: -

1. Abstraction: The SCI abstracts the underlying hardware details, allowing developers to write applications without needing to manage hardware-specific code.
2. Consistency: The interface ensures that the same system calls can be used across different hardware platforms, enhancing portability.
3. Security: The SCI enforces security measures by validating requests and ensuring that applications have the necessary permissions to perform operations.
4. Efficiency: The interface is designed to minimize overhead and optimize performance, allowing for efficient communication between user applications and the kernel.

Types of System Calls

System calls can be categorized based on the services they provide.

1. Process Control:
   • Manage processes, including creating, terminating, and scheduling them.
   • Examples: fork() , exec() , wait() , exit() .
2. File Management:
   • Handle file operations such as creating, opening, reading, writing, and deleting files.
   • Examples: open() , read() , write() , close() .
3. Device Management:
   • Manage input/output devices, allowing processes to read from and write to devices.
   • Examples: ioctl() , read() , write() .
4. Information Maintenance:
   • Obtain or set system information, such as the current time or system status.
   • Examples: getpid() , time() .
5. Communication:
   • Facilitate communication between processes, including sending and receiving messages.
   • Examples: pipe() , msgget() , send() .