What is meant by system call? How can it be used? How does an application program use these calls during execution?

System Call:

A system call is a software mechanism that allows a user-level program to request a service from the kernel. It provides a way for a program to interact with the operating system (OS) and access its resources. System calls are essential for any program that needs to perform tasks such as input/output (I/O), memory management, process management, and file system operations.

How System Calls Work:

1. User-Level Program: The user-level program makes a system call by executing a special instruction, known as a system call instruction. This instruction causes a trap to the kernel.

2. Trap to Kernel: When a system call instruction is executed, the hardware generates a trap and transfers control to the kernel. The kernel then identifies the system call being requested.

3. Kernel Services: The kernel performs the requested service. This may involve accessing hardware devices, managing memory, or scheduling processes.

4. Return to User-Level Program: Once the service is complete, the kernel returns control to the user-level program. The program can then continue execution.

Types of System Calls:

There are many different types of system calls, each providing a specific service. Common system calls include:

• File System Operations: Open, close, read, write, delete, etc.
• Process Management: Create process, terminate process, wait for process, etc.
• Memory Management: Allocate memory, free memory, change memory protection, etc.
• Device Management: Open device, close device, read from device, write to device, etc.
• Networking: Send data, receive data, connect to a network, etc.

Using System Calls in Application Programs:

Application programs use system calls to interact with the OS and perform various tasks. Here are some examples:

• To read data from a file, a program would use the open() system call to open the file, the read() system call to read data from the file, and the close() system call to close the file.
• To create a new process, a program would use the fork() system call. The fork() system call creates a new process that is a copy of the calling process.
• To allocate memory, a program would use the malloc() system call. The malloc() system call allocates a block of memory of the specified size.

System calls are an essential part of any operating system, providing a way for user-level programs to interact with the OS and access its resources. By understanding how system calls work, developers can write programs that can perform a wide variety of tasks.