What are system calls? Explain any five system calls.

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written 8.5 years ago by

priyad84 • 350

System calls provide an interface to the services made available by an operating system.
We know that there are two main modes in the operation of the system, that is the user mode and kernel mode.
- User mode: All user processes are executed.
- System mode: All privileged operations are executed.
However many a times user-mode programs need to execute some privileged operations in kernel-mode. Since processor prevents direct access to kernel-mode functions, we need to use an interface.
Definition: System calls provide an interface to the services made available by an operating system.
System calls expose all kernel functionalities that user-mode programs require.
The main advantage of system calls is security. Due to system calls a user program is not able to enter into the OS or any other users region.
Similarly, I/O devices are free from any misuse by the user.

Calls	Description
Open()	A program initializes access to a file in a file system using the open system call.
Read()	A program that needs to access data from a file stored in a file system uses the read system call.
Write()	It writes data from a buffer declared by the user to a given device, maybe a file. This is primary way to output data from a program by directly using a system call.
Exec()	exec is a functionality of an operating system that runs an executable file in the context of an already existing process, replacing the previous executable
Fork()	fork is an operation whereby a process creates a copy of itself. Fork is the primary method of process creation on Unix-like operating systems.

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