System Calls

An operating system provided various types of functions and services. The user processes need to access those OS services but cannot use or execute it directly.  A system call provides an interface between a process and operating system that allow user process to request service of the operating system. It is defined as a programmatic way in which a program request services from the  operating or interact with the operating system via Application Programming Interface. These system calls are programs written in C and C++  and certain low level task where hardware is to be accessed directly  are written using assembly language. 

A system call interface check function calls in the API and invokes the necessary system call within the OS. 

For example, suppose a student want to see his / her final exam answer sheet. For this he / she writes the application and fill form. After filling form, with the permission of examination controller he/she can see examination sheet. Here we can say that, fill form is a system call to access service provided by examination cell.

Type of System Calls:

Process Control System calls

• End, abort
• Load, execute
• Create process, terminate process
• Get and set process attriburtes
• Wait for time
• Wait event
• Allocate and free memory

File Management System Calls

• Create or delete file
• Open or close
• Read, write or reposition
• Get or set file attribute

Device Management System Calls

• Request device or  release device
• Read, write or reposition
• Get or set device attributes
• Attach or detach device

Information Maintenance System Calls

• Get or set date and time
• Get or set system data
• Get process, file or device attribute
• Set process, file or device attributes

Communication

• Create or delete connection
• Send or receive message
• Transfer status in formation
• Attach or detach remote device