What is system call interface?

Question

I am new to Operating system subject. And I am having trouble in understanding system calls interface. If you could help, I will be thankful. thanks

Answer 1

I have tried to simply explain the whole process of making a system call.

The kernel provides a set of interfaces by which processes running in user-space can interact with the system. These interfaces give applications controlled access to hardware, a mechanism with which to create new processes and communicate with existing ones, and the capability to request other operating system resources. 

These APIs (Application Programming Interfaces) act as the messengers between applications and the kernel, the applications issues various requests and the kernel fulfills them (or returns an error). System calls provide a layer between the hardware and user-space processes.

But System calls and APIs are not the same thing. APIs basically are function definitions that specify "how to obtain a specific service". You generally don't make system calls directly, instead you use an API.

Each system call has a corresponding Wrapping routine, that specifies the API that the application program must use to invoke that system call. (Wrapper Routines are function definitions whose only purpose is to issue a system call). However, an API does not have to correspond to a system call, an API can offer its services directly in User mode, without making any system calls, or a single API function can make several different system calls, more so different API functions can invoke the same system call.

An API defines a set of programming interfaces used by applications. Those interfaces can be implemented as a system call, implemented through multiple system calls, or implemented without the use of system calls at all. The same API can exist on multiple systems and provide the same interface to applications while the implementation of the API itself can differ greatly from system to system. 

From the programmer's perspective, the distinction b/w API and System call is irrelevant, to them its just another function call all he/she needs to think about is the function name, parameter type and return values. From the kernel designer's point of view the distinction obviously is very significant.

Further, when a User Mode process invokes a system call, the CPU switches to Kernel Mode and starts the execution of a Kernel Function(which happens to be a assembly language function) called the System Call Handler. This System Call Handler has a similar structure to that of other "Exception Handlers".

This System Call Handler first saves the content of the registers in the kernel mode stack. Then based on the system call number (each system call has a number associated with it and the user mode process must pass this number as a parameter so that the requested call can be identified) the System Call Handler calls the relevant System Call Service Routine which in Linux happens to be a C function that actually goes on to implement the functionality requested by the User Process. After that's done, registers are loaded back to their previous values and the CPU switches back to User Mode.

The same process can also be represented in a different manner.

Answer 2

A system call interface is a set of functions for requesting a service from the kernel on the operating system they are executed on.It provides an essential interface between the process and the operating system.

For example:

open();

Is a system call used to provide access to a file in a file system and so on.