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Difference Between System Call and library call

Last Updated : 05 Sep, 2024
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For anyone dealing with computer programming especially when it comes to performance issues or dealing with operations systems, it is very important to know the difference between system calls and library calls. System calls can be said as a way through which programs can ask the Kernel of an operating system for a certain service on the other hand we have library calls which are usually in the form of functions that a programmer can call from a library that may or may not involve the Kernel. Although they may be quite similar, their mode of functioning as well as the result they produce is quite distinct.

What is a System Call?

There are two modes in the computer system one is user mode and another is kernel mode. In a computer system, there are different types of processes that are running on a computer system. When a user runs an application it is said to be in user mode or the computer is in user mode.  When there is a requirement of hardware resource, the process sends a request to the kernel to get the process access and then the computer enters kernel mode. These requests are sent by using system call. The computer switches between these two modes frequently. Whenever the task is completed the computer goes back to the user mode from kernel mode. This mode of transition is called context switching.

Advantages of System Calls

  • Direct Access to OS Services: System calls offer a basic level interface with the operating system and are often useful for carrying out tasks that demand fine-tuning of the hardware.
  • Security and Control: Due to the fact that system calls are provided through the kernel, they provide a certain level of security so that the unauthorized code cannot interfere with system calls.

Disadvantages of System Calls

  • Performance Overhead: Because a system call yields the processor from the user mode to kernel mode and vice versa then it incurs more time compared to function call between two functions in a program.
  • Complexity: Working with system calls itself involves writing code that is more complex and prone to containing mistakes because of the operations’ lower level.

What is Library Call?

A Library call is a kind of request to use a specific piece of function that is defined in a programming library. A library file contains code data file, object file that are packed into one file for use. To make a library call, library should be imported first. The library call may be depended on the system call to complete the task.

Advantages of Library Calls

  • Ease of Use: Library calls are even easier because they wrap complex functionality into simple function call.
  • Efficiency: This is because library calls don’t necessarily need to shift from user mode to kernel mode when the call is made and as a result they may take less time as compared to system calls.
  • Portability: Being more of system independent, library functions can be more portable because they cover the system specifics.

Disadvantages of Library Calls

  • Limited Control: The library calls are quite different from the system calls and they might provide even less control on it since they are often operate on a lower level.
  • Dependency on External Libraries: Using library calls make the functional program dependent on the libraries and may be less efficient since it has to rely on the libraries in the system or on a variety of libraries for implementation.

Difference Between System Call And Library Call

SYSTEM CALL

LIBRARY CALL
A system call is a request made by the program to enter into kernel mode to access a process.. A library call is a request made by the program to access a library function defined in a programming library.
In kernel mode the programs are directly accessible to the memory and hardware resources. In user mode, the programs cannot directly accessible to the memory and hardware resources.
In system call, the mode is executed or switches from user mode to Kernel mode. In library call, the mode is executed in user mode only.
In system call the execution process speed is slower than the library call because there is a mode of transition called context switching. In library call the execution process speed is faster than the system call because there is no mode of context switching.
A system call is a function provided by the kernel to enter into the kernel mode to access the hardware resources. A Library call is a function provided by the programming library to perform a task.
System call are the entry points of the kernel, and therefore they are not linked to the program. Library functions are linked into your program.
A system call is not portable. A library call is portable.
System call have more privileges than library calls because it runs in a supervisory mode. Library call have less privileges than system calls because it is runs in a user mode only. 
System calls are provided by the system and are executed by the system kernel. Library calls included the ANSI C standard library. 
In system call all functions are a part of the kernel.  In library call all library functions are a part of the standard library file of programming languages. 
Whenever a program requires the memory or hardware resources, it directly sends a request to the kernel to get the process access by using a system call.

Whenever a programmer or a developer uses a specific library function, the programmer has to invoke or call the library function first by including a header file into his program. The preprocessor ( # ) directives helps to include header files. Some useful header files are :-

1. #include<stdio.h>

2. #include<math.h>

3. #include<conio.h>

Examples of system calls are – 

1. fork() 

2. exec() 

Example of Library call are –

1. fopen()

2. fclose()

3. scanf()

4. printf() 

Conclusion

System calls are important to software development while library calls also have their uses in the process of software development. Whereas system calls provide a direct way of interacting with the operating system is suitable to carry out tasks that closely involve the system, it also has system call overhead and complexity. While on the other hand library calls are easy to use, fast and portable but it may not be as powerful as a system call. Knowledge about when each type should be applied can help to write more efficient and easy to maintain code.



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