Difference between the user-defined and the library function in C

This topic will discuss the difference between the user-defined and the library function in the C programming language. Before going to the topic, let's understand the function in the C language. A function is a self-contained group of subprograms used to perform specific, well-defined tasks. The functions are divided into two parts: the user-defined and the library function.

User-defined Function

As the name suggests, a user-defined function is a function written by the user to write any program code and execute specific actions. These user-defined functions can be modified and executed according to the requirements of the programmer. A programmer can change the user-defined function, but these functions are not defined in the C header files. A user-defined function is made up using the function declaration, function definition, and the function call.

Function Definition

The function definition defines the function's actual body that performs specific tasks in a program.

Library Function

C programming language provides some library functions to perform some predefined tasks. These functions are also called the built-in or predefined functions in the C header files whose meaning cannot change. When we use these functions on any program, we call the function name with appropriate header files because these functions are defined inside the header files. In other words, we are not required to write the complete code to perform a specific task; instead, we can directly call the function in our program whenever it is required. For example: printf(), scanf(), getch(), etc., are the predefined library functions.