What are the types of function in c language?

In C, one can essentially think of two primary types of functions.

First, you have the library functions. These are already built for you. They are part of the standard C library. Think of common ones like printf() or strlen(). You do not write their code. You just include the proper header file to call upon them. They are very convenient for standard operations.

Then, you have the user-defined functions. These are the ones we, the programmers, create. We write them to perform a specific task, which helps keep our code clean and reusable.

Of course, one can also classify functions structurally, based on whether they accept arguments or return a value. But the main distinction is always library versus user-defined.