A Function In A Function In C & C++

C is all about functions. Right? Well, it’s structured around functions. A program written in C always starts with a function called main and then calls various functions along the way. Some of the functions are part of the standard library and some are user defined. I wanted to know if you could write a function within a function. It happens all the time in other languages. Why not C?

I started with a bare bones main function.

int main()

{

     // return some value

     return 0;

} // end main

That code works. It compiles. It runs. But nothing happens.

I then added a function fn() inside this main function.

int main()

{

     // Prototype declaration.

     int fn();

     // return some value

     return 0;

     int fn()

     {

          return 0;

     }

} // end main

It wouldn’t compile. I got this compile error:

error C2601: 'fn' : local function definitions are illegal

End of that story. You can’t define a function inside of another function in C. Not sure why. I expect there is some sort of explanation. Must be. But for now, C is a series of functions, all separate, one after another with the focus on the main function.