Extern in function declaration

The extern keyword is not required for function declaration. Because:

Int func ();

Int func ()
{
}

The two above are very clear. The first one is declaration, and the second is definition, because function declaration is followed by semicolons (;) and Function Definition parameters are followed by curly brackets, this is a good judgment for both people and compilers.

If the above method is used to declare and define variables, they cannot be clearly defined.

Int I;

Therefore, for variable declaration, the extern keyword must be used, that is:

Extern int I;

In this way, it is differentiated from the definition.

To be consistent, you can also use the extern keyword for the function declaration, but this is not necessary and is not recommended.

The above content is taken from the ansi c language standard description.