Why would you use `extern void my_func();` rather than including `my_utils.h`?

Question

I'm working on some code I didn't write and noticed that there are many extern void my_func();.

My understanding is that extern in for global variables, not for functions.

Is there a practical reason to declare a function as extern rather than putting it in a header file and including that? Or is this just a stylistic choice?

Solution

This is only needed if, for some reason, the header file doesn't declare the function. And extern is always unnecessary for functions, as functions are always extern by default.

Comments

[Donotalo]

functions are always extern by default in C? are you sure??

[Oliver Charlesworth]

@Donotalo: Pretty sure, but it is midnight, so who knows what tiredness has done to my brain.

[Donotalo]

i'm working for micro controller for 2 years. the compiler i use is C. though not completely ANSI C, and many features of C99 are absent, but to call a non-static function defined in another file we need to extern its prototype, or include the header.

[Oliver Charlesworth]

@Donotalo: Then your compiler is broken. #include-ing a header file with function prototypes is equivalent to writing the prototypes directly into your source file, and you don't see header files full of explicit extern functions!

[Donotalo]

no we don't put extern functions in header. we have to put it in .c files where they are called.

[Donotalo]

i got it what do you mean by: functions are always extern by default. at first i thought functions can be called across implementation files without including header and without externing it. i was wrong.

[Oliver Charlesworth]

As discussed in the comments to my answer, use of extern is unnecessary.

[Donotalo]

@Oli: I've just tested this feature using VS2008. It didn't work except extern. Any idea why? I have 3 files: main.cpp (contains main()), functions.h and functions.cpp. functions.cpp defines a function (named fun()) that just returns 0. from main(), i called cout << fun() << endl; compiler says fun() is undeclared. then i wrote extern int fun(); before main(). then it worked.

[0xbadf00d]

@Donotalo A function declaration R func(T arg) tells the compiler that there is a function called func which returns a value of type R and has a formal parameter of type T called arg. This allows the use of the function name in the context of the declaration. The linker is responsible for resolving such a name and find the corresponding function definition which might can be found in a different compilation unit.