CODEWITHSUNDEEP
cextern
bodacydo
bodacydo

Reputation: 79539

Should functions be made "extern" in header files?

Should functions be made extern in header files? Or are they extern by default?

For example, should I write this:

// birthdays.h
struct person find_birthday(const char* name);

or this:

// birthdays.h
extern struct person find_birthday(const char* name);

Upvotes: 57

Views: 50923

Answers (6)

dW1972
dW1972

Reputation: 1

"Should functions be made extern in header files? Or are they extern by default?"

Should functions be made extern in header files?

  • That is an opinion and differs even among open source.

The reason why I do is simple. If not declared in a header, you can declare that function extern in the file you wish to use it, right before using it. If you put it in the header file, that will be included from the file in which the function is used.

In that respect, I think it is more correct to have the extern in the header. However, compilers do that intentionally because there were bad programmers who didn't know any better.

Or are they extern by default?

  • Function declarations are implicitly extern by default.

Upvotes: 0

StuartLC
StuartLC

Reputation: 107387

No, functions declared in header files do not need to be declared extern.

But variables defined in a .h header and then #included in multiple .c files will need to be declared extern.

Upvotes: 17

tur1ng
tur1ng

Reputation: 3309

They are implicitly declared with "extern".

Upvotes: 21

supercat
supercat

Reputation: 81307

I never bother with the "extern" in my source code, but some people do. To my mind, having extern before variables but not functions makes it more visually obvious which things are functions and which things are variables (possibly including function pointers). I think a lot probably depends on how the declarations in the .h file are created, and how they relate to the main .c file. I usually start by typing in the .h file prototypes, and then copy/paste to the .c file and add the function body (striking the semicolon at the end of the prototype), so "extern" would require have to be added to the header file or struck from the main .c file after the copy/paste.

Upvotes: 1

Jonathan Leffler
Jonathan Leffler

Reputation: 755010

Functions declared in headers are normally (unless you work really hard) extern. Personally, I prefer to see the explicit keyword there - but the compiler doesn't need it. It reminds the readers that they are extern, and since humans are more fallible than computers, I find the reminder helps.

With variables, it is important to use the extern keyword (and no initializer) in the header file. Consequently, for symmetry with the (very few) global variables declared in headers, I use extern with the function too - even though it is strictly not necessary.

Upvotes: 22

Matthew Flaschen
Matthew Flaschen

Reputation: 285037

From The C Book:

If a declaration contains the extern storage class specifier, or is the declaration of a function with no storage class specifier (or both), then:

  • If there is already a visible declaration of that identifier with file scope, the resulting linkage is the same as that of the visible declaration;
  • otherwise the result is external linkage.

So if this is the only time it's declared in the translation unit, it will have external linkage.

Upvotes: 25

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