How does extern "C" work in C++?

Question

I see some code in C++ using extern "C" at the beginning of the file like this:

#ifdef __cplusplus 
extern "C" {} 
#endif

What does this mean? How does it work?

Good information on extern "c" here: http://stackoverflow.com/questions/1041866/what-does-extern-c-in-c-source — Chen Harel, Feb 08 '12 at 09:22

ksming · Answer 1 · 2012-02-08T09:44:44.080

5

It is used to inform the compiler to disable C++ name mangling for the functions defined within the braces. http://en.wikipedia.org/wiki/Name_mangling

edited Feb 08 '12 at 09:44

answered Feb 08 '12 at 09:20

ksming

+1 you said in one line what I was going to say in 100. Well done. – Stephen Quan Feb 08 '12 at 09:46

Luchian Grigore · Accepted Answer · 2012-02-08T10:26:20.970

4

It's probably not like that, but more like:

#ifdef __cplusplus 
extern "C" {
#endif

//some includes or declarations

#ifdef __cplusplus 
}
#endif

It tells the compiler to use C name mangling for whatever is declared inside the directives.

The way you have it now:

#ifdef __cplusplus 
extern "C" {} 
#endif

is just dead code.

edited Feb 08 '12 at 10:26

answered Feb 08 '12 at 09:20

Luchian Grigore

but the code I saw is:#ifdef __cplusplus extern "C" {} #endif , so is it wrong code? – Feb 08 '12 at 09:21
1

@ratzip it's not wrong, it's just dead code. It does nothing. – Luchian Grigore Feb 08 '12 at 09:23
3

It tells the compiler (not the linker) to use C conventions for the functions, rather than C++. This involves not only name mangling, but the conventions for passing arguments, and possibly other things as well. – James Kanze Feb 08 '12 at 09:24
@JamesKanze are you saying the linker doesn't need to know about name mangling? – Luchian Grigore Feb 08 '12 at 09:29
Not at all. The compiler mangles names. The linker joins up whatever names it's given to whatever names its's asked-for. – spraff Feb 08 '12 at 10:24
@spraff ah I see. Thanks for the clarification. – Luchian Grigore Feb 08 '12 at 10:26
@LuchianGrigore Not necessarily. About the only time most linkers take name mangling into account is when outputting error messages. – James Kanze Feb 08 '12 at 10:42

score 0 · Answer 3 · edited May 30 '16 at 05:36

0

It specifies a linkage specification.
It tells the linker how to link the code.

It is useful when you want to mix C and C++ code.

edited May 30 '16 at 05:36

Jonathan Leffler

answered Feb 08 '12 at 09:19

Alok Save

score 0 · Answer 4 · answered Feb 08 '12 at 09:20

0

Extern "C" - notify the compiler,that the noted function is compiled in C style.

answered Feb 08 '12 at 09:20

Oyeme

4 Answers4