How does extern "C" work in C++? [duplicate]

A

4

7

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?

Aleshia answered 8/2, 2012 at 9:17 Comment(1)

Good information on extern "c" here: #1042366 – Constipate 8/2, 2012 at 9:22

M

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.

Matta answered 8/2, 2012 at 9:20 Comment(6)

but the code I saw is:#ifdef __cplusplus extern "C" {} #endif , so is it wrong code? – Aleshia 8/2, 2012 at 9:21

@ratzip it's not wrong, it's just dead code. It does nothing. – Matta 8/2, 2012 at 9:23

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. – Tropho 8/2, 2012 at 9:24

@JamesKanze are you saying the linker doesn't need to know about name mangling? – Matta 8/2, 2012 at 9:29

Not at all. The compiler mangles names. The linker joins up whatever names it's given to whatever names its's asked-for. – Krissykrista 8/2, 2012 at 10:24

@LuchianGrigore Not necessarily. About the only time most linkers take name mangling into account is when outputting error messages. – Tropho 8/2, 2012 at 10:42

M

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

Montpelier answered 8/2, 2012 at 9:20 Comment(1)

+1 you said in one line what I was going to say in 100. Well done. – Meadors 8/2, 2012 at 9:46

M

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.

Matta answered 8/2, 2012 at 9:20 Comment(6)

but the code I saw is:#ifdef __cplusplus extern "C" {} #endif , so is it wrong code? – Aleshia 8/2, 2012 at 9:21

@ratzip it's not wrong, it's just dead code. It does nothing. – Matta 8/2, 2012 at 9:23

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. – Tropho 8/2, 2012 at 9:24

@JamesKanze are you saying the linker doesn't need to know about name mangling? – Matta 8/2, 2012 at 9:29

Not at all. The compiler mangles names. The linker joins up whatever names it's given to whatever names its's asked-for. – Krissykrista 8/2, 2012 at 10:24

@LuchianGrigore Not necessarily. About the only time most linkers take name mangling into account is when outputting error messages. – Tropho 8/2, 2012 at 10:42

F

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.

Fransen answered 8/2, 2012 at 9:19 Comment(1)

@Nawaz: And who said so? The "C"is. – Fransen 8/2, 2012 at 9:24

A

0

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

Aureliaaurelian answered 8/2, 2012 at 9:20 Comment(0)

Recommended topics

Hot tags