extern "C"

Hello everyone,


Just to confirm what is the most correct way beyond a just working function.

1.

We need to add extern "C" to both variable/function definition/declaration? Or only need to add to the variable/function declaration?

2.

How about extern? Declaration only or both declaration and definition are required?

BTW: previously, I only add to declaration, but after reading more and more code which add to both declaration and definition, I come to here to ask this question.


thanks in advance,
George

extern "C" basically tells the compiler not to mangle the names so you can call them from C instead of C++.

Quote:

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Originally Posted by Elysia

Yes, you need to add it to both declaration/definition.
extern "C" basically tells the compiler not to mangle the names so you can call them from C instead of C++.

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hmmm never used extern "C" in the cpp file - only in h-file

Quote:

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If a function has more than one linkage specification, they must agree; it is an error to declare functions as having both C and C++ linkage. Furthermore, if two declarations for a function occur in a program — one with a linkage specification and one without — the declaration with the linkage specification must be first. Any redundant declarations of functions that already have linkage specification are given the linkage specified in the first declaration.

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So enough to declare the function as extern "C" once in a header and include this header before any other declaration/definition of the function to make its name C-styled

I don't know if compilers can treat it differently, though?
Perhaps the declaration is enough.

The declaration is enough. It just has to be the first declaration.

C functions should be placed in separate .c files. extern "C" is not enough to make the function compile as a C function; it only links it as a C function.

It's possible to call the a C++ function through C if you add extern "C".

Depends on how you mean. You can use "extern C" to make a function that is callable from C, and this function then does C++ stuff, e.g.:

Code:

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// someheader.h
extern "C" void func1(void);

// somefile.c
class X
{
public:
  void doStuff();
  ...
}

void func1(void)
{
  X x;
  x.doStuff();
}

---

--
Mats

Yes. That's how I see the extern "C" should be used.
Otherwise if you're just going to compile C code, you might as well just compile a C dll.

Quote:

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somefile.c

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That should be somefile.cpp, of course. .c will most likely be compiled as C, and no 'extern "C"' in the world will make a C compiler recognize classes.

Quote:

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Originally Posted by CornedBee

That should be somefile.cpp, of course. .c will most likely be compiled as C, and no 'extern "C"' in the world will make a C compiler recognize classes.

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Naturally, sorry for the typo.

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Mats

Hello everyone,


From your discussion above, I learned a lot. But are there a conclusion now, whether we should add extern or extern "C" to declaration only or both declaration and definition?

Quote:

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Originally Posted by matsp

Naturally, sorry for the typo.

--
Mats

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regards,
George

Declaration only.

We are in unison - declaration only.

Looks like only add extern "C" to declaration is not safe, see http://www.glenmccl.com/ansi028.htm.

Quote:

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Originally Posted by Elysia

We are in unison - declaration only.

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regards,
George

Yes, in strange places, you may need to "do more" to make it work - it's pretty obvious when you do, so unless you are compiling HUGE chunks of code, it should (now that you are aware of it) be possible to rectify pretty easily.

Of course, it's also a good idea to make typedefs of your function pointer types anyways - because it makes the function pointer easier to use generally, so the second pattern in the linked article is probably a good idea.

--
Mats

If you feel it unsafe, you can always add extern "C" to your definitions as well. Then there will be no problems such as mentioned in the article.

Thanks Mats and Elysia,


Let us discuss another topic of this thread -- extern -- other than extern "C". Currently, my understanding is,

1. We should add extern to declaration, so what we know we are referring a function/variale from other compile unit;
2. We should also add extern to definition so that this function/variale is visible to other compile unit. In this case, extern is on the other side of static.

Right?


regards,
George

Quote:

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Originally Posted by matsp

Yes, in strange places, you may need to "do more" to make it work - it's pretty obvious when you do, so unless you are compiling HUGE chunks of code, it should (now that you are aware of it) be possible to rectify pretty easily.

Mats

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Quote:

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Originally Posted by George2

1. We should add extern to declaration, so what we know we are referring a function/variale from other compile unit;

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Yes and no. Functions are already visible in all compilation units, so no extern is required at all.
For variables, you do indeed need to add a declaration with extern before it.

Quote:

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2. We should also add extern to definition so that this function/variale is visible to other compile unit. In this case, extern is on the other side of static.

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No. All variables and functions have external linkage by default. That means you can access them from other compilation units by default. You can override this by adding "static" before them. In that case, you can't use the functions/variables in other compilation units.

I'm only 90% sure about the functions side of the argument, but the variables should be right.

Thanks Elysia,


My question is answered.

Quote:

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Originally Posted by Elysia

Yes and no. Functions are already visible in all compilation units, so no extern is required at all.
For variables, you do indeed need to add a declaration with extern before it.


No. All variables and functions have external linkage by default. That means you can access them from other compilation units by default. You can override this by adding "static" before them. In that case, you can't use the functions/variables in other compilation units.

I'm only 90% sure about the functions side of the argument, but the variables should be right.

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regards,
George