extern "C"

[George2]

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

[weaknessforcats]

Do you know what extern "C" is for??

[RedSon]

Do you know what extern "C" is for??

I think we can all answer that for the OP...

[George2]

Sure, weaknessforcats. Used for stopping mangling name for function and variable, any comments to my original question?

Do you know what extern "C" is for??

regards,
George

[George2]

Thanks RedSon!


Any comments to my post #4?

I think we can all answer that for the OP...

regards,
George

[RedSon]

Sure, weaknessforcats. Used for stopping mangling name for function and variable, any comments to my original question?





regards,
George

Actually it is more complicated then that. It's true that "extern c" does prevent mangling but that is a side effect of c linking. What compiler are you using?

[hdanw]

Use the extern keyword so that they can be linked outside of its normal frame of reference.

The "c" tells the compiler how to push information on the stack.

The keyword should be added to the Definition of functions, so they can be called by other files.

And declaring extern on a declaration means that the actual declaration exists outside of this file. And should be visible to all linked object files.

// myfile.cpp
int globalint
 
extern myfunc()
{
  // do some stuff
}
 

// myfile.hpp
 
extern int globalint;
extern myfunc();
 

// other.cpp
 
#include "myfile.hpp"
 
globalint = 10;
myfunc();
 
 

[George2]

Thanks RedSon,


I am using Visual Studio 2008 to develop C/C++. Any advice about whether to add extern "C" to declaration only or both declaration and definition?

Actually it is more complicated then that. It's true that "extern c" does prevent mangling but that is a side effect of c linking. What compiler are you using?

regards,
George

[George2]

Thanks hdanw,


Please review whether I am correct.

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

Use the extern keyword so that they can be linked outside of its normal frame of reference.

The "c" tells the compiler how to push information on the stack.

The keyword should be added to the Definition of functions, so they can be called by other files.

And declaring extern on a declaration means that the actual declaration exists outside of this file. And should be visible to all linked object files.

// myfile.cpp
int globalint
 
extern myfunc()
{
  // do some stuff
}
 

// myfile.hpp
 
extern int globalint;
extern myfunc();
 

// other.cpp
 
#include "myfile.hpp"
 
globalint = 10;
myfunc();
 
 

[weaknessforcats]

I am using Visual Studio 2008 to develop C/C++. Any advice about whether to add extern "C" to declaration only or both declaration and definition?

I just did a Google on "extern "C" " and got 309,000 hits.

Maybe you could read a few of these pages. What you are asking is covered in any C++ reference material.

You are supposed to post questions when you are stuck in the code and not as a substitute for doing your own research.

Next:

All function prototypes and definitions have a default storage class specifier of extern. That is, the functions have external linkage. This means you do not need to specify extern. You can override the default with static. However, using the "C" qualifier will require the extern spcifier.

[RedSon]

Educate yourself ...

[George2]

Thanks weaknessforcats,


I think we should add extern "C" to both declaration and definition, right?

http://www.glenmccl.com/ansi028.htm

I just did a Google on "extern "C" " and got 309,000 hits.

Maybe you could read a few of these pages. What you are asking is covered in any C++ reference material.

You are supposed to post questions when you are stuck in the code and not as a substitute for doing your own research.

Next:

All function prototypes and definitions have a default storage class specifier of extern. That is, the functions have external linkage. This means you do not need to specify extern. You can override the default with static. However, using the "C" qualifier will require the extern spcifier.

regards,
George

[George2]

Hi RedSon,


I do not think the same in the MSDN page is good enough. When demonstrating extern, it should use more than one cpp files (more than one compile unit) right?

// specifying_linkage1.cpp
int i = 1;
void other();
 
int main() {
   // Reference to i, defined above:
   extern int i;
}
 
void other() {
   // Address of global i assigned to pointer variable:
   static int *external_i = &i;
 
   // i will be redefined; global i no longer visible:
   // int i = 16;
}
 

Educate yourself ...

regards,
George

[RedSon]

Hi RedSon,


I do not think the same in the MSDN page is good enough. When demonstrating extern, it should use more than one cpp files (more than one compile unit) right?

regards,
George

The point of providing the link was to give you a starting point into your investigation into the extern keyword. Did you follow any of the links in the see also section?

[George2]

Sure, RedSon.


Any comments to my post #13? I think to demonstrate the usage of extern, we should use more than one compile unit (cpp file) to demonstrate, right?

The point of providing the link was to give you a starting point into your investigation into the extern keyword. Did you follow any of the links in the see also section?

regards,
George

[oler1s]

I think to demonstrate the usage of extern, we should use more than one compile unit (cpp file) to demonstrate, right?

Not necessary, if you understand the concepts of linkage. Of course, the assumption is that you do your reading and actually understand the background material. Otherwise, you have to rely on unnecessarily complicated demonstrations to cover up such lack of knowledge.

[George2]

Hi oler1s,


Why it is needed to use extern even if we are in the same cpp file? Any more description please?

(I think extern is useful for cross-compile unit cross-reference usage, if only in one compile unit, is it very helpful?)

Not necessary, if you understand the concepts of linkage. Of course, the assumption is that you do your reading and actually understand the background material. Otherwise, you have to rely on unnecessarily complicated demonstrations to cover up such lack of knowledge.

regards,
George

[oler1s]

Why it is needed to use extern even if we are in the same cpp file?

What do you mean by that question? You use extern to specify external linkage. That is you are simply the declaring the existence of something, and indicating that the actual definition is in some other source file or library. You use extern if that's what you need to indicate.

(I think extern is useful for cross-compile unit cross-reference usage, if only in one compile unit, is it very helpful?

Again, let me repeat myself. If you need something to have external linkage, it should either implicity or explicitly be declared as extern. For example, you don't indicate extern with function declarations, because it is implicitly implied. You do with variables, because you need to say so explicitly. IF something doesn't need external linkage, you don't need to use the extern keyword. Just because the extern keyword exists doesn't mean you need to use it, you realize?

But why are you having fundamental issues understanding it, when you declare your understanding on Cboard, continue with a second thread at CodeGuru, and yet another on a Usenet group?

[George2]

Hi oler1s,


Are there any practical benefits if we use extern keyword to qualify a variable defined in the same cpp file? This is my question.

What do you mean by that question? You use extern to specify external linkage. That is you are simply the declaring the existence of something, and indicating that the actual definition is in some other source file or library. You use extern if that's what you need to indicate.

Again, let me repeat myself. If you need something to have external linkage, it should either implicity or explicitly be declared as extern. For example, you don't indicate extern with function declarations, because it is implicitly implied. You do with variables, because you need to say so explicitly. IF something doesn't need external linkage, you don't need to use the extern keyword. Just because the extern keyword exists doesn't mean you need to use it, you realize?

But why are you having fundamental issues understanding it, when you declare your understanding on Cboard, continue with a second thread at CodeGuru, and yet another on a Usenet group?

regards,
George

[RedSon]

Hi oler1s,

Are there any practical benefits if we use extern keyword to qualify a variable defined in the same cpp file? This is my question.

regards,
George

If you took the time to read about what extern means and try to learn from the text. Then ask questions about the text, for example say, "I don't understand what this text means when it says "blah, blah blah blah, blah", we might be more willing to help you.

You continue to ask the same question that would have been answered if you would have investigated more.

But in answer to your question, it depends on the scope of your variable.