template with C linkage [Error]

[kumararajas]

Hi,

I am linking C library, So I have included below in my header file.

ifdef __cplusplus
extern "C" {
#endif

Now, I am getting an error that says "template with C linkage". On clicking this error, it takes me to Q_OBJECT inside the class.

Not sure about this error. Any helps?

Thanks,
Kumara

[mcosta]

Hi,

If you use this header, you're defining a header that can be included in C source code; so no C++ inside.

you don't need to specify that in a C++ header

[kumararajas]

@mcosta

ifdef __cplusplus
extern "C" {
#include "db.h"
#endif

The use case is that, db.h is a C library. I want to include C library in my C++ code. I think, I should be using extern C to do that?

However, even with extern C, I am getting C compatibility related issues.

[JohanSolo]

AFAIK you need the

#ifdef __cplusplus
extern "C" {
#endif

stuff only if you want to ensure that the code enclosed by extern "C" is pure C, especially if you're using a C++ compiler. You can include as many pure C headers as you want in your C++ code without doing anything special.

In your example, you're getting in trouble because your own code (which contain templates) is enclosed by extern "C", and pure C has no support of templates.

[jsulm]

Well, you should change it:

ifdef __cplusplus
extern "C" {
#include "db.h"
} // Add this
#endif

Else all your C++ stuff will be inside Extern "C"!

And if you do it in C++ code you do not have to check whether it is C++ or not, just do:

extern "C" {
#include "db.h"
}

[kumararajas]

@jsulm That's correct. I was suppose to close it immediately after including C header.

But I am still getting error when I do this

extern "C" {
#include "db.h"
}

Here is the error list

/usr/include/c++/4.6/bits/cpp_type_traits.h:73:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:86:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:90:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:96:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:104:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:111:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:119:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:126:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:136:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:146:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:153:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:160:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:167:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:175:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:199:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:206:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:213:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:220:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:227:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:234:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:241:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:248:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:258:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:266:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:273:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:280:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:290:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:297:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:307:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:314:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:325:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:333:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:341:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:349:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:356:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:364:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:372:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:379:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:386:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:393:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:403:3: error: template with C linkage

[jsulm]

What is in db.h?
Is it really pure C?
And what does db.h include (cpp_type_traits.h?)?

[mcosta]

Hi,

as I said in my previous post you can use extern "C" only if your header file does not contain C++ code.
It must be used to use C code inside C++

[kumararajas]

@jsulm To explain about my db.h,

db.h is an interface, containing set of APIs. And it's a C code.
db.h interfaces C++ code (odb.h)which written for accessing database.

C++ code, gets accessed in C library.
And in my C++ code and using that C library.

(Sorry if I confused you)

In short, db.h is a C header file and I am linking C library (db.a) in my project file.

[kumararajas]

@mcosta Thanks!
My db.h contains C code, basically a set of global functions acting as an interface.
It does not contain any C++ code, but it invokes C++ code.

Example -
In db.h

IfcFunc()
{
// C Code
   OtherFunc();
}

In odb.h

OtherFunc()
{
// C++ code
}

I am linking db library and including the header file.

[kumararajas]

I have removed extern "C" { } now.
Because, db.h has extern "C" in it.

After removing that, I don't get template with C linkage error.

But when I use an API in my code, which comes from the library, it says, undefined reference.

I am linking as

LIBS += -L$(PATH)/lib -ldb

Am I missing something?

[bsomervi]

Seems to me that OtherFunc() has C++ linkage, you cannot mention it in a C compatible header file. If you need to access C++ from a C library that has a C compatible header then you must only access the C++ code from within the implementation files of the C library.

Do some research on how C++ deals with function overloading with name mangling and you should then understand how C++ linkage differs from C linkage and why symbols with C++ linkage cannot be in C compatible header files.

[kumararajas]

I could get rid of issue. I did not want extern C

Reason is that, my code is accessing a library and C compatibility is taken care in the library.

Here is a bird eye view -
myLibrary->linking libdb

I do have a different problem now. That is,
I have an application that is linking myLibrary which is accessing libdb

MyApp->linkingmyLibrary->linking libdb

I am getting 'undefined reference to' when my Library calls an API from libDB.

I see it as a linkage order issue. Is there any thoughts on this?

Thank you,
Kumara

[kumararajas]

Hello all,

Problem is solved.

Issue is that, I was accessing a library, and that link multiple libraries.

As an application point of view, I should be linking those libraries as well, which I was missing to do.
By linking those dependencies as well, I could build my application successfully.

Thanks,
Kumara