Extern "C" problem

[CodeBugs]

Hello,

I'm a a pretty new programmer just beginning to play around with C and C++. I use Linux and am compiling with gcc/g++ 4.3.2 however I'm coming across a strange problem that I just can't resolve.

I'm trying to write a simple program to just see the possibilities of calling C++ code from C. The main() function is in C :

#include <stdio.h>
#include <stdlib.h>
#include "cpp_main.h"

/*
 * 
 */
int main(int argc, char** argv) {
    int return_value;
    return_value = cpp_main(argc,argv);
    return (return_value);
}

The function cpp_main() is written in C++, and is in file cpp_main.cpp :

#include "cpp_main.h"
//#include "testclass.h"

//int cpp_main(int argc, char** argv);
/*extern "C" {
    int cpp_main(int argc, char** argv);
}*/

int cpp_main(int argc, char** argv){
    char *testtext = "Some Test Text to display.\n";
    //TestClass::WriteMessage(testtext);
    
    return(0);
}

And cpp_main.h is :

//#ifndef _CPP_MAIN_H
//#define _CPP_MAIN_H

// Declare cpp_main() function
//int cpp_main(int argc, char** argv);
extern "C" {
   void cpp_main(int argc, char** argv);
};

//#endif	/* _CPP_MAIN_H */ 

However when I try to build the executable I receive this error :

cpp_main.h:6: error: expected identifier or ‘(’ before string constant


gcc is telling me there is something wrong with extern "C"... ?!?!!?

If I take the line from the header file and paste it directly into cpp_main.cpp everything compiles and links ok... but why won't it work with the header file ?

Any help is much appreciated

Steve

[melkiy]

I'll try to guess the problem.

The block
extern "C"
{
...
}
need not a final semicolon.

You also may use the difinition of the form
extern "C" void cpp_main(int argc, char** argv);
if you declare the only function.

But worse thing is that the definition in the cpp_main.h differs from that in the cpp_main.cpp in the returned type (void vs int).

[firedraco]

Also, extern "C" is only defined in C++ compiliers, so if you are compiling with a C compilier, you can't compile it. Put #ifdef _cplusplus (I think) around it with correct #endif s.

[CodeBugs]

Firedraco :

OK, well as in my original post, can you explain why this compiles :

//#include "cpp_main.h"
//#include "testclass.h"
//=========================================
// Include the header in cpp_main.cpp
//=========================================
//int cpp_main(int argc, char** argv);
extern "C" {
    int cpp_main(int argc, char** argv);
}

int cpp_main(int argc, char** argv){
    char *testtext = "Some Test Text to display.\n";
    //TestClass::WriteMessage(testtext);
    
    return(0);
}

Yet if I separate decelerations into a header file, this does NOT compile, giving the error I originally stated :

cpp_main.h

// Declare cpp_main() function
//int cpp_main(int argc, char** argv);
extern "C" {
   int cpp_main(int argc, char** argv);
};

cpp_main.cpp

#include "cpp_main.h"


int cpp_main(int argc, char** argv){
    char *testtext = "Some Test Text to display.\n";
    //TestClass::WriteMessage(testtext);
    
    return(0);
}

I'm using gcc and it has the capability to determine the source type (C or C++) from the file extension and passes it to the appropriate compiler in the toolchain, therefore I do not believe it's a problem due to trying to compile C++ with a C compiler.

melkiy:

I was playing round with the return type and forgot to put it back before quoting the code here in the forum. Also it's not because I didn't have a semi-colon after the 'extern' decleration.

[melkiy]

i succeeded in compiling your code with *.h and *.cpp files
with gcc version 3.4.6 20060404 (Red Hat 3.4.6-10)

Are you sure the problem is in compiling (gcc -c cpp_main.cpp), not in linking? Otherwise it's another problem.

[CodeBugs]

melkiy:

I managed to solve the problem - I had included a C++ header in the C file which contains main() (as you'll see from my original post)

I had a misunderstanding of the use of headers - I thought they were there to enable the linker to resolve references to functions in other files. I was therefore skeptical at first removing the C++ #include expecting an 'unresolved function' error - but no gcc doesn't necessarily use headers to resolve

Thanks for your help though.. tis much appreciated by a newbie !