Thread: Extern "C" problem

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 :

Code:

#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 :

Code:

#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 :

Code:

//#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

Originally Posted by MK27

I use // all the time and neither me nor my programs know a note of C++.

It was added to C with C99.

Originally Posted by brewbuck

The C code has no idea what extern "C" means. You need this:

Code:

#ifdef __cplusplus
extern "C" {
#endif

/* Your C-callable prototypes */

#ifdef __cplusplus
}
#endif

Also, do NOT use // style comments in this header. It's shared between C and C++ code, and C has no idea what a // comment is.

EDIT: Oops. I was using #ifndef where it should have been #ifdef.

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

Code:

//#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

Code:

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

cpp_main.cpp

Code:

#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.

Originally Posted by laserlight

Recall that once you separated the code into a header, you included the header in a C source file.

If you mean I included if in cpp_main.cpp, then no, cpp_main is a C++ file - and what I mean by this is that the file has the extension .cpp so gcc will interpret that file as a C++ file.

Or do you mean that I included a .h file in a C++ source file - .h files a C headers and shouldn't be used in C++ ?

Originally Posted by laserlight

So, what is the name of the file that contains this code?

Code:

#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);
}

OK.. point taken.. thanks laserlight .. and apologies too !

OK, now I've removed the #include cpp_main.h from the main() file and compiled it.. expecting it to throw a new error i.e. cpp_main not found... but no.. the compiler found it ! So what's the point of declaring functions then ? (You'll have to forgive me as I'm a relatively new C programmer)