Question

To teach myself a little C++, I decided to write a little Program to write text to my Saitek X52 Pro joystick display.

I wanted to use Eduards C-library
http://plasma.hasenleithner.at/x52pro/

I know I have to place an “extern C” around the methods if I want to use them in my C++ program. But that means changing the Header file of the library – and then it wouldn’t build anymore.
What would be the correct approach in this case?

EDIT: the suggested method worked partially.

Comm.cpp:

...
extern "C"{
#include <x52pro.h>
}
using namespace std;
int main ( int argc, char *argv[] ) {
    cout<<"entered main"<<endl;
    char *String;
    strcpy(String,"testing");
    struct x52 *hdl = x52_init();
    x52_settext(hdl, 0,String , 7);
    x52_close(hdl);
    return EXIT_SUCCESS;
}

Error Message:

Comm.o: In function `main': 
Comm.cpp|38| undefined reference to `x52_init' 
Comm.cpp|39| undefined reference to `x52_settext' 
Comm.cpp|40| undefined reference to `x52_close'

which are all methods defined in x52pro.h

Answer 1

To use extern "C" in C header files, wrap it so

#ifdef __cplusplus
extern "C" {
#endif
...
#ifdef __cplusplus
}
#endif

or you can wrap the #includes with extern "C"

extern "C" {
#include <chdr1.h>
#include <chdr2.h>
}

When linking your application, you must tell the linker what library to use and where the library is. From your link, you must add libusb as well. This looks roughly like this

g++ -o app_name Comm.o -L /path/to/library -lx52pro -lusb

When the library is installed in the system lib directory, you can omit the -L /path/... part. If you use a Makefile, you define this in some variables, usually

LDFLAGS = -L /path/to/library
LDLIBS = -lx52pro -lusb

See also Compiling and Linking and Wikipedia – Linker (computing)