trouble using c library in c++

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

defined in x52pro.h or declared in? if declared in, then you need to link to the library that actually contains the definition. what is with all these answers saying to add extern "C"? It's already there!! — thang
– thang, Commented Jan 18, 2013 at 23:50
declared in it. defined in the shared library. Do I need to tell the compiler where to find the shared library? (it had a Makefile which hat an install target though) I've added the extern "C" because of the additions. — mish
– mish, Commented Jan 19, 2013 at 0:01
I think it would benefit your question if you post the relevant parts of the make file. It may be a missing -I parameter. — StoryTeller - Unslander Monica
– StoryTeller - Unslander Monica, Commented Jan 19, 2013 at 0:14
You will very likely need to tell the compiler which libraries are needed. You may or may not need to specify the location of the library. If you do, how that is done varies on the platform, so you should consult the appropriate documentation for your compiler. — jxh
– jxh, Commented Jan 19, 2013 at 0:14
I adjusted the makefile accordingly - it was indeed the missing part. — mish
– mish, Commented Jan 19, 2013 at 10:23

Olaf Dietsche · Accepted Answer · 2013-01-19 07:10:15Z

4

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)

edited Jan 19, 2013 at 7:10

answered Jan 18, 2013 at 22:37

Olaf Dietsche

74.4k9 gold badges113 silver badges214 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

jxh · Accepted Answer · 2013-01-18 22:36:30Z

0

In your C++ code, you can surround the included header file with extern "C" like this:

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

Then, you would not need to modify the header file of the third party library.

answered Jan 18, 2013 at 22:36

jxh

70.9k8 gold badges116 silver badges205 bronze badges

Comments

A--C · Accepted Answer · 2013-01-18 22:39:52Z

0

#ifdef __cplusplus
   extern C {
#endif 
    ...

#ifdef __cplusplus
   }
#endif

edited Jan 18, 2013 at 22:39

A--C

36.4k10 gold badges109 silver badges92 bronze badges

answered Jan 18, 2013 at 22:36

user1252446

Collectives™ on Stack Overflow

trouble using c library in c++

3 Answers 3

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related