error when using extern "C" to include a header in c++ program

Question

I am working on a school project which requires to work with sheepdog. Sheepdog provides a c api which enables you to connect to a sheepdog server. First i create c source file(test.c) with the following content :

#include "sheepdog/sheepdog.h"
#include <stdio.h>
int main()
{
struct sd_cluster *c = sd_connect("192.168.1.104:7000");
if (!c) {
    fprintf(stderr, "failed to connect %m\n");
    return -1;
}else{
    fprintf(stderr, "connected successfully %m\n");
}
    return 0;
}

then i compile with no error using the following command

gcc -o test test.c -lsheepdog -lpthread

But what i need is to use it with c++ project so i created a cpp file(test.cpp) with the following content :

extern "C"{
#include "sheepdog/sheepdog.h"
}
#include <stdio.h>
int main()
{
    struct sd_cluster *c = sd_connect("192.168.1.104:7000");
    if (!c) {
       fprintf(stderr, "failed to connect %m\n");
       return -1;
    }else{
       fprintf(stderr, "connected successfully %m\n");
    }
    return 0;
}

now, when i compiled using the following command :

g++ -o test test.cpp -lsheepdog -lpthread

I got this error :

Try with g++ -o test test.cpp -L/Absolut/PATH/TO/sheepdog -lpthread — Phiber
– Phiber, Commented Aug 19, 2016 at 14:28
Just for grins, did you try it without the extern "C" first? Is this file the sheepdog/sheepdog.h? (I don't see a sheepdog/sheepdog.h under include/ in GitHub.) — cxw
– cxw, Commented Aug 19, 2016 at 14:49
it seems also you have problem with include headers, try with: g++ -I /sheepdog/headers OR set CPLUS_INCLUDE_PATH env variable — Phiber
– Phiber, Commented Aug 19, 2016 at 14:52

Pete Becker · Accepted Answer · 2016-08-19 14:55:38Z

6

You can't just wrap extern "C" around a header and expect it to compile in a C++ program. For example, the header sheepdog_proto.h uses an argument named new; that's a keyword in C++, so there's no way that will compile as C++. The library was not designed to be called from C++.

answered Aug 19, 2016 at 14:55

Pete Becker

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Community · Accepted Answer · 2017-05-23 12:00:46Z

1

I agree with @PeteBecker. From a quick look around Google, I am not sure there is an easy solution. Sheepdog is using C features and names that don't port well to C++. You might need to hack sheepdog fairly extensively. For example:

move the inline functions out of sheepdog_proto.h into a new C file, leaving prototypes in their place. This should take care of the offsetof errors, e.g., discussed in this answer.
#define new not_a_keyword_new in sheepdog/sheepdog.h

and whatever other specific changes you have to make to get it to compile. More advice from the experts here.

edited May 23, 2017 at 12:00

CommunityBot

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answered Aug 19, 2016 at 15:02

cxw

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5 Comments

Ilyas Lahmer Over a year ago

@PeteBecker if i create .so library in c using this library which provide me the functions that i need in my c++ program then link it with my c++ program .will this work

Pete Becker Over a year ago

#define new ... produces undefined behavior.

cxw Over a year ago

@PeteBecker in this situation? For sheepdog, I would think new would just be an identifier susceptible to redefinition like any other identifier. Sheepdog has no calls to operator new, so the compiler wouldn't have a reason to treat something called foo_new specially. Am I missing something? --- I also found this answer to a vaguely-related question :) .

Pete Becker Over a year ago

@cxw - redefining any C++ keyword with a macro produces undefined behavior. It might be that that turns out to be harmless, but that doesn't change what it is.

cxw Over a year ago

For future readers of this post - more about #define new and friends

Serge Ballesta · Accepted Answer · 2016-08-19 15:31:21Z

As sheepdog was not designed to be useable from C++ you should build a tiny wrapper in C language to call the functions from sheepdog and only call the wrapper from your c++ code. Some hints to write such a wrapper:

void * is great to pass opaque pointers

extractors can help to access badly named members. If a struct has a member called new (of type T), you could write:

T getNew(void *otherstruct); // declaration in .h

and

T getNew(void *otherstruct) {        // implementation in a c file
    return ((ActualStruct *) otherstruct)->new;
}

Depending on the complexity of sheepdog (I do not know it) and the part you want to use, it may or not be an acceptable solution. But it is the way I would try facing such a problem.

Anyway, the linker allows mixing modules compiled in C and in C++, either in static linking or dynamic linking.

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