Sharing global variable from C++ library to C main program

Question

I have gstdsexample.so, a C++ library. Inside, it has two global variables that I'd like to share between the library and the main C program.

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
int *ptr;

Test two scenarios.

Scenario 1

sharedata.h

#ifndef __SHARE_DATA_H__
#define __SHARE_DATA_H__
#include <stdio.h>
#include <pthread.h>

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
int *ptr;

#endif /* __SHARE_DATA_H__ */

Include sharedata.h in gstdsexample.cpp and main.c. Compilation OK but I get a segmentation fault when gstdsexample.cpp writes data to *ptr.

Scenario 2

Declare two variables in

gstdsexamle.cpp

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
int *ptr;

Then declare as extern in main.c.

extern pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
extern int *ptr;

Now I have undefined reference errors to the two variables when compiling main.c.

Scenario 3:

#ifndef __SHARE_DATA_H__
#define __SHARE_DATA_H__
#include <stdio.h>
#include <pthread.h>

extern "C" {
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
int *ptr;
}


#endif /* __SHARE_DATA_H__ */

Then include sharedata.h in gstdsexample.cpp and main.c. Compiling for cpp lib is fine. But compiling for main.c has errors as

error: expected identifier or ‘(’ before string constant
 extern "C" {
        ^~~
deepstream_app_main.c: In function ‘all_bbox_generated’:
deepstream_app_main.c:98:24: error: ‘mutex’ undeclared (first use in this function); did you mean ‘GMutex’?
   pthread_mutex_lock( &mutex );
                        ^~~~~
                        GMutex
deepstream_app_main.c:98:24: note: each undeclared identifier is reported only once for each function it appears in
deepstream_app_main.c:101:21: error: ‘ptr’ undeclared (first use in this function); did you mean ‘puts’?
     printf("%d ", *(ptr+x));

How to share variables between C++ and C source files?

The variables need to be in extern "C" { ... } blocks if you want to share them with C code. — Barmar
– Barmar, Commented Nov 24, 2021 at 4:00
I tested extern "C" {} in sharedata.h. Added Scenario 3 in original post. But error compiling main.c. cpp library is fine. — batuman
– batuman, Commented Nov 24, 2021 at 4:41
The C compiler is complaining about extern "C" (which is a C++ construct). You need to check for __cplusplus to avoid that. (and just use extern instead) — robthebloke
– robthebloke, Commented Nov 24, 2021 at 4:44
#ifdef __cplusplus extern "C" { pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER; int *ptr; } #endif /* __SHARE_DATA_H__ */ I did like that but still same error — batuman
– batuman, Commented Nov 24, 2021 at 4:47

robthebloke · Accepted Answer · 2021-11-24 04:47:45Z

1

in a header file... gstdsexamle.h

// disable name mangling in C++
#ifdef __cplusplus
extern "C" {
#endif

// declare your two vars in the header file as extern. 
extern pthread_mutex_t mutex;
extern int *ptr;


#ifdef __cplusplus
}
#endif

in gstdsexamle.c

#include "gstdsexamle.h"

/* only initialise here */
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
int *ptr;

in main.c

#include "gstdsexamle.h"

Thats all you need. mutex & ptr are now available in main.cpp/main.c

answered Nov 24, 2021 at 4:47

robthebloke

9,88911 silver badges13 bronze badges

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

2 Comments

batuman Over a year ago

gstdsexample lib is fine. But main.c has undefined errors as `undefined reference to mutex and undefined reference to ptr'

batuman Over a year ago

I also need to declare this in main.c pthread_mutex_t mutex; int *ptr;

Luis Colorado · Accepted Answer · 2023-03-02 11:30:55Z

0

Scenario 1 and scenario 2 are invalid because C++ mangles the names of the C++ identifiers to allow for identifier overloading.

Scenario 3 fails because the extern "C" { } constructor is a syntactic construction valid only in C++ and it is not legal in C. To be able to do this, you need to parse

extern "C" {
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
int *ptr;
}

to indicate the C++ compiler that they are variables to be compatibilized with C calling schema.

But the C language is not aware of something coming from a different language so in C, those declarations must appear as:

extern pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
extern int *ptr;

in the header file, but without the invalid syntax.

An approach has been exposed in anotheer answer, so I will not extend explaining what is said there, just to say that __cplusplus macro is defined by the compiler automatically when it is acting as a C++ compiler, so that can be used as in the other answer to allow the header file to share C and C++ declarations.

edited Mar 2, 2023 at 11:30

answered Nov 25, 2021 at 7:26

Luis Colorado

13.1k1 gold badge19 silver badges34 bronze badges

2 Comments

kirsch Over a year ago

I've seen __cplusplus__ macro referenced in older (25+ years) code, but I think compilers define __cplusplus nowadays. Do you know where the __cplusplus__ with trailing underscores comes from?

Luis Colorado Over a year ago

That's a typo, my apologies for that. Now it's correct. It's possible that old compilers define it, but today, the standard only talks of the one without trailing underscores.

Collectives™ on Stack Overflow

Sharing global variable from C++ library to C main program

2 Answers 2

2 Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related