CODEWITHSUNDEEP
c++cglobal-variables
batuman
batuman

Reputation: 7304

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

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?

Upvotes: 0

Views: 938

Answers (2)

Luis Colorado
Luis Colorado

Reputation: 12668

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.

Upvotes: 0

robthebloke
robthebloke

Reputation: 9682

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

Upvotes: 1

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