build issue when adding C function inside C++ in Qt

Question

I'm building a Qt/C++ app. This App must connect to an android device through MTP. during a mtp copy, I had to provide a C callback to the MTP API (C-only)

I have declared this callback is below:

DeviceMngr.cpp

int fileprogress(const uint64_t sent, const uint64_t total, void const * const data) {
    int percent = (sent * 100) / total;

    if (Transfer_Cancelled == true)
        return 1;

    return 0;
}

DeviceMngr.h

extern  bool Transfer_Cancelled;

extern "C" {    
int fileprogress(const uint64_t sent, const uint64_t total, void const * const data);
}

And it's called in the method below:

uint32_t DeviceMngr::CreateFile(QString filename, uint32_t parent_id) {
...
    ret = LIBMTP_Send_File_From_File(Device->device, strdup(AbsolutePath), genfile, fileprogress, NULL);
...

The Transfer_Cancelled is used :

void DeviceMngr::CancelTransfer() {
    Transfer_Cancelled = true;
}

and

DeviceMngr::DeviceMngr()
{
    ...
    Transfer_Cancelled = false;
}

And also in the method instantiation to make sure it's init to false.

Here is the issue:

Undefined symbols for architecture x86_64:
  "_Transfer_Cancelled", referenced from:
      DeviceMngr::DeviceMngr() in devicemngr.o
      DeviceMngr::CreateFile(QString, unsigned int) in devicemngr.o
      _fileprogress in devicemngr.o
      DeviceMngr::CancelTransfer() in devicemngr.o
ld: symbol(s) not found for architecture x86_64

TransferCancel is only define DeviceMngr.c and any other place.

Any idea ?

Answer 1

It has nothing to do with the function, it's the variable Transfer_Cancelled that is the problem. It's a problem because you define it in the header file, and since you define it in the header file it will be defined in all source files ( translation units ) where the header file is included.

You should only declare the variable in the header file, by doing eg

extern bool Transfer_Cancelled;

Answer 2

Add ifndef to avoid several include

#ifndef FOO_H                                                
#define FOO_H
extern "C" {
    bool Transfer_Cancelled;

    int fileprogress(const uint64_t sent, const uint64_t total, void const * const data);
}  
#endif