Re: variable scope/access in threads

"Bob" <nospam@does.not.exist>
Sat, 24 Mar 2007 23:17:36 CST
<> wrote:

{ Please confine replies, as far as possible, to std C++. -mod/aps }

If the problem actually had anything to do with multithreading, that
would be a little tough.

But I consider the problem is more basic, and has nothing to do with
the program being multithreaded.

Hi All,

I have a question on the scope of variables for threads. The program
has a main thread which creates a few worker threads. The main thread
can access another class stored in another file, however, the worker
threads can't. I got link errors for the code inside of the worker
threads. The following contains code segments.

Multi_Queue is a class and getXXX() is a public member function inside
of Multi_Queue which is in another file.

// inside of the main thread
//global data
static Multi_Queue *test1;

int abc;
abc = test1->getXXX(); // no link error

// create worker threads
for (i=0;i<numWorker;i++)
       x = pthread_create(&workers[i], NULL, &workerThread, NULL);


// This is the worker thread
void *workerThread(void *inPtr)
int xyz = test1->getXXX(); // link error on getXXX

My question is why there is no link error in the main thread but in
the worker thread it has. A global variable, in this case
Multi_Queus, created in the main thread should be available in other
threads. Appreciate any insights.

You haven't really described things properly. Essentially, you have
incorrectly assumed that the problem is related to your code being
multithreaded, so you have left out information about what (I suspect)
is the real problem. The real problem is that the linker is
not being supplied with necessary definitions of functions or objects.

My guess, based on insuffient information, is that you haven't included
a definition of the member function Multi_Queue::getXXX() in the link.
This will yield a linker error about getXXX(). And some linkers will
only complain about an error once, even if they encounter it multiple

If Multi_Queue::getXXX() is defined in a file.cpp, then that file.cpp
needs to be compiled to a file.obj (or .o depending on your development
environment). That file.obj needs to be included in the link.

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