variable scope in threads

[chunghorng_lung@hotmail.com]

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<numWorke r;i++)
{
x = pthread_create( &workers[i], NULL, &workerThrea d, NULL);
...
}

}

// This is the worker thread
void *workerThread(v oid *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.

Regards,
Chung

[Larry Smith]

Re: variable scope in threads

chunghorng_lung @hotmail.com wrote:

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<numWorke r;i++)
{
x = pthread_create( &workers[i], NULL, &workerThrea d, NULL);
...
}
>
}
>
// This is the worker thread
void *workerThread(v oid *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.
>
Regards,
Chung
>

Only if all of the code (main thread & worker threads)
is in ONE source file.

You declared 'test1' as 'static', so it is NOT global;
it's only visible within the source file where it
is declared.

One (of several) solutions might look like this:

// in main.cpp
Multi_Queue *test1;

// in worker.cpp
extern Multi_Queue *test1;

When main.cpp & worker.cpp are compiled and then
Linked together, code in 'worker.cpp' will have
access to 'test1'.