Hallo, Phil!

*** Am 06. November 1996 um 23:00 schrieb Phil Crown an All:

 PC> I have two threads that call this function.  It seems to me that it is
 PC> possible that the first thread can be pre-empted before completing, then
 PC> a second thread calls this function and completes it before the first
 PC> function has returned a value.
 PC>                        // called by two or more threads.

 PC> release.MutexSem();
 PC> return rc; // A calling thread is pre-empted here before returning rc

In theory, it is possible if the priorities of the two threads differ that
much so that the second call to the function "overcomes" the
first one. Since two threads of the same process have the same priority if
not explicitly changed by the parent, this is quite improbable.

Each thread has its own stack, though, so that function-local data is not
disturbed. Your problem comes into reality if you have to modify a global
return value variable.

I realize by now, that the called function is not the thread function
itself. You don't have a problem here either, since the value to return is
stored on the stack, i.e. a new instance will be created for each call to
the function.

This is a design problem anyway, I think. I would rather protect the call
in the thread function with a semaphore as to prevent the thread function
from calling the other function more than once.

void thread(...) {
  request();
  rc=function();
  release();
}


Tschau...Thomas

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