Subj : Re: Memory visibility with Pthreads
To   : comp.programming.threads
From : David Schwartz
Date : Wed Aug 10 2005 04:59 pm


"Christoph Bartoschek" <bartoschek@gmx.de> wrote in message 
news:7ucqs2-3pc.ln1@barney.wesseling.pontohonk.de...

> I am confused about memory visibility with Pthreads. Especially I am not
> sure which practices are guaranteed to be safe. What does the Pthread
> standard assure about visibility of memory changes? What can be taken for
> granted after some of the pthread functions have been executed?

    Certain specific functions are guaranteed to fully synchronize memory, 
such as the mutex and condition variable functions.

> For example I would assume that a thread which calls pthread_join can see
> all changes done by the thread which is joined to the calling thread.

    Correct.

> However what about the changes of other threads?  I guess this changes are
> undetermined because one does not know in which state they are. But what
> happens in the following example:
>
> There are two global variables and a shared mutex:
>
> int A = 0;
> int B = 0;
>
> Thread 1 executes the following code:
>
> A = 10;
> pthread_mutex_lock(mutex);
> B = 10;
> pthread_mutex_unlock(mutex);
>
> Thread 2 executes another codesequence:
>
> pthread_mutex_lock(mutex);
> if (B == 10) {
>  printf("The value of A is %d\n", A);
> } else {
>  printf("B is not 10\n");
> }
> pthread_mutex_unlock(mutex);
>
> One question is, whether it is legal to use the variable A in thread 2? 
> The
> other question is, whether one can be sure that the value is always 10 
> when
> the if condition is true?

    Yes, the mutex functions are defined by POSIX to fully synchronize 
memory.

> I have another question on reading shared data. For example I have an
> integer which is set before the first call to pthread_create is made.
> Afterwards all threads only read from this integer. Is this legal? Or does
> one have to use a synchronisation mechanism?

    A thread is guaranteed to see any variables set by the thread that 
created it prior to a call to pthread_create.

    DS

.