Subj : Memory visibility with Pthreads
To   : comp.programming.threads
From : Christoph Bartoschek
Date : Thu Aug 11 2005 01:50 am

Hi,

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?

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.
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?

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?

Christoph Bartoschek

.