New subject: [opensuse-programming] pthread_mutex vs semaphore.h ?

4 Jun 2018

      Roger Oberholtzer wrote:
...
On Mon, Jun 4, 2018 at 8:45 AM, Per Jessen mailto:per@computer.org> wrote:
I'm reviving some old code, dates back to 2010.  In one place I see I
    have replaced use of pthread_mutexes with sem_wait() and sem_post()
    (posix semaphores).  I just can't see why I did it.  :-(
The semantics are slightly different, but can anyone suggest why one
    should use one over the other?  (or vice versa).
They would act differently. The pthread_mutexes typically wait for 
access to a variable. They do not wait for the variable itself to 
change. The sem_wait(), well, waits for the semaphore to change. Is the 
semaphore value itself the one you are interested in? Or does it's 
change mean that some variable you are interested in has changed? IMHO, 
pthread_mutexes are safer because when you access the protected 
variable, it should not be in the process of changing.
Thanks for your input, Roger.

Here is what I wrote -

    //pthread_mutex_lock( &mux );
    while( EINTR==sem_wait( &mux ) );

    do some queue manip.

    //pthread_mutex_unlock( &mux );
    sem_post( &mux );

    mux is either a pthread_mutex_t or a sem_t.

The queue manipulation is key, the mux is uninteresting.  The reason I am 
looking at this is that a thread got hung up (over the weekend) - in the core 
dump, a signal handler was called and tried to write to the queue whilst someone 
was reading from the queue.  It's possible that is why I had to move to using 
semaphores - I wonder if pthread_mutex_lock is async-safe.

/Per
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Re: [opensuse-programming] pthread_mutex vs semaphore.h ?

Per Jessen

Per Jessen

Per Jessen

Roger Oberholtzer

Per Jessen

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