Re: light weight user level semaphores

[Ulrich Drepper <drepper@redhat.com>]

Alan Cox <alan@lxorguk.ukuu.org.uk> writes:

> mknod foo p. Or use sockets (although AF_UNIX sockets are higher latency)
> Thats why I suggested using flock - its name based. Whether you mkstemp()
> stuff and pass it around isnt something I care about
> 
> Files give you permissions for free too

I don't want nor need file permissions.  A program would look like this:


  process 1:


   fd = open("somefile")
   addr = mmap(fd);
   
   pthread_mutexattr_init(&attr);
   pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);

   pthread_mutex_init ((pthread_mutex_t *) addr, &attr);

   pthread_mutex_lock ((pthread_mutex_t *) addr);

   pthread_mutex_destroy((pthread_mutex_t *) addr);

  process 2:

   fd = open("somefile")
   addr = mmap(fd);

   pthread_mutex_lock ((pthread_mutex_t *) addr);


The shared mem segment can be retrieved in whatever way.  The mutex in
this case is anonymous.  Everybody who has access to the shared mem
*must* have access to the mutex.


For semaphores it looks similarly.  First the anonymous case:

 process 1:


   fd = open("somefile")
   addr = mmap(fd);

   sem_init ((sem_t *) addr, 1, 10);	// 10 is arbitrary

   sem_wait ((sem_t *) addr);

   sem_destroy((sem_t *) addr);


  process 2:

   fd = open("somefile")
   addr = mmap(fd);

   sem_wait ((sem_t *) addr);

Note that POSIX semaphores could be implemented with global POSIX
mutexes.


Finally, named semaphores:

   semp = sem_open("somefile", O_CREAT|O_EXCL, 0600)

   sem_wait (semp);

   sem_close(semp);
   sem_unlink(semp);


This is the only semaphore kind which maps nicely to a pipe or socket.
All the others don't.  And even for named semaphores it is best to
have a separate name space like the shmfs.

> So you have unix file permissions on them ?

See above.  Permissions are only allowed for named semaphores.

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