RE: [PHP] semaphores are broken

This is on a simple-ish embedded system. And yes, I did see the code -- that's when I kind of choked :) And, yes, I did implement the interrupt handler, but liked the design that it would just set a sem that the main loop can wait for. But that means that the main loop is waiting on a sem that will get and EINTR now and then. So there's the problem. I have switched to the approach of putting code in the interrupt handler... which I guess is fine. It's just annoying that I wasted time with the 1st approach, that I think is a very reasonable design. And, I have always been hesitant to put much code in an interrupt handler -- I have always thought that has issues too. Jeremy -----Original Message----- From: Rasmus Lerdorf [mailto:rasmus@lerdorf.com] Sent: Sunday, May 08, 2011 11:38 AM To: Jeremy Greene Cc: php-general@lists.php.net Subject: Re: [PHP] semaphores are broken On Fri, May 6, 2011 at 12:57 PM, Jeremy Greene <jeremy@zeevee.com> wrote: > Then.... I find out that sem_acquire() actually returns **OK** when the > underlying semop call gets an EINTR!!! Holy cow. How can a "php system" > call loose an error?! That's just crazy. Generally you don't care about an EINTR on a semop() call. If you do you will have installed a signal handler to handle that interruption. The low-level code has: while (semop(sem_ptr->semid, &sop, 1) == -1) { if (errno != EINTR) { php_error_docref(NULL TSRMLS_CC, E_WARNING, "failed to %s key 0x%x: %s", acquire ? "acquire" : "release", sem_ptr->key, strerror(errno)); RETURN_FALSE; } } which ignores the EINTR as you can see. If you really want to get in there to handle the uncaught interruption you can do: pcntl_signal(SIGEINTR, function($signo) { echo "Interrupted\n"; }); sem_acquire(...); pcntl_signal_dispatch(); But in general, using semaphores in a Web app is a bad idea. Any sort of locking is bound to get you in trouble which is also why PHP resets the semaphore at the end of a request. Generally you will want to think beyond a single server and look at a message passing mechanism to communicate between a PHP app and a server process. I think my favourite way to implement this today is through ZeroMQ. There is a good guide to it at: http://zguide.zeromq.org/page:all http://www.zeromq.org/bindings:php Or if you want something higher level you could have a look at Gearman. Your server could register itself as a Gearman worker and you could use the nice Gearman API to communicate with the server. -Rasmus