Re: sysvsem extention question

There probably should be a full implementation of semaphores in php. If you have a need for this, we should discuss exactly how it should be implemented. I will have some free time available soon, so I can start working on this. Though I have a couple other projects as well. If you would be interested in contributing a CVS account can be arranged. This should probably start as a separate module, and then eventually replace the sysvsem extension as it is no long being actively maintained. I have a great desire for php to function well as a standalone scripting language as well as web, so I am always interested as projects like this. Is there anyone else who would find this useful? -Jason ----- Original Message ----- From: "Tom May" <tom.may@infospace.com> To: "Chris Chabot" <chabotc@reviewboard.com> Cc: <php-dev@lists.php.net>; <php-general@lists.php.net>; <jason@inetgurus.net> Sent: Tuesday, August 21, 2001 1:58 PM Subject: Re: sysvsem extention question > First off, before you get the wrong impression of my answer, let me > say that your observation "It all seems kinda kludgy and quick-hack > for a specific > project, and not 'sysv semaphores implimented in > php'" is right on. > > Chris Chabot <chabotc@reviewboard.com> writes: > > > I have been playing with the idea of making some application backend > > services using pcntl (for singaling and fork)+ sysvsem's (for > > synchronisation). > > Cool. But see below. > > > However while reading thru the sysvsem source code, i noticed the > > behaviour of the semaphores in PHP is basicly as a 'lock' (as one would > > use a file lock w/ flock()). From what i understand from my advanced > > unix programming book, and the linux man pages, this is not what > > semaphores are supposed to be (though they can be abused to be so). > > > > The way one would -expect- semaphores to function on a unix platform > > is best described in "Linux Programmers Guide" at > > > > http://uiarchive.uiuc.edu/mirrors/ftp/ftp.ibiblio.org/pub/Linux/docs/linux-doc-project/programmers-guide/lpg-0.4.pdf > > (page 46 and on). > > > > A short sumary would be: a semaphore is a resource counter (not an > > absolute lock). > > A lock is just a degenerate case. A slightly less degenerate case > (that can't be implemented with flock) is when you want to allow N > users of the resource instead of just one. > > > The main this is that a process other then the process that acquired > > the semaphore can decrease the semaphore count, thus releasing it for > > another client. Another big difference is that a process can set the > > semaphore count lower then zero (0 == locked). This sometimes can be > > usefull for Client / Server synchonisation. > > > > Example usage for a typical Client / Server program flow using > > semaphores / signals : > > I admit I haven't looked at this hard enough to understand all the > details, but could message queues help you out here? You could get > higher throughput since the server fills the buffer and calls msgsnd() > beforehand, whereas in your signalling implementation clients will > stall between the time they signal the server and the time the server > has filled a buffer for them. Also, you need some additional overhead > to manage the buffer handoff from client to server. And doing things > in signal handlers can get dicey. > > If a message queue isn't big enough for you, you can pass references > to shared memory in your messages. > > > Server : > > create sem > > sem acquire > > setup envirioment > > fork children > > > > Multiple Clients: > > repeat (wait for sem acquire) { > > send signal to Server > > communicate with server, get 'buffer' > > process buffer > > } > > > > Server: > > > > while (Fill data into buffer) { > > semaphore release (!); > > Sleep(infinite or untill time out); > > } > > > > -> Sleeps untill interupted by signal (signal send by Client/Child) > > -> In signal handler: > > Send 'buffer' to client that acquired semaphore > > return; > > will cause the program to go back to main loop, sleep was interupted, > > so goes to while (fill buffer) again. Also note that the Client -never- > > calls semaphore release. The server does that once the resource is > > available again. > > > > Rinse and Repeat till end of time, or end of data :-) This will > > distribute the data to be processed over all the different clients > > (since semaphores guarantee a linear processing of clients, so all > > clients get there equal share). > > > > Last, i don't see why the implimentation as exists, requires 3 > > semaphores. > > I don't remember why either. I did that code somewhere around the end > of 1998 . . . > > > It all seems kinda kludgy and quick-hack for a specific > > project, and not 'sysv semaphores implimented in php'. (please forgive > > my rude assumptions) > > Bingo. > > > Does the maintainer (Tom May) want to work on this, or anyone else? I'd > > be happy to help out, but my last C coding days are over 6 years behind > > me, so i don't feel very confident to lead this project.. So any/all > > help and/or comments would be apreciated. > > I no longer use php or maintain any of the stuff I helped with (aside > from answering the occaisional question, like this one. > > > Also i noticed a potential security hole in the exisiting source, at > > line 190 of sysvsem.c it uses > > > > semget(key, 3, perm|IPC_CREATE); > > > > However, perm is not zero'd to 7 bits before being used, thus allowing > > extra flags being added to the call, which presumably is unintentional, > > since it allows nasty flags to be passed to this call. perm is gotton > > from arg_perm, which is a lval. What i imagine you 'should' do is zero > > out all non-relivant bits, basicly AND perm with 0x777. this will clear > > all bits other then the (file style) permission flags. > > AND with 0777. Good catch. > > > -- Chris Chabot > > > > Ps. please CC me in replies, since im not subscribed to the php lists. > > Me either. > > Tom.