Re: Shared Memory

The p ----- Original Message ----- From: "Sander Pilon" <sander@3dnews.net> To: "Ilia Alshanestky" <iliaa@home.com>; "Jeff Davis" <jdavis@wasabimg.com>; <php-general@lists.php.net> Sent: Saturday, October 14, 2000 5:18 PM Subject: RE: [PHP] Shared Memory > PHP has several problems related to shared memory. > > 1) PHP sticks a php-signature on a shared memory block, last time I checked > you could not just 'dump' data in a block. > So, in order to communicate with a C program you'd need to use the 'php > method' of reading and writing shared memory. (Which can be quite a PITA, > since you'd have to dig into the php sourcecode to figure out the block > layout and encoding.) > This is no longer an issue if you use the new shmop() of shared memory functions that have been added in php 4.0.3. Shmop doesn't add any extra headers to the shared memory segment, which means you don't have the headache of creating headers etc... > 2) Synchronisation. Usually, when dealing with a shared piece of memory > there is some kind of synchronisation, and very often that is done using > semaphores. Now, when you enter a WAIT state the process is suspended, and > if the process waiting is never signalled the process *stays* suspended. PHP > execution timers do *not* terminate processes that are in suspended state. > This is a very nasty effect that makes semaphores a very dangerous tool in > php. Deadlocks are not a good thing. > > So, if you're absolutely sure what you're doing, shared memory may be a very > fast way of transferring data. But I would advise using alternative methods > of communication if you want to communicate with other php scripts or other > processes. (For example, pipes or unix domainsockets) > > -Sander > > > > > When you create a shared memory segment the system needs a way to identify > > your shared memory segment from all other shared memory segments > > out there. > > This is accomplished by assigning each shared memory segment an identifier > > of sorts. The system then uses that identifier as a pointer to your shared > > memory segment. The key should be a unique number so that the > > system doesn't > > confuse your shared memory segment with someone else's memory segment. > > The access modes for shared memory are the same as for files, ex 644 would > > mean that the shared memory segment is world readable but only the owner > > (person who created the memory segment) can write to it. > > Shared memory is the fastest IPC (Inter Process Communication) > > method which > > means its generally used to quickly transfer data between > > separate process', > > ex. I use shared memory to transfer date between a C daemon running on a > > system and a php client. > > > > > > Ilia Alshanetsky > > iliaa@home.com > > > > > > > > ----- Original Message ----- > > From: "Jeff Davis" <jdavis@wasabimg.com> > > To: <php-general@lists.php.net> > > Sent: Saturday, October 14, 2000 4:05 PM > > Subject: [PHP] Shared Memory > > > > > > > I read the following at the php.net manual: > > > > > > shm_open() takes 4 parameters: key, which is the system's id for the > > > shared memory block, this parameter can be passed as a decimal or hex. > > > > > > I am interested in using shared memory, but I don't know what to supply > > > for the key. I also don't know how the access mode (ie 0644) works with > > > memory. What kinds of situations might shared memory be used for? Is it > > > to hold data from one script to the next, or should it not be sued for > > > that? > > > > > > Thanks, > > > Jeff Davis >