I have been playing with the idea of making some application backend
services using pcntl (for singaling and fork)+ sysvsem's (for
synchronisation).
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).
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 :
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. It all seems kinda kludgy and quick-hack for a specific
project, and not 'sysv semaphores implimented in php'. (please forgive
my rude assumptions)
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.
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.
-- Chris Chabot
Ps. please CC me in replies, since im not subscribed to the php lists.
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