[PHP3] Re: File Locking Question From: Martin Pool (mbp <email protected>)
Date: 09/30/98

Date: Wed, 30 Sep 1998 01:06:00 -0700
To: php3 <email protected>
From: pnet <email protected>
Subject: [PHP3] File Locking Question
Message-Id: <v02140b04b2371ee7e825@[209.142.59.218]>

 pnet> What is going to happen?

 pnet> Will PHP report an error "unable to open file..." or will linux
 pnet> suspend the CGI and keep trying to execute fopen() for awhile
 pnet> before returning an error?

 pnet> Do I need to handle such possibilities in my PHP script with
 pnet> something like: while( ! fopen($fp, "w") );

There are a few levels to the answer:

Firstly, in general fopen does not do any locking: if two processes
open the file to write then they will both be able to write over each
other's data, which is probably a bad thing. (By the way, if you open
the file with mode "w" you will "truncate" the file, deleting any
existing contents.)

Secondly, you can do fs-level locking using flock(2) or fcntl(2),
which I imagine have PHP equivalents. They do
shared-read/exclusive-write locking of files. In this case, you'll
have to write your own code to cope with contention by backing off or
whatever.

Thirdly, if you expect to handle concurrent write access to files then
I'd strongly recommend you look at using a database library that can
handle that for you, because it's not a simple problem once you have
multiple resources or complex processing to do. Try
Sleepycat/Berkeley DB2.0 or an RDBMS.

 pnet> My script needs to first read the file, then increment the
 pnet> count before writing it back out. I need to be sure that
 pnet> process2 doesn't try to read from the file after process1 has
 pnet> read it, but before process1 can increment it and write the new
 pnet> value back out to the file.

If your app is that simple, then flock and fopen will probably do fine
to get atomic counter updates. Pseudocode:

  open file read/write
  retries = 5
  while retries-- > 0
    try to get an exclusive lock on the file
    if it was locked:
      do your update
      close & unlock
      return
    else
      sleep one second
      try again
  log "can't lock the file!"
  try to go on without it 

--
Martin Pool

As for the M$ pundits claim that the problems result from an
incorrect setup/configuration: they are perfectly correct, if you
hadn't installed an M$ OS, then the crashes wouldn't be happening.
             -- Jeff Dutky on c.o.l.a

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