How to Lock Files (/etc/passwd /var/spool/[user]

Larry James · 12-02-2000, 06:48 PM

Can someone tell me how to lock files while processing them. In a previous Unix version I could use /usr/spool/usr.lock to lock a mail file while processing it.

I'm writing utilities to allow users to change their passwords and read their mail over the web. During the brief moment while the file is being updated, in the case of the shadow file if the user changes it, or the mail file once the user connects to it, I wouldn't want to risk currupting the file during a clash update by more than one program.

Thanks in advance for any comments.

-- L. James

Larry James · 12-03-2000, 07:32 PM

I would like to thank everyone who thought about my question about file locking. The key is fcntl() for C and the library module Fcntl ':flock' for Perl. I'm going to have to look around and see what I see for PHP later. It appears the file.lock that I was using, wasn't really doing the trick. I was just kind of luck that my apps had went in did it work and exited before damage from other programs were done.

-- L. James

jeremy · 12-03-2000, 07:45 PM

Sorry Larry - for some reason I did not see this thread. For PHP it would look something like

Code:

<?php 

$file_name = "data.dat"; 

$file_pointer = fopen($file_name, "a"); 

$lock = flock($file_pointer, LOCK_EX); 
// Use 2 instead of LOCK_EX if you use < PHP4.0.2 

if ($lock) { 

 fseek($file_pointer, 0, SEEK_END); 
 // Use this if < PHP4.0RC1 : fseek($file_pointer, filsize($file_name)); 

 fwrite($file_pointer, "what u wanna write"); 
 flock($file_pointer, LOCK_UN); 
 // Use 3 instead of LOCK_UN if you use < PHP4.0.2 

} 

fclose($file_pointer); 

print "data written to file successfuly"; 

?>

Larry James · 12-04-2000, 11:06 AM

Jeremy. Thanks a bunch for the code. I'm gong to work on setting it up and testing it out as soon as I take a break from the Perl project. I'll let you know how it works out. At a glance it seems to contain just about everything I need.

-- L. James