topic Re: 2 program writing to same file in Operating System - Linux

2 program writing to same file

Henry Chua — Sun, 17 Jul 2005 18:20:50 GMT

Hi Guys,

I have 2 programs written in C in my process that needs to write to a log file, can I know how I can facilitate these programs so that no contention occurs?

Best regards
Henry

Re: 2 program writing to same file

Vibhor Kumar Agarwal — Sun, 17 Jul 2005 21:36:12 GMT

This is one of the basic problems.

Gather some information about Semaphores.

Re: 2 program writing to same file

Rajeev Shukla — Sun, 17 Jul 2005 21:53:40 GMT

One option is you create a LOCK file.
When ever a program writes to the log file it creates a LOCK file and so that the other program waits till it sees the LOCK file is no more there and then writes back and once done it removes the LOCK file.

Cheers
Rajeev

Re: 2 program writing to same file

Bejoy C Alias — Mon, 18 Jul 2005 00:50:40 GMT

Check fcntl()

Re: 2 program writing to same file

Ralph Grothe — Mon, 18 Jul 2005 01:05:53 GMT

Your IPC programs must device some sort of locking to avoid race conditions.
Advisory locking is quite often done through the fcntl() syscall (see manpage),
or the flock() function in Perl.
As the term advisory implies this only works if your processes obey the locks at all.

Re: 2 program writing to same file

Muthukumar_5 — Mon, 18 Jul 2005 02:05:09 GMT

You have to use file locking with fcntl() call. It will control this problem. When one process is trying then next process will check what is the status of log file (is it writable or not). If get the access starts to access it.

hth.

Re: 2 program writing to same file

Daavid Turnbull — Tue, 19 Jul 2005 16:30:26 GMT

fcntl() is obviously a better option but the simple aproach of using a lock file is better implimented by creating a directory instead of a file as the create and test for existance can be implimented as a single call and more efficient that using something like open() with the appropriate options.