Different threads writing to the same file

Hi all!

My program is formed by three threads. What happens if all these threads have write access to a log file? What would be the worst case? I don't care if one thread writes its log before the others but I'm afraid of loosing any log. I didn't want to develop a complicated mechanism just to write logs

Thanks,

Daniel.

Hi estratos,

Originally Posted by estratos Hi all! My program is formed by three threads. What happens if all these threads have write access to a log file? What would be the worst case? I don't care if one thread writes its log before the others but I'm afraid of loosing any log. I didn't want to develop a complicated mechanism just to write logs

your log file may be wrong. you can use mutexes(pthread_mutex_lock/unlock) or flock.

You can establish a fairly simple locking system.

while lock file exists
  wait
end while
touch lock file
write logs
delete lock file

That way you will not lose any log records. However this is really only acceptable if writing to log records is infrequent, otherwise it might lead to one thread bing locked out.

Anything that threads do with respect to any shared object should be synchronized explicitly. The amount of time needed to do that is completely inconsequential (the I/O operation dwarfs it), and for reliability it's essential.

You might consider whether you want multiple threads to be writing to the same file anyway. That's a "big, fat, slow thing" to be synchronizing against and it could easily wipe out whatever advantages having multiple threads may give you!

Thank you very much for your replies.

I have a last question: what about calling a function from multimple threads? This function would be defined in my code and doesn't alter any shared object.

Sorry for this trivial question.

Daniel.

You could have a FIFO-stack which is emptied by a fourth logging-thread. This way only one thread will write to your log file. Just an idea...

Originally Posted by Ztyx You could have a FIFO-stack which is emptied by a fourth logging-thread. This way only one thread will write to your log file. Just an idea...

Hi Ztyx.

I'll use the FIFO stack but the logging-thread will be one of my threads already running. I don't want to create a fourth one.

Thank you very much,

Daniel.

Originally Posted by graemef You can establish a fairly simple locking system. while lock file exists wait end while touch lock file write logs delete lock file That way you will not lose any log records. However this is really only acceptable if writing to log records is infrequent, otherwise it might lead to one thread bing locked out.

There is still a risk of simultaneous writes with this code, as the control can be passed from one thread to the other just before the former was about to create the lock file, resulting in two threads believing they own the log file at the same time.
This risk is higher with multicore/multi CPU O/Ses.