file locking

kjpstudios

i have a script that recursively watches a directory and then moves the files from that directory onto another server - the files come via an ftp upload.

how can i make sure that the file i am copying is not being written to by the ftp daemon? (as a user uploads)

the copy() function seems to copy partial files... so, i tried the following to see if i could get some kind of flag telling me that the file is still being written to:

open an file pointer to the file with the "w" option to see if it throws an error
try to flock() the file to see if that throws an error
did a stat() on the file to see if that tells me anything

anyone have any ideas on how to make sure that a file isn't being written to before copying it?

thanks in advance!

kenny

ednark

ok there are things called semaphores... they mean lots of things depending on what/where/when you are using them... but basically it is the solution that is used to block access to shared resources...

if you want to lock a whole directory (or a single file, or a block of memroy) you do this... the file dir.lock in this is considered a semaphore

1 - open up the directory
2 - look for a file called dir.lock
3 - if the file exists, wait
4 - when the file does not exist
5 - create the file
6 - do your dirty work
7 - remove the file dir.lock

this works file assuming your program does not die between creating and destroying the dir.lock file, this can be gotten around by simple ignoring a dir.lock file beyond a certain age. assuming that if it got to be 5 seconds old, whatever process made it must have died, because it shouldn't take 5 seconds.

also this system is voluntary, this only works if everything that is going to use this directory plays by the same rules.

however this should work for the system you described

[edit]

scratch that... i didn't read your post carefully enough, you would have to make your ftp daemon follow these rules also, so i don't think my solution would work...

kjpstudios

hmmm... interesting - i will read up on whether my ftp uses that dir.lock file - I use proftpd

one thing that i am going to try is to open a pointer to the file with write access, then actually try to write an empty string to the end of the file - i would think that i would get some kind of error if that file is in the process of being written to

i will post my findings in a bit