Why doesn't inotify update?

Question

I'm writing an inotify watcher in C for a Minecraft server. Basically, it watches server.log, gets the latest line, parses it, and if it matches a regex; performs some actions.

The program works fine normally through "echo string matching the regex >> server.log", it parses and does what it should. However, when the string is written to the file automatically via Minecraft server, it doesn't work until I shut down the server or (sometimes) log out.

I would post code, but I'm wondering if it doesn't have something to do with ext4 flushing data to disk or something along those lines; a filesystem problem. It would be odd if that were the case though, because "tail -f server.log" updates whenever the file does.

Answer 1

Solved my own problem. It turned out the server was writing to the log file faster than the watcher could read from it; so the watcher was getting out of sync.

I fixed it by adding a check after it processes the event saying "if the number of lines currently in the log file is more than the recorded length of the log, reprocess the file until the two are equal."

Thanks for your help!

Answer 2

Presumably that is because you are watching for IN_CLOSE events, which may not occur until the server shuts down (and closes the log file handle). See man inotify(7) for valid mask parameters for the inotify_add_watch() call. I expect you'll want to use IN_WRITE.

Answer 3

Your theory is more than likely correct, the log file is being buffered by the OS, and the log writer has no flushing of that buffer, so everything will remain in the buffer till the file is closed or the buffer is full. A fast way to test is to start up the log to the point where you know it would have written events to the log, then forcibly close it so it cannot close the handle, if the log is empty is definitly the buffer. If you can get hold of the file handle/descriptor, you can use setbuf to remove buffering, at the cost of performance.