Inotify vim modification - inotify

Question

I'm trying to use inotify to watch a file for being modified in an infinite loop. I'm having some problems with it:
1) I have a while(1), and the read doesn't work unless i create a new file descriptor and a new watch descriptor for every iteration of the while (what i wanted to do is to open those descriptors before the infinite loop, but if the other solution is acceptable, then i can use it). This is the version that works:
while(1){
int file_descriptor = inotify_init();
if (file_descriptor < 0) {
perror("inotify_init");
}
int watch_descriptor = inotify_add_watch(file_descriptor, "/home/user/hello.cfg", IN_CLOSE_WRITE);
....
2) I tryed using the mask IN_MODIFY, but I read that it doesn't work well with vim, so instead i use IN_CLOSE_WRITE. The problem is that when i modify the file with vim, the event is read, but the mask of the event is IN_IGNORED (Mask 0x00008000). When I use gedit, sometimes the mask of the event is IN_IGNORED, and sometimes is IN_CLOSE_WRITE (Mask 0x0000008). I would like to know why i get IN_IGNORED if i'm modifying the file, and why the event is not IN_CLOSE_WRITE. Is there another way to watch for modifications of a single file? Is IN_CLOSE_WRITE the correct mask?

Answer 1

I found the same behavior (kernel 3.14.4) while writing a daemon that automatically reloads its configuration file if changed while running. What I found was that some programs (vim among them) cause inotify to generate IN_IGNORED because they delete the original file and replace it with a "swap" file.
From the inotify documentation,
IN_IGNORED
Watch was removed explicitly (inotify_rm_watch(2)) or
automatically (file was deleted, or filesystem was
unmounted). See also BUGS.
You can see this by running
strace -o log vi myfile.txt
and examining the strace result in log, which will show creation and deletion of .myfile.txt.swp.
The solution is to catch the IN_IGNORED event and re-add the watch for your file, i.e.
int watch_descriptor = inotify_add_watch(file_descriptor, "/home/user/hello.cfg", (IN_CLOSE_WRITE | IN_IGNORED));
IN_CLOSE_WRITE is the correct event to handle, but there is a gotcha with it as well. Some programs open a file for reading and writing, but only read from it. When they close the file, IN_CLOSE_WRITE is generated even though the file was not modified.
I found the best approach was to maintain a flag for every file I am watching and toggle it when I catch IN_MODIFY. Then, when IN_CLOSE_WRITE is generated, the flag can be examined to decide if IN_CLOSE_WRITE was meaningful or not.