inotify not firing notifications on file modification

Question

I have been tweaking the example here to make it repeatedly watch a file for 'modifications'. My code is here. In my tests the inotify notification is only firing the first time the file is 'modified'(i.e. touch'ed). Any subsequent modifications to the file are not causing any notifications to fire. stat shows that the 'Modify' time has changed. Also, modifying the code to remove the watch and re-add each time a notification fires(i.e. move inotify_add_watch and inotify_rm_watch to be inside the while(1) loop in my sample) did not help resolve this issue.

I was wondering if any one here could help with what I may be doing wrong. Also, although I have added a watch for IN_ALL_EVENTS, I really only care about IN_MODIFY events. Not sure if that makes any difference.

Also, does this use case not work? Should I change my approach to watch the directory instead? Please advice.

TIA.

Edit 1: As noted by themel, the handling of i needed some fixing. However even the fixed version is not firing notifications for subsequent filesystem 'events'. Also, adding a watch on the directory as opposed to the file is exhibiting similar non-deterministic behavior.

Edit 2: I would like to get this asio + inotify example based on this answer to work. Unfortunately that example hasn't been working for me at all. Any help would be much appreciated. TIA.

score 9 · Accepted Answer · answered Mar 06 '13 at 02:21

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After themel's fix, your code works fine when watching a directory, in my tests. When watching a file, event->len is zero, and your code ignores the notifications.

With the test for event->len removed and all event->name replaced by file_path in the printf statements, it works fine when watching a file as well.

PS: Just noticed you mention touch.

touch sends the following events:

IN_OPEN
IN_ATTRIB
IN_CLOSE_WRITE

no IN_MODIFY

Also, don't test modification by editing it with vim as I just did - it deletes the file while shuffling the working copy and the swap, which removes the watch. pico works.

answered Mar 06 '13 at 02:21

Cubbi

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Although I wonder why `touch` does not cause an `IN_MODIFY` notification. `stat` seems to suggest `touch` has modified the `Modify` time of the file. – decimus phostle Mar 06 '13 at 02:45
IN_MODIFY refers to actual modifications to the file data. Time is part of meta-data, which is an attribute of the file system. – FractalSpace Jun 11 '21 at 14:27
Appreciate the vim comment. Was smacking my head against that one. – ca2longoria Aug 02 '22 at 19:34

score 3 · Answer 2 · answered Mar 05 '13 at 18:24

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Your handling of i is broken, you never reset it to 0 in the loop. This causes any later inotify events to be considered only when they are longer than the longest event before them, which is not likely what you want.

Fixed version.

answered Mar 05 '13 at 18:24

themel

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You're right. The handling of `i` was certainly broken - thanks for catching that. However even the fixed version seems to miss later notifications(after the first notification fires). Any thoughts on fixing this? – decimus phostle Mar 06 '13 at 00:33
Hm, I keep getting inotify events with that code (but none of your conditions fire because a) when watching an individual file, event->name will always be empty and b) touch will give you IN_ATTRIB events, not IN_MODIFY). – themel Mar 06 '13 at 10:40

score 1 · Answer 3 · answered Dec 06 '13 at 21:25

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when monitoring an individual file event->len will be 0 because the file name is not returned. I noticed a lot of the example programs have this issue.

answered Dec 06 '13 at 21:25

user3076087

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inotify not firing notifications on file modification

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