CODEWITHSUNDEEP
c++inotify
cheng
cheng

Reputation: 2136

inotify stops monitoring file when the file is deleted and created again

I encounter some problem when using inotify. I use inotify to monitor changes on files. Here is my code:

int fd = inotify_init();
int wd = inotify_add_watch(fd, "/root/temp", IN_ALL_EVENTS);
int bufSize = 1000;
char *buf = new char[bufSize];
memset(buf, 0, sizeof(buf));
int nBytes = read(fd, buf, bufSize - 1);
cout << nBytes << " bytes read" << endl;
inotify_event *eventPtr = (inotify_event *)buf;
int offset = 0;
while (offset < nBytes)
{
    cout << eventPtr->mask << endl;
    offset += sizeof(inotify_event) + eventPtr->len;
    eventPtr = (inotify_event *)(buf + offset);
}
delete []buf;

If I delete "/root/temp" and re-create such a file, any changes to this file is not monitored by inotify, anyone how is this? Thanks.

cheng

Upvotes: 2

Views: 6368

Answers (3)

Matthew Flaschen
Matthew Flaschen

Reputation: 284786

The other two answers are correct. Another useful point is that inotify tells you when the watch is invalidated. 

mask & IN_IGNORED

will be non-zero. IN_IGNORED is set when:

"Watch was removed explicitly (inotify_rm_watch(2)) or automatically (file was deleted, or file system was unmounted)."

So, as noted, when this is set, you can rewatch the file (and/or the directory if the file has not yet been re-created).

Upvotes: 6

jweyrich
jweyrich

Reputation: 32240

That's because inotify monitors the underlying inode, not the filename. When you delete that file, the inode you're currently watching becomes invalid, therefore, you must invoke inotify_rm_watch. If you want to monitor a new file with the same name, but a different inode, you must detect when it's created by monitoring its parent folder.

Upvotes: 7

Zan Lynx
Zan Lynx

Reputation: 54325

Whenever you use an API, READ THE DOCUMENTATION.

inotify works using the unique file identifer inode, not a filename. The entire Linux kernel works with inodes in fact. Filenames are only a means to look up inodes.

To get what you want you need to monitor the /root directory. It will report a creation event when a file is added. If that file is named "temp" then you can add a watch on that file.

Upvotes: 4

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