Inotify event in C

Question

Program:

#include <stdio.h>
#include <sys/inotify.h>

int main()
{
    int fd = inotify_init();
    int wd1 = inotify_add_watch(fd, "/home/guest/a", IN_MODIFY);
    struct inotify_event *event = (struct inotify_event*) malloc(sizeof(struct inotify_event));
    read(fd, event, 1000);
    if (event->mask & IN_MODIFY) {
        printf("File '%s' is modified\n", event->name);
    }
}

Output:

$ ./a.out 
File '' is modified
$

I expected that the above program will notify with the filename if file a is modified. But it notify without filename. So, how to get the filename if the file is modified using inotify.

[Do not cast the return value of `malloc()`, and `void *` in c in general](http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc). And you really don't need to `malloc()` anything at all. Just `struct inotify_event event; read(fd, &event, sizeof(event))`. will do — Iharob Al Asimi, Dec 22 '15 at 14:55

score 4 · Answer 1 · answered Dec 22 '15 at 14:59

4

The documentation says:

The name field is present only when an event is returned for a file inside a watched directory; it identifies the file pathname relative to the watched directory. This pathname is null-terminated, and may include further null bytes ('\0') to align subsequent reads to a suitable address boundary.

So your problem is that you're expecting inotify to "echo" the name back to you, but that is not how it works.

answered Dec 22 '15 at 14:59

John Zwinck

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Is there any way to achieve that? – mohangraj Dec 22 '15 at 15:00
1

Yes, you can keep your own mapping of watch descriptors to filenames. This has been discussed a bit here: http://stackoverflow.com/a/9313324/4323 – John Zwinck Dec 22 '15 at 15:02
It is the only way to get the filename. But this scenario will also fail in some condition like if more than one file is added to fd passed to notify_add_watch function. – mohangraj Dec 22 '15 at 15:12
@mrg: No. Every time you add a watch, you get a new watch descriptor. There is therefore a mapping from watch descriptors to watches, and one watch on one file has a filename. – John Zwinck Dec 22 '15 at 15:16
1

@iharob: There is no "watched directory" in the OP's case: the code watches `/home/guest/a` and the OP says that `a` is a file. No directory is watched. Therefore no names are reported. Because they are meant to be inferred from the watch descriptor. – John Zwinck Dec 22 '15 at 16:24
That clears things up. It doesn't make sense to return the name of the watched "*file*" because you know the name or else, how did you start watchng it in the first place? I Appologize, let me remove the comments. – Iharob Al Asimi Dec 23 '15 at 17:13

Inotify event in C

1 Answers1