CODEWITHSUNDEEP
linuxinotify
enthu
enthu

Reputation: 69

Why is it required to call read() twice with inotify_add_watch()

I am trying to get notifications using inotify_add-watch() for a file when it is modified (inotify_add_watch (fd, filename.c_str(), IN_MODIFY);) on linux file system(linux kernel 4.9.0).

But after notification, read() is expected to call twice until I get notification for the next modification on the file /etc/temp. Can someone clarify why do I need to call read() twice? Thank you.

int fd, wd;
fd = inotify_init ();

if (fd < 0)
{
    perror ("inotify_init () = ");
}
else
{
    std::string filename = "/etc/test";
    wd = inotify_add_watch (fd, filename.c_str(), IN_MODIFY);

    if (wd < 0)
    {
    perror ("inotify_add_watch");
    }
    else
    {
        char* buffer = new char[1024];
        while(true)
        {
            //first read blocks until the /etc/temp file is modified, 
            //it returns 16 which is sizeof(struct inotify_event)
            printf("first read %d), read( fd, buffer, 1024));

            //second read() does not block and read returns 16 again
            printf("second read %d), read( fd, buffer, 1024));
         }
     }

}

Upvotes: 0

Views: 856

Answers (1)

that other guy
that other guy

Reputation: 123400

You have to consume all pending events before it'll start blocking again.

When you e.g. do echo foo > /etc/test, you may get two events: one for the truncation, and one for the write.

If you don't consume both, the next will be returned immediately.

Upvotes: 2

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