How to use inotify in C?

Question

I searched for questions related to inotify, and this one is somewhat different...

I use the following code to monitor change of one file (not directory). In testing, the read() does return when I save the target file which means it works. But event->mask is 32768 which is not IN_MODIFY and name is empty. Another issue: it cannot monitor continuously. When I change the file the second time, it has no response. Thank you for the help!

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

#define EVENT_SIZE  (sizeof (struct inotify_event))
#define BUF_LEN        (16 * (EVENT_SIZE + 16))

int main()
{
    int fd;
    fd = inotify_init();
    if (fd < 0)
        perror("inotify_init()");

    int wd;
    wd = inotify_add_watch(fd, "target.txt", IN_MODIFY);
    if (wd < 0)
        perror("inotify_add_watch");

    char buf[BUF_LEN];
    int len;

start:

    len = read(fd, buf, BUF_LEN);

    if (len > 0)
    {
        int i = 0;
        while (i < len)
        {
            struct inotify_event *event;
            event = (struct inotify_event *) &buf[i];

            printf("wd=%d mask=%x cookie=%u len=%u\n",
                event->wd, event->mask,
                event->cookie, event->len);

            if (event->mask & IN_MODIFY)
                printf("file modified %s", event->name);

            if (event->len)
                printf("name=%s\n", event->name);

            i += EVENT_SIZE + event->len;
        }
    }

    goto start;

    return 0;
}

Answer 1

The 0x8000 corresponds to IN_IGNORED. Its presence in the mask indicates that the inotify watch had been removed because the file had been removed. Your editor probably removed the old file and put a new file in its place. Changing the file a second time had no effect because the watch had been removed.

The name is not being returned because you are not watching a directory.

From the inotify man page.

The name field is only present when an event is returned for a file inside a watched directory; it identifies the file pathname relative to the watched directory.

...

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

Answer 2

event->mask 32768 is equivalent to 0x8000 that is IN_IGNORED For more information : "/usr/include/linux/inotify.h"

    if (event->mask & IN_IGNORED) {
        /*Remove watch*/ inotify_rm_watch(fileDescriptor,watchDescriptor)
        /*Add watch again*/ inotify_add_watch
    }