How could I detect when a directory is mounted with inotify?

Question

I\'m using Linux Inotify to detect FS events on my program.

How could I be notified when a device is mounted on a monitored directory?

Answer 1

inotify only tells you about unmounts, and uevents no longer tells you about mount/unmount.

The way to do is to poll on /proc/mounts, read in the contents, and keep track of the mounts you've seen, and then reparse when the poll wakes up. The poll will wake up on ERR/PRI when any filesystem is mounted or unmounted.

#include 
#include 
#include 
#include 
#include 

int main()
{
    int fd;
    struct pollfd ev;
    int ret;
    ssize_t bytesread;
    char buf[8192];

    fd = open("/proc/mounts", O_RDONLY);
    printf("########################################\n");
    while ((bytesread = read(fd, buf, sizeof(buf))) > 0)
        write(1, buf, bytesread);

    do {

        ev.events = POLLERR | POLLPRI;
        ev.fd = fd;
        ev.revents = 0;
        ret = poll(&ev, 1, -1);
        lseek(fd, 0, SEEK_SET);
        if (ev.revents & POLLERR) {
            printf("########################################\n");
            while ((bytesread = read(fd, buf, sizeof(buf))) > 0)
                write(1, buf, bytesread);
        }
    } while (ret >= 0);
    close(fd);

    return 0;
}

The above code just prints out the mount points on startup, and then on any mount/unmount. It's up to you to "diff" them to find out what got added/removed.

Note, all these techniques has been both unstable and/or broken in past Linux versions. It all got stable around the end of Linux 2.6.35 (or maybe a bit earlier).