Reading events from an inotify file descriptor¶

To determine what events have occurred, an application read(2)s from the inotify file descriptor. If no events have so far occurred, then, assuming a blocking file descriptor, read(2) will block until at least one event occurs (unless interrupted by a signal, in which case the call fails with the error EINTR; see signal(7)).

Each successful read(2) returns a buffer containing one or more of the following structures:

struct inotify_event {


    int      wd;       /* Watch descriptor */


    uint32_t mask;     /* Mask describing event */


    uint32_t cookie;   /* Unique cookie associating related


                          events (for rename(2)) */


    uint32_t len;      /* Size of name field */


    char     name[];   /* Optional null-terminated name */
};

wd identifies the watch for which this event occurs. It is one of the watch descriptors returned by a previous call to inotify_add_watch(2).

mask contains bits that describe the event that occurred (see below).

cookie is a unique integer that connects related events. Currently, this is used only for rename events, and allows the resulting pair of IN_MOVED_FROM and IN_MOVED_TO events to be connected by the application. For all other event types, cookie is set to 0.

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

The len field counts all of the bytes in name, including the null bytes; the length of each inotify_event structure is thus sizeof(struct inotify_event)+len.

The behavior when the buffer given to read(2) is too small to return information about the next event depends on the kernel version: before Linux 2.6.21, read(2) returns 0; since Linux 2.6.21, read(2) fails with the error EINVAL. Specifying a buffer of size

sizeof(struct inotify_event) + NAME_MAX + 1

will be sufficient to read at least one event.

inotify events¶

The inotify_add_watch(2) mask argument and the mask field of the inotify_event structure returned when read(2)ing an inotify file descriptor are both bit masks identifying inotify events. The following bits can be specified in mask when calling inotify_add_watch(2) and may be returned in the mask field returned by read(2):

Inotify monitoring is inode-based: when monitoring a file (but not when monitoring the directory containing a file), an event can be generated for activity on any link to the file (in the same or a different directory).

When monitoring a directory:

•

the events marked above with an asterisk (*) can occur both for the directory itself and for objects inside the directory; and

•

the events marked with a plus sign (+) occur only for objects inside the directory (not for the directory itself).

Note: when monitoring a directory, events are not generated for the files inside the directory when the events are performed via a pathname (i.e., a link) that lies outside the monitored directory.

When events are generated for objects inside a watched directory, the name field in the returned inotify_event structure identifies the name of the file within the directory.

The IN_ALL_EVENTS macro is defined as a bit mask of all of the above events. This macro can be used as the mask argument when calling inotify_add_watch(2).

Two additional convenience macros are defined:

The following further bits can be specified in mask when calling inotify_add_watch(2):

The following bits may be set in the mask field returned by read(2):

Examples¶

Suppose an application is watching the directory dir and the file dir/myfile for all events. The examples below show some events that will be generated for these two objects.

Suppose an application is watching the directories dir1 and dir2, and the file dir1/myfile. The following examples show some events that may be generated.

Suppose that dir1/xx and dir2/yy are (the only) links to the same file, and an application is watching dir1, dir2, dir1/xx, and dir2/yy. Executing the following calls in the order given below will generate the following events:

Suppose an application is watching the directory dir and (the empty) directory dir/subdir. The following examples show some events that may be generated.

/proc interfaces¶

The following interfaces can be used to limit the amount of kernel memory consumed by inotify:

/proc/sys/fs/inotify/max_queued_events

The value in this file is used when an application calls inotify_init(2) to set an upper limit on the number of events that can be queued to the corresponding inotify instance. Events in excess of this limit are dropped, but an IN_Q_OVERFLOW event is always generated.

/proc/sys/fs/inotify/max_user_instances

This specifies an upper limit on the number of inotify instances that can be created per real user ID.

/proc/sys/fs/inotify/max_user_watches

This specifies an upper limit on the number of watches that can be created per real user ID.

Limitations and caveats¶

The inotify API provides no information about the user or process that triggered the inotify event. In particular, there is no easy way for a process that is monitoring events via inotify to distinguish events that it triggers itself from those that are triggered by other processes.

Inotify reports only events that a user-space program triggers through the filesystem API. As a result, it does not catch remote events that occur on network filesystems. (Applications must fall back to polling the filesystem to catch such events.) Furthermore, various pseudo-filesystems such as /proc, /sys, and /dev/pts are not monitorable with inotify.

The inotify API does not report file accesses and modifications that may occur because of mmap(2), msync(2), and munmap(2).

The inotify API identifies affected files by filename. However, by the time an application processes an inotify event, the filename may already have been deleted or renamed.

The inotify API identifies events via watch descriptors. It is the application's responsibility to cache a mapping (if one is needed) between watch descriptors and pathnames. Be aware that directory renamings may affect multiple cached pathnames.

Inotify monitoring of directories is not recursive: to monitor subdirectories under a directory, additional watches must be created. This can take a significant amount time for large directory trees.

If monitoring an entire directory subtree, and a new subdirectory is created in that tree or an existing directory is renamed into that tree, be aware that by the time you create a watch for the new subdirectory, new files (and subdirectories) may already exist inside the subdirectory. Therefore, you may want to scan the contents of the subdirectory immediately after adding the watch (and, if desired, recursively add watches for any subdirectories that it contains).

Note that the event queue can overflow. In this case, events are lost. Robust applications should handle the possibility of lost events gracefully. For example, it may be necessary to rebuild part or all of the application cache. (One simple, but possibly expensive, approach is to close the inotify file descriptor, empty the cache, create a new inotify file descriptor, and then re-create watches and cache entries for the objects to be monitored.)

If a filesystem is mounted on top of a monitored directory, no event is generated, and no events are generated for objects immediately under the new mount point. If the filesystem is subsequently unmounted, events will subsequently be generated for the directory and the objects it contains.

Dealing with rename() events¶

As noted above, the IN_MOVED_FROM and IN_MOVED_TO event pair that is generated by rename(2) can be matched up via their shared cookie value. However, the task of matching has some challenges.

These two events are usually consecutive in the event stream available when reading from the inotify file descriptor. However, this is not guaranteed. If multiple processes are triggering events for monitored objects, then (on rare occasions) an arbitrary number of other events may appear between the IN_MOVED_FROM and IN_MOVED_TO events. Furthermore, it is not guaranteed that the event pair is atomically inserted into the queue: there may be a brief interval where the IN_MOVED_FROM has appeared, but the IN_MOVED_TO has not.

Matching up the IN_MOVED_FROM and IN_MOVED_TO event pair generated by rename(2) is thus inherently racy. (Don't forget that if an object is renamed outside of a monitored directory, there may not even be an IN_MOVED_TO event.) Heuristic approaches (e.g., assume the events are always consecutive) can be used to ensure a match in most cases, but will inevitably miss some cases, causing the application to perceive the IN_MOVED_FROM and IN_MOVED_TO events as being unrelated. If watch descriptors are destroyed and re-created as a result, then those watch descriptors will be inconsistent with the watch descriptors in any pending events. (Re-creating the inotify file descriptor and rebuilding the cache may be useful to deal with this scenario.)

Applications should also allow for the possibility that the IN_MOVED_FROM event was the last event that could fit in the buffer returned by the current call to read(2), and the accompanying IN_MOVED_TO event might be fetched only on the next read(2), which should be done with a (small) timeout to allow for the fact that insertion of the IN_MOVED_FROM+IN_MOVED_TO event pair is not atomic, and also the possibility that there may not be any IN_MOVED_TO event.

Example output¶

$ ./a.out /tmp /home/user/temp
Press enter key to terminate.
Listening for events.
IN_OPEN: /home/user/temp/foo [file]
IN_CLOSE_WRITE: /home/user/temp/foo [file]
IN_OPEN: /tmp/ [directory]
IN_CLOSE_NOWRITE: /tmp/ [directory]
Listening for events stopped.

Program source¶

#include <errno.h>
#include <poll.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/inotify.h>
#include <unistd.h>
#include <string.h>
/* Read all available inotify events from the file descriptor 'fd'.


   wd is the table of watch descriptors for the directories in argv.


   argc is the length of wd and argv.


   argv is the list of watched directories.


   Entry 0 of wd and argv is unused. */
static void
handle_events(int fd, int *wd, int argc, char* argv[])
{


    /* Some systems cannot read integer variables if they are not


       properly aligned. On other systems, incorrect alignment may


       decrease performance. Hence, the buffer used for reading from


       the inotify file descriptor should have the same alignment as


       struct inotify_event. */


    char buf[4096]


        __attribute__ ((aligned(__alignof__(struct inotify_event))));


    const struct inotify_event *event;


    ssize_t len;


    /* Loop while events can be read from inotify file descriptor. */


    for (;;) {


        /* Read some events. */


        len = read(fd, buf, sizeof(buf));


        if (len == -1 && errno != EAGAIN) {


            perror("read");


            exit(EXIT_FAILURE);


        }


        /* If the nonblocking read() found no events to read, then


           it returns -1 with errno set to EAGAIN. In that case,


           we exit the loop. */


        if (len <= 0)


            break;


        /* Loop over all events in the buffer. */


        for (char *ptr = buf; ptr < buf + len;


                ptr += sizeof(struct inotify_event) + event->len) {


            event = (const struct inotify_event *) ptr;


            /* Print event type. */


            if (event->mask & IN_OPEN)


                printf("IN_OPEN: ");


            if (event->mask & IN_CLOSE_NOWRITE)


                printf("IN_CLOSE_NOWRITE: ");


            if (event->mask & IN_CLOSE_WRITE)


                printf("IN_CLOSE_WRITE: ");


            /* Print the name of the watched directory. */


            for (size_t i = 1; i < argc; ++i) {


                if (wd[i] == event->wd) {


                    printf("%s/", argv[i]);


                    break;


                }


            }


            /* Print the name of the file. */


            if (event->len)


                printf("%s", event->name);


            /* Print type of filesystem object. */


            if (event->mask & IN_ISDIR)


                printf(" [directory]\n");


            else


                printf(" [file]\n");


        }


    }
}
int
main(int argc, char* argv[])
{


    char buf;


    int fd, i, poll_num;


    int *wd;


    nfds_t nfds;


    struct pollfd fds[2];


    if (argc < 2) {


        printf("Usage: %s PATH [PATH ...]\n", argv[0]);


        exit(EXIT_FAILURE);


    }


    printf("Press ENTER key to terminate.\n");


    /* Create the file descriptor for accessing the inotify API. */


    fd = inotify_init1(IN_NONBLOCK);


    if (fd == -1) {


        perror("inotify_init1");


        exit(EXIT_FAILURE);


    }


    /* Allocate memory for watch descriptors. */


    wd = calloc(argc, sizeof(int));


    if (wd == NULL) {


        perror("calloc");


        exit(EXIT_FAILURE);


    }


    /* Mark directories for events


       - file was opened


       - file was closed */


    for (i = 1; i < argc; i++) {


        wd[i] = inotify_add_watch(fd, argv[i],


                                  IN_OPEN | IN_CLOSE);


        if (wd[i] == -1) {


            fprintf(stderr, "Cannot watch '%s': %s\n",


                    argv[i], strerror(errno));


            exit(EXIT_FAILURE);


        }


    }


    /* Prepare for polling. */


    nfds = 2;


    fds[0].fd = STDIN_FILENO;       /* Console input */


    fds[0].events = POLLIN;


    fds[1].fd = fd;                 /* Inotify input */


    fds[1].events = POLLIN;


    /* Wait for events and/or terminal input. */


    printf("Listening for events.\n");


    while (1) {


        poll_num = poll(fds, nfds, -1);


        if (poll_num == -1) {


            if (errno == EINTR)


                continue;


            perror("poll");


            exit(EXIT_FAILURE);


        }


        if (poll_num > 0) {


            if (fds[0].revents & POLLIN) {


                /* Console input is available. Empty stdin and quit. */


                while (read(STDIN_FILENO, &buf, 1) > 0 && buf != '\n')


                    continue;


                break;


            }


            if (fds[1].revents & POLLIN) {


                /* Inotify events are available. */


                handle_events(fd, wd, argc, argv);


            }


        }


    }


    printf("Listening for events stopped.\n");


    /* Close inotify file descriptor. */


    close(fd);


    free(wd);


    exit(EXIT_SUCCESS);
}