There are many problems with this interface. It can only watch directories. If one wants to watch many directories,it takes many file descriptors. Moreover,the open file pins the filesystem so that it cannot be unmounted. When something happens it is unknown what,and a?stat()?on all files of interest is needed. The communication mechanism,signals,is unfortunate. Dnotify is obsolete now.

12.7?inotify

(Since 2.6.13.) Inotify is implemented using three new system calls and the usual?read(),?poll(),?close()?calls:

int inotify_init(void);
int inotify_add_watch (int fd,const char *pathname,int mask);
int inotify_rm_watch (int fd,int wd);

The first returns a file descriptor:?fd = inotify_init(). The second tells what to watch,and what to watch for,and returns a watch descriptor:?wd = inotify_add_watch(fd,"/home/aeb",IN_CREATE | IN_DELETE). The file descriptor?fd?can be used in a?read()?call,and then returns an array of struct inotify_event's. One can use?select()?and?poll()on it. A watch is removed by?inotify_rm_watch(fd,wd). The inotify instance is closed by?close(fd).

An inotify_event is defined by

struct inotify_event {
        int      wd;       /* Watch descriptor */
        uint32_t mask;     /* Mask of events */
        uint32_t cookie;   /* Unique cookie associating related
                              events (for rename(2)) */
        uint32_t len;      /* Size of 'name' field */
        char     name[];   /* Optional null-terminated name */
};

The?name?field defines the file involved,when one is watching a directory.

There is a?/proc?interface with settable limits:

% ls /proc/sys/fs/inotify/
max_queued_events  max_user_instances  max_user_watches
% cat $_/*
16384
128
8192

Applications are?,??and?.

/* inotify demo,mimicking the above dnotify one */
#include 
#include 
#include 
#include 
#include 
define BUFSZ   16384
static void errexit(char *s) {

fprintf(stderr,"%sn",s);

exit(1);

}
int main(void) {

int ifd,wd,i,n;

char buf[BUFSZ];
    ifd = inotify_init();
    if (ifd < 0)
            errexit("cannot obtain an inotify instance");

    wd = inotify_add_watch(ifd,".",IN_MODIFY|IN_CREATE|IN_DELETE);
    if (wd < 0)
            errexit("cannot add inotify watch");

    while (1) {
            n = read(ifd,buf,sizeof(buf));
            if (n <= 0)
                    errexit("read problem");

            i = 0;
            while (i < n) {
                    struct inotify_event *ev;

                    ev = (struct inotify_event *) &amp;buf[i];
                    if (ev->len)
                            printf("file %s %sn",ev->name,(ev->mask &amp; IN_CREATE) ? "created" :
                                   (ev->mask &amp; IN_DELETE) ? "deleted" :
                                   "modified");
                    else
                            printf("unexpected event - wd=%d mask=%dn",ev->wd,ev->mask);

                    i += sizeof(struct inotify_event) + ev->len;
            }
            printf("---n");
    }
}

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