notify_inval_entry.c File Reference

#include <fuse_lowlevel.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <assert.h>
#include <signal.h>
#include <stddef.h>
#include <sys/stat.h>
#include <unistd.h>
#include <pthread.h>

Go to the source code of this file.

Detailed Description

This example implements a file system with a single file whose file name changes dynamically to reflect the current time.

It illustrates the use of the fuse_lowlevel_notify_inval_entry() and fuse_lowlevel_notify_expire_entry() functions.

To see the effect, first start the file system with the --no-notify

$ notify_inval_entry --update-interval=1 --timeout 30 --no-notify mnt/

Observe that ls always prints the correct directory contents (since readdir output is not cached)::

$ ls mnt; sleep 1; ls mnt; sleep 1; ls mnt
Time_is_15h_48m_33s  current_time
Time_is_15h_48m_34s  current_time
Time_is_15h_48m_35s  current_time

However, if you try to access a file by name the kernel will report that it still exists:

$ file=$(ls mnt/); echo $file
Time_is_15h_50m_09s
$ sleep 5; stat mnt/$file
  File: ‘mnt/Time_is_15h_50m_09s’
  Size: 32                Blocks: 0          IO Block: 4096   regular file
Device: 2ah/42d     Inode: 3           Links: 1
Access: (0444/-r--r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 1969-12-31 16:00:00.000000000 -0800
Modify: 1969-12-31 16:00:00.000000000 -0800
Change: 1969-12-31 16:00:00.000000000 -0800
 Birth: -

Only once the kernel cache timeout has been reached will the stat call fail:

$ sleep 30; stat mnt/$file
stat: cannot stat ‘mnt/Time_is_15h_50m_09s’: No such file or directory

In contrast, if you enable notifications you will be unable to stat the file as soon as the file system updates its name:

$ notify_inval_entry --update-interval=1 --timeout 30 --no-notify mnt/
$ file=$(ls mnt/); stat mnt/$file
  File: ‘mnt/Time_is_20h_42m_11s’
  Size: 0                 Blocks: 0          IO Block: 4096   regular empty file
Device: 2ah/42d     Inode: 2           Links: 1
Access: (0000/----------)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 1969-12-31 16:00:00.000000000 -0800
Modify: 1969-12-31 16:00:00.000000000 -0800
Change: 1969-12-31 16:00:00.000000000 -0800
 Birth: -
$ sleep 1; stat mnt/$file
stat: cannot stat ‘mnt/Time_is_20h_42m_11s’: No such file or directory

To use the function fuse_lowlevel_notify_expire_entry() instead of fuse_lowlevel_notify_inval_entry(), use the command line option –only-expire

Compilation

gcc -Wall notify_inval_entry.c `pkg-config fuse3 --cflags --libs` -o notify_inval_entry

Source code

/*
  FUSE: Filesystem in Userspace
  Copyright (C) 2016 Nikolaus Rath <Nikolaus@rath.org>
 
  This program can be distributed under the terms of the GNU GPLv2.
  See the file COPYING.
*/
 
#define FUSE_USE_VERSION 34
 
#include <fuse_lowlevel.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <assert.h>
#include <signal.h>
#include <stddef.h>
#include <sys/stat.h>
#include <unistd.h>
#include <pthread.h>
 
#define MAX_STR_LEN 128
static char file_name[MAX_STR_LEN];
static fuse_ino_t file_ino = 2;
static int lookup_cnt = 0;
static pthread_t main_thread;
 
/* Command line parsing */
struct options {
    int no_notify;
    float timeout;
    int update_interval;
    int only_expire;
};
static struct options options = {
    .timeout = 5,
    .no_notify = 0,
    .update_interval = 1,
    .only_expire = 0,
};
 
#define OPTION(t, p)                           \
    { t, offsetof(struct options, p), 1 }
static const struct fuse_opt option_spec[] = {
    OPTION("--no-notify", no_notify),
    OPTION("--update-interval=%d", update_interval),
    OPTION("--timeout=%f", timeout),
    OPTION("--only-expire", only_expire),
    FUSE_OPT_END
};
 
static int tfs_stat(fuse_ino_t ino, struct stat *stbuf) {
    stbuf->st_ino = ino;
    if (ino == FUSE_ROOT_ID) {
        stbuf->st_mode = S_IFDIR | 0755;
        stbuf->st_nlink = 1;
    }
 
    else if (ino == file_ino) {
        stbuf->st_mode = S_IFREG | 0000;
        stbuf->st_nlink = 1;
        stbuf->st_size = 0;
    }
 
    else
        return -1;
 
    return 0;
}
 
static void tfs_lookup(fuse_req_t req, fuse_ino_t parent,
                       const char *name) {
    struct fuse_entry_param e;
    memset(&e, 0, sizeof(e));
 
    if (parent != FUSE_ROOT_ID)
        goto err_out;
    else if (strcmp(name, file_name) == 0) {
        e.ino = file_ino;
        lookup_cnt++;
    } else
        goto err_out;
 
    e.attr_timeout = options.timeout;
    e.entry_timeout = options.timeout;
    if (tfs_stat(e.ino, &e.attr) != 0)
        goto err_out;
    fuse_reply_entry(req, &e);
    return;
 
err_out:
    fuse_reply_err(req, ENOENT);
}
 
static void tfs_forget (fuse_req_t req, fuse_ino_t ino,
                        uint64_t nlookup) {
    (void) req;
    if(ino == file_ino)
        lookup_cnt -= nlookup;
    else
        assert(ino == FUSE_ROOT_ID);
    fuse_reply_none(req);
}
 
static void tfs_getattr(fuse_req_t req, fuse_ino_t ino,
                        struct fuse_file_info *fi) {
    struct stat stbuf;
 
    (void) fi;
 
    memset(&stbuf, 0, sizeof(stbuf));
    if (tfs_stat(ino, &stbuf) != 0)
        fuse_reply_err(req, ENOENT);
    else
        fuse_reply_attr(req, &stbuf, options.timeout);
}
 
struct dirbuf {
    char *p;
    size_t size;
};
 
static void dirbuf_add(fuse_req_t req, struct dirbuf *b, const char *name,
                       fuse_ino_t ino) {
    struct stat stbuf;
    size_t oldsize = b->size;
    b->size += fuse_add_direntry(req, NULL, 0, name, NULL, 0);
    b->p = (char *) realloc(b->p, b->size);
    memset(&stbuf, 0, sizeof(stbuf));
    stbuf.st_ino = ino;
    fuse_add_direntry(req, b->p + oldsize, b->size - oldsize, name, &stbuf,
                      b->size);
}
 
#define min(x, y) ((x) < (y) ? (x) : (y))
 
static int reply_buf_limited(fuse_req_t req, const char *buf, size_t bufsize,
                             off_t off, size_t maxsize) {
    if (off < bufsize)
        return fuse_reply_buf(req, buf + off,
                              min(bufsize - off, maxsize));
    else
        return fuse_reply_buf(req, NULL, 0);
}
 
static void tfs_readdir(fuse_req_t req, fuse_ino_t ino, size_t size,
                        off_t off, struct fuse_file_info *fi) {
    (void) fi;
 
    if (ino != FUSE_ROOT_ID)
        fuse_reply_err(req, ENOTDIR);
    else {
        struct dirbuf b;
 
        memset(&b, 0, sizeof(b));
        dirbuf_add(req, &b, file_name, file_ino);
        reply_buf_limited(req, b.p, b.size, off, size);
        free(b.p);
    }
}
 
static const struct fuse_lowlevel_ops tfs_oper = {
    .lookup     = tfs_lookup,
    .getattr    = tfs_getattr,
    .readdir    = tfs_readdir,
    .forget     = tfs_forget,
};
 
static void update_fs(void) {
    time_t t;
    struct tm *now;
    ssize_t ret;
 
    t = time(NULL);
    now = localtime(&t);
    assert(now != NULL);
 
    ret = strftime(file_name, MAX_STR_LEN,
                   "Time_is_%Hh_%Mm_%Ss", now);
    assert(ret != 0);
}
 
static void* update_fs_loop(void *data) {
    struct fuse_session *se = (struct fuse_session*) data;
    char *old_name;
 
 
    while(!fuse_session_exited(se)) {
        old_name = strdup(file_name);
        update_fs();
 
        if (!options.no_notify && lookup_cnt) {
            if(options.only_expire) { // expire entry
                int ret = fuse_lowlevel_notify_expire_entry
                   (se, FUSE_ROOT_ID, old_name, strlen(old_name));
 
                // no kernel support
                if (ret == -ENOSYS) {
                    printf("fuse_lowlevel_notify_expire_entry not supported by kernel\n");
                    printf("Exiting...\n");
 
                    fuse_session_exit(se);
                    // Make sure to exit now, rather than on next request from userspace
                    pthread_kill(main_thread, SIGPIPE);
 
                    break;
                }
                // 1) ret == 0: successful expire of an existing entry
                // 2) ret == -ENOENT: kernel has already expired the entry /
                //                    entry does not exist anymore in the kernel
                assert(ret == 0 || ret == -ENOENT);
            } else { // invalidate entry
                assert(fuse_lowlevel_notify_inval_entry
                      (se, FUSE_ROOT_ID, old_name, strlen(old_name)) == 0);
            }
        }
        free(old_name);
        sleep(options.update_interval);
    }
    return NULL;
}
 
static void show_help(const char *progname)
{
    printf("usage: %s [options] <mountpoint>\n\n", progname);
    printf("File-system specific options:\n"
               "    --timeout=<secs>       Timeout for kernel caches\n"
               "    --update-interval=<secs>  Update-rate of file system contents\n"
               "    --no-notify            Disable kernel notifications\n"
               "    --only-expire            Expire entries instead of invalidating them\n"
               "\n");
}
 
int main(int argc, char *argv[]) {
    struct fuse_args args = FUSE_ARGS_INIT(argc, argv);
    struct fuse_session *se;
    struct fuse_cmdline_opts opts;
    struct fuse_loop_config config;
    pthread_t updater;
    int ret = -1;
 
    if (fuse_opt_parse(&args, &options, option_spec, NULL) == -1)
        return 1;
 
    if (fuse_parse_cmdline(&args, &opts) != 0)
        return 1;
    if (opts.show_help) {
        show_help(argv[0]);
        fuse_cmdline_help();
        fuse_lowlevel_help();
        ret = 0;
        goto err_out1;
    } else if (opts.show_version) {
        printf("FUSE library version %s\n", fuse_pkgversion());
        fuse_lowlevel_version();
        ret = 0;
        goto err_out1;
    }
 
    /* Initial contents */
    update_fs();
 
    se = fuse_session_new(&args, &tfs_oper,
                          sizeof(tfs_oper), &se);
    if (se == NULL)
        goto err_out1;
 
    if (fuse_set_signal_handlers(se) != 0)
        goto err_out2;
 
    if (fuse_session_mount(se, opts.mountpoint) != 0)
        goto err_out3;
 
    fuse_daemonize(opts.foreground);
 
    // Needed to ensure that the main thread continues/restarts processing as soon
    // as the fuse session ends (immediately after calling fuse_session_exit() ) 
    // and not only on the next request from userspace
    main_thread = pthread_self();
 
    /* Start thread to update file contents */
    ret = pthread_create(&updater, NULL, update_fs_loop, (void *)se);
    if (ret != 0) {
        fprintf(stderr, "pthread_create failed with %s\n",
                strerror(ret));
        goto err_out3;
    }
 
    /* Block until ctrl+c or fusermount -u */
    if (opts.singlethread) {
        ret = fuse_session_loop(se);
    } else {
        config.clone_fd = opts.clone_fd;
        config.max_idle_threads = opts.max_idle_threads;
        ret = fuse_session_loop_mt(se, &config);
    }
 
    fuse_session_unmount(se);
err_out3:
    fuse_remove_signal_handlers(se);
err_out2:
    fuse_session_destroy(se);
err_out1:
    free(opts.mountpoint);
    fuse_opt_free_args(&args);
 
    return ret ? 1 : 0;
}
 
 

Definition in file notify_inval_entry.c.