notify_store_retrieve.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 <stddef.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 contents change dynamically: it always contains the current time.

While notify_inval_inode.c uses fuse_lowlevel_notify_inval_inode() to let the kernel know that it has to invalidate the cache, this example actively pushes the updated data into the kernel cache using fuse_lowlevel_notify_store().

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

$ notify_store_retrieve --update-interval=1 --no-notify mnt/

Observe that the output never changes, even though the file system updates it once per second. This is because the contents are cached in the kernel:

$ for i in 1 2 3 4 5; do
>     cat mnt/current_time
>     sleep 1
> done
The current time is 15:58:18
The current time is 15:58:18
The current time is 15:58:18
The current time is 15:58:18
The current time is 15:58:18

If you instead enable the notification functions, the changes become visible:

$ notify_store_retrieve --update-interval=1 mnt/
$ for i in 1 2 3 4 5; do
>     cat mnt/current_time
>     sleep 1
> done
The current time is 15:58:40
The current time is 15:58:41
The current time is 15:58:42
The current time is 15:58:43
The current time is 15:58:44

Compilation

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

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 GPL.
  See the file COPYING.
*/

#define FUSE_USE_VERSION 31

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

/* We can't actually tell the kernel that there is no
   timeout, so we just send a big value */
#define NO_TIMEOUT 500000

/* We cannot check directly if e.g. O_RDONLY is set, since this is not
 * an individual bit (cf. open(2)) */
#define ACCESS_MASK (O_RDONLY | O_WRONLY | O_RDWR)

#define MAX_STR_LEN 128
#define FILE_INO 2
#define FILE_NAME "current_time"
static char file_contents[MAX_STR_LEN];
static int lookup_cnt = 0;
static size_t file_size;

/* Keep track if we ever stored data (==1), and
   received it back correctly (==2) */
static int retrieve_status = 0;

/* Command line parsing */
struct options {
    int no_notify;
    int update_interval;
};
static struct options options = {
    .no_notify = 0,
    .update_interval = 1,
};

#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),
    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 | 0444;
        stbuf->st_nlink = 1;
        stbuf->st_size = file_size;
    }

    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 = NO_TIMEOUT;
    e.entry_timeout = NO_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, NO_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 void tfs_open(fuse_req_t req, fuse_ino_t ino,
                     struct fuse_file_info *fi) {

    /* Make cache persistent even if file is closed,
       this makes it easier to see the effects */
    fi->keep_cache = 1;

    if (ino == FUSE_ROOT_ID)
        fuse_reply_err(req, EISDIR);
    else if ((fi->flags & ACCESS_MASK) != O_RDONLY)
        fuse_reply_err(req, EACCES);
    else if (ino == FILE_INO)
        fuse_reply_open(req, fi);
    else {
        // This should not happen
        fprintf(stderr, "Got open for non-existing inode!\n");
        fuse_reply_err(req, ENOENT);
    }
}

static void tfs_read(fuse_req_t req, fuse_ino_t ino, size_t size,
                     off_t off, struct fuse_file_info *fi) {
    (void) fi;

    assert(ino == FILE_INO);
    reply_buf_limited(req, file_contents, file_size, off, size);
}

static void tfs_retrieve_reply(fuse_req_t req, void *cookie, fuse_ino_t ino,
                               off_t offset, struct fuse_bufvec *data) {
    struct fuse_bufvec bufv;
    char buf[MAX_STR_LEN];
    char *expected;
    ssize_t ret;

    assert(ino == FILE_INO);
    assert(offset == 0);
    expected = (char*) cookie;

    bufv.count = 1;
    bufv.idx = 0;
    bufv.off = 0;
    bufv.buf[0].size = MAX_STR_LEN;
    bufv.buf[0].mem = buf;
    bufv.buf[0].flags = 0;

    ret = fuse_buf_copy(&bufv, data, 0);
    assert(ret > 0);
    assert(strncmp(buf, expected, ret) == 0);
    free(expected);
    retrieve_status = 2;
    fuse_reply_none(req);
}


static struct fuse_lowlevel_ops tfs_oper = {
    .lookup     = tfs_lookup,
    .getattr    = tfs_getattr,
    .readdir    = tfs_readdir,
    .open       = tfs_open,
    .read       = tfs_read,
    .forget     = tfs_forget,
    .retrieve_reply = tfs_retrieve_reply,
};

static void update_fs(void) {
    struct tm *now;
    time_t t;
    t = time(NULL);
    now = localtime(&t);
    assert(now != NULL);

    file_size = strftime(file_contents, MAX_STR_LEN,
                         "The current time is %H:%M:%S\n", now);
    assert(file_size != 0);
}

static void* update_fs_loop(void *data) {
    struct fuse_session *se = (struct fuse_session*) data;
    struct fuse_bufvec bufv;
    int ret;

    while(1) {
        update_fs();
        if (!options.no_notify && lookup_cnt) {
            /* Only send notification if the kernel
               is aware of the inode */
            bufv.count = 1;
            bufv.idx = 0;
            bufv.off = 0;
            bufv.buf[0].size = file_size;
            bufv.buf[0].mem = file_contents;
            bufv.buf[0].flags = 0;

            /* This shouldn't fail, but apparently it sometimes
               does - see https://github.com/libfuse/libfuse/issues/105 */
            ret = fuse_lowlevel_notify_store(se, FILE_INO, 0, &bufv, 0);
            if (-ret == ENODEV) {
                // File system was unmounted
                break;
            }
            else if (ret != 0) {
                fprintf(stderr, "ERROR: fuse_lowlevel_notify_store() failed with %s (%d)\n",
                        strerror(-ret), -ret);
                abort();
            }

            /* To make sure that everything worked correctly, ask the
               kernel to send us back the stored data */
            ret = fuse_lowlevel_notify_retrieve(se, FILE_INO, MAX_STR_LEN,
                                                0, (void*) strdup(file_contents));
            if (-ret == ENODEV) { // File system was unmounted
                break;
            }
            assert(ret == 0);
            if(retrieve_status == 0)
                retrieve_status = 1;
        }
        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"
               "    --update-interval=<secs>  Update-rate of file system contents\n"
               "    --no-notify            Disable kernel notifications\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;
    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), NULL);
    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);

    /* 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
        ret = fuse_session_loop_mt(se, opts.clone_fd);

    assert(retrieve_status != 1);
    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_store_retrieve.c.