notify_inval_inode.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_store_retrieve.c uses fuse_lowlevel_notify_store() to actively push the updated data into the kernel cache, this example uses fuse_lowlevel_notify_inval_inode() to notify the kernel that the cache has to be invalidated - but the kernel still has to explicitly request the updated data on the next read.

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

$ notify_inval_inode –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_inval_inode --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


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

Source code

FUSE: Filesystem in Userspace
Copyright (C) 2016 Nikolaus Rath <>
This program can be distributed under the terms of the GNU GPLv2.
See the file COPYING.
#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
#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;
/* 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),
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;
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;
} 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);
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;
assert(ino == FUSE_ROOT_ID);
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);
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,
#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));
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);
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 & O_ACCMODE) != 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 const struct fuse_lowlevel_ops tfs_oper = {
.lookup = tfs_lookup,
.getattr = tfs_getattr,
.readdir = tfs_readdir,
.open = tfs_open,
.read = tfs_read,
.forget = tfs_forget,
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;
while(1) {
if (!options.no_notify && lookup_cnt) {
/* Only send notification if the kernel
is aware of the inode */
(se, FILE_INO, 0, 0) == 0);
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"
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) {
ret = 1;
goto err_out1;
if (opts.show_help) {
ret = 0;
goto err_out1;
} else if (opts.show_version) {
printf("FUSE library version %s\n", fuse_pkgversion());
ret = 0;
goto err_out1;
/* Initial contents */
se = fuse_session_new(&args, &tfs_oper,
sizeof(tfs_oper), NULL);
if (se == NULL)
goto err_out1;
goto err_out2;
if (fuse_session_mount(se, opts.mountpoint) != 0)
goto err_out3;
/* 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",
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);
return ret ? 1 : 0;
int fuse_set_signal_handlers(struct fuse_session *se)
Definition: fuse_signals.c:62
const char * fuse_pkgversion(void)
Definition: fuse.c:5124
void fuse_remove_signal_handlers(struct fuse_session *se)
Definition: fuse_signals.c:79
int fuse_daemonize(int foreground)
Definition: helper.c:253
void fuse_session_destroy(struct fuse_session *se)
#define FUSE_ROOT_ID
Definition: fuse_lowlevel.h:43
int fuse_reply_open(fuse_req_t req, const struct fuse_file_info *fi)
int fuse_reply_err(fuse_req_t req, int err)
int fuse_reply_buf(fuse_req_t req, const char *buf, size_t size)
struct fuse_req * fuse_req_t
Definition: fuse_lowlevel.h:49
int fuse_session_loop(struct fuse_session *se)
Definition: fuse_loop.c:19
int fuse_reply_entry(fuse_req_t req, const struct fuse_entry_param *e)
void fuse_session_unmount(struct fuse_session *se)
void fuse_cmdline_help(void)
Definition: helper.c:130
struct fuse_session * fuse_session_new(struct fuse_args *args, const struct fuse_lowlevel_ops *op, size_t op_size, void *userdata)
void fuse_reply_none(fuse_req_t req)
void fuse_lowlevel_help(void)
int fuse_lowlevel_notify_inval_inode(struct fuse_session *se, fuse_ino_t ino, off_t off, off_t len)
int fuse_session_mount(struct fuse_session *se, const char *mountpoint)
void fuse_lowlevel_version(void)
uint64_t fuse_ino_t
Definition: fuse_lowlevel.h:46
size_t fuse_add_direntry(fuse_req_t req, char *buf, size_t bufsize, const char *name, const struct stat *stbuf, off_t off)
int fuse_reply_attr(fuse_req_t req, const struct stat *attr, double attr_timeout)
void fuse_opt_free_args(struct fuse_args *args)
Definition: fuse_opt.c:34
int fuse_opt_parse(struct fuse_args *args, void *data, const struct fuse_opt opts[], fuse_opt_proc_t proc)
Definition: fuse_opt.c:398
#define FUSE_ARGS_INIT(argc, argv)
Definition: fuse_opt.h:123
#define FUSE_OPT_END
Definition: fuse_opt.h:104
int argc
Definition: fuse_opt.h:111
char ** argv
Definition: fuse_opt.h:114
Definition: fuse_lowlevel.h:59
fuse_ino_t ino
Definition: fuse_lowlevel.h:67
unsigned int keep_cache
Definition: fuse_common.h:69
void(* lookup)(fuse_req_t req, fuse_ino_t parent, const char *name)

Definition in file notify_inval_inode.c.