libfuse
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),
};
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);
}
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;
}
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 apparenly 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]);
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 */
update_fs();
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;
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);
err_out3:
err_out2:
err_out1:
free(opts.mountpoint);
return ret ? 1 : 0;
}

Definition in file notify_store_retrieve.c.