Add a new daemonize API

Existing example/ file systems do the fuse_daemonize() after
fuse_session_mount() - i.e. after the mount point is already
established. Though, these example/ daemons do not start
extra threads and do not need network initialization either.

fuse_daemonize() also does not allow to return notification
from the forked child to the parent.

Complex fuse file system daemons often want the order of
1) fork - parent watches, child does the work

Child:
    2) start extra threads and system initialization (like network
       connection and RDMA memory registration) from the fork child.
    3) Start the fuse session after everything else succeeded

Parent:
    Report child initialization success or failure

A new API is introduced to overcome the limitations of
fuse_daemonize()

fuse_daemonize_start() - fork, but foreground process does not
terminate yet and watches the background.

fuse_daemonize_success() / fuse_daemonize_fail() - background
daemon signals to the foreground process success or failure.

fuse_daemonize_active() - helper function for the high level
interface, which needs to handle both APIs. fuse_daemonize()
is called within fuse_main(), which now needs to know if the caller
actually already used the new API itself.

The object 'struct fuse_daemonize *' is allocated dynamically
and stored a global variable in fuse_daemonize.c, because
- high level fuse_main_real_versioned() needs to know
if already daemonized
- high level daemons do not have access to struct fuse_session
- FUSE_SYNC_INIT in later commits can only be done if the new
API (or a file system internal) daemonization is used.

Signed-off-by: Bernd Schubert <bernd@bsbernd.com>

example/passthrough_hp.cc

@@ -55,6 +55,7 @@
 #include <errno.h>
 #include <ftw.h>
 #include <fuse_lowlevel.h>
+#include <fuse_daemonize.h>
 #include <inttypes.h>
 #include <string.h>
 #include <sys/file.h>
@@ -1580,6 +1581,7 @@ int main(int argc, char *argv[])
 {
 	struct fuse_loop_config *loop_config = NULL;
 	void *teardown_watchog = NULL;
+	unsigned int daemon_flags = 0;
 
 	// Parse command line options
 	auto options{ parse_options(argc, argv) };
@@ -1638,10 +1640,14 @@ int main(int argc, char *argv[])
 
 	fuse_loop_cfg_set_clone_fd(loop_config, fs.clone_fd);
 
-	if (fuse_session_mount(se, argv[2]) != 0)
+	/* Start daemonization before mount so parent can report mount failure */
+	if (fs.foreground)
+		daemon_flags |= FUSE_DAEMONIZE_NO_BACKGROUND;
+	if (fuse_daemonize_early_start(daemon_flags) != 0)
 		goto err_out3;
 
-	fuse_daemonize(fs.foreground);
+	if (fuse_session_mount(se, argv[2]) != 0)
+		goto err_out4;
 
 	if (!fs.foreground)
 		fuse_log_enable_syslog("passthrough-hp", LOG_PID | LOG_CONS,
@@ -1650,7 +1656,7 @@ int main(int argc, char *argv[])
 	teardown_watchog = fuse_session_start_teardown_watchdog(
 		se, fs.root.stop_timeout_secs, NULL, NULL);
 	if (teardown_watchog == NULL)
-		goto err_out3;
+		goto err_out4;
 
 	if (options.count("single"))
 		ret = fuse_session_loop(se);
@@ -1659,6 +1665,9 @@ int main(int argc, char *argv[])
 
 	fuse_session_unmount(se);
 
+err_out4:
+	if (fuse_daemonize_early_is_active())
+		fuse_daemonize_early_fail(ret);
 err_out3:
 	fuse_remove_signal_handlers(se);
 err_out2:

include/fuse_daemonize.h

@@ -0,0 +1,67 @@
+/*
+ * FUSE: Filesystem in Userspace
+ * Copyright (C) 2026 Bernd Schubert <bsbernd.com>
+ *
+ * This program can be distributed under the terms of the GNU LGPLv2.
+ * See the file COPYING.LIB.
+ *
+ */
+
+#ifndef FUSE_DAEMONIZE_H_
+#define FUSE_DAEMONIZE_H_
+
+#include <stdint.h>
+#include <stdbool.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Flags for fuse_daemonize_start()
+ */
+#define FUSE_DAEMONIZE_NO_CHDIR      (1 << 0)
+#define FUSE_DAEMONIZE_NO_BACKGROUND (1 << 1)
+
+/**
+ * Start daemonization process.
+ *
+ * Unless FUSE_DAEMONIZE_NO_BACKGROUND is set, this forks the process.
+ * The parent waits for a signal from the child via fuse_daemonize_success()
+ * or fuse_daemonize_fail().
+ * The child returns from this call and continues setup.
+ *
+ * Unless FUSE_DAEMONIZE_NO_CHDIR is set, changes directory to "/".
+ *
+ * Must be called before fuse_session_mount().
+ *
+ * @param flags combination of FUSE_DAEMONIZE_* flags
+ * @return 0 on success, negative errno on error
+ */
+int fuse_daemonize_early_start(unsigned int flags);
+
+/**
+ * Signal daemonization failure to parent and cleanup.
+ *
+ * To be called from the child process on any kind of error.
+ *
+ * @param err Error code passed to the parent and used as process exit code.
+
+ */
+void fuse_daemonize_early_fail(int err);
+
+/**
+ * Check if daemonization is active and waiting for signal.
+ *
+ * Can be called from the child process to check state of daemonization.
+ *
+ * @return true if active, false otherwise
+ */
+bool fuse_daemonize_early_is_active(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* FUSE_DAEMONIZE_H_ */
+

include/meson.build

@@ -1,4 +1,5 @@
 libfuse_headers = [ 'fuse.h', 'fuse_common.h', 'fuse_lowlevel.h',
-	            'fuse_opt.h', 'cuse_lowlevel.h', 'fuse_log.h' ]
+	            'fuse_opt.h', 'cuse_lowlevel.h', 'fuse_log.h',
+	            'fuse_daemonize.h' ]
 
 install_headers(libfuse_headers, subdir: 'fuse3')

lib/fuse_daemonize.c

@@ -0,0 +1,304 @@
+/*
+ * FUSE: Filesystem in Userspace
+ * Copyright (C) 2026 Bernd Schubert <bsbernd.com>
+ *
+ * This program can be distributed under the terms of the GNU LGPLv2.
+ * See the file COPYING.LIB.
+ */
+
+#define _GNU_SOURCE
+
+#include "fuse_daemonize.h"
+#include "fuse_daemonize_i.h"
+
+#include <fcntl.h>
+#include <poll.h>
+#include <pthread.h>
+#include <stdatomic.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/types.h>
+#include <unistd.h>
+#include <stdbool.h>
+#include <errno.h>
+#include <err.h>
+
+/**
+ * Status values for fuse_daemonize_success() and fuse_daemonize_fail()
+ */
+#define FUSE_DAEMONIZE_SUCCESS 0
+#define FUSE_DAEMONIZE_FAILURE 1
+
+	/* Private/internal data  */
+	struct fuse_daemonize {
+	unsigned int flags;
+	int signal_pipe_wr;	/* write end for signaling parent */
+	int death_pipe_rd;	/* read end, POLLHUP when parent dies */
+	int stop_pipe_rd;	/* read end for stop signal */
+	int stop_pipe_wr;	/* write end for stop signal */
+	pthread_t watcher;
+	bool watcher_started;
+	_Atomic bool active;
+	_Atomic bool daemonized;
+	_Atomic bool mounted;
+};
+
+/* Global daemonization object pointer */
+static struct fuse_daemonize daemonize = {
+	.signal_pipe_wr = -1,
+	.death_pipe_rd = -1,
+	.stop_pipe_rd = -1,
+	.stop_pipe_wr = -1,
+};
+
+/* Watcher thread: polls for parent death or stop signal */
+static void *parent_watcher_thread(void *arg)
+{
+	struct fuse_daemonize *di = arg;
+	struct pollfd pfd[2];
+
+	pfd[0].fd = di->death_pipe_rd;
+	pfd[0].events = POLLIN;
+	pfd[1].fd = di->stop_pipe_rd;
+	pfd[1].events = POLLIN;
+
+	while (1) {
+		int rc = poll(pfd, 2, -1);
+
+		if (rc < 0)
+			continue;
+
+		/* Parent died - death pipe write end closed */
+		if (pfd[0].revents & (POLLHUP | POLLERR))
+			_exit(EXIT_FAILURE);
+
+		/* Stop signal received */
+		if (pfd[1].revents & POLLIN)
+			break;
+	}
+	return NULL;
+}
+
+static int start_parent_watcher(struct fuse_daemonize *dm)
+{
+	int rc;
+
+	rc = pthread_create(&dm->watcher, NULL, parent_watcher_thread,
+			    dm);
+	if (rc != 0) {
+		fprintf(stderr, "fuse_daemonize: pthread_create: %s\n",
+			strerror(rc));
+		return -rc;
+	}
+	dm->watcher_started = true;
+	return 0;
+}
+
+static void stop_parent_watcher(struct fuse_daemonize *dm)
+{
+	char byte = 0;
+
+	if (dm && dm->watcher_started) {
+		/* Signal watcher to stop */
+		if (write(dm->stop_pipe_wr, &byte, 1) != 1)
+			perror("fuse_daemonize: stop write");
+		pthread_join(dm->watcher, NULL);
+		dm->watcher_started = false;
+	}
+}
+
+static int daemonize_child(struct fuse_daemonize *dm)
+{
+	int stop_pipe[2], err = 0;
+
+	if (pipe(stop_pipe) == -1) {
+		err = -errno;
+		perror("fuse_daemonize_start: stop pipe");
+		return err;
+	}
+	dm->stop_pipe_rd = stop_pipe[0];
+	dm->stop_pipe_wr = stop_pipe[1];
+
+	if (setsid() == -1) {
+		err = -errno;
+		perror("fuse_daemonize_start: setsid");
+		goto err_close_stop;
+	}
+
+	/* Close stdin immediately */
+	int nullfd = open("/dev/null", O_RDWR, 0);
+
+	if (nullfd != -1) {
+		(void)dup2(nullfd, 0);
+		if (nullfd > 0)
+			close(nullfd);
+	}
+
+	/* Start watcher thread to detect parent death */
+	err = start_parent_watcher(dm);
+	if (err)
+		goto err_close_stop;
+
+	dm->daemonized = true;
+	return 0;
+
+err_close_stop:
+	close(dm->stop_pipe_rd);
+	close(dm->stop_pipe_wr);
+	return err;
+}
+
+/* Fork and daemonize. Returns 0 in child, never returns in parent. */
+static int do_daemonize(struct fuse_daemonize *dm)
+{
+	int signal_pipe[2], death_pipe[2], err;
+
+	if (pipe(signal_pipe) == -1) {
+		err = -errno;
+		perror("fuse_daemonize_start: signal pipe");
+		return err;
+	}
+
+	if (pipe(death_pipe) == -1) {
+		err = -errno;
+		perror("fuse_daemonize_start: death pipe");
+		close(signal_pipe[0]);
+		close(signal_pipe[1]);
+		return err;
+	}
+
+	switch (fork()) {
+	case -1:
+		err = -errno;
+		perror("fuse_daemonize_start: fork");
+		close(signal_pipe[0]);
+		close(signal_pipe[1]);
+		close(death_pipe[0]);
+		close(death_pipe[1]);
+		return err;
+
+	case 0:
+		/* Child: signal write end, death read end */
+		close(signal_pipe[0]);
+		close(death_pipe[1]);
+		dm->signal_pipe_wr = signal_pipe[1];
+		dm->death_pipe_rd = death_pipe[0];
+		return daemonize_child(dm);
+
+	default: {
+		/* Parent: signal read end, death write end (kept open) */
+		int status;
+		ssize_t res;
+
+		close(signal_pipe[1]);
+		close(death_pipe[0]);
+
+		res = read(signal_pipe[0], &status, sizeof(status));
+		close(signal_pipe[0]);
+		close(death_pipe[1]);
+
+		if (res != sizeof(status))
+			_exit(EXIT_FAILURE);
+		_exit(status);
+	}
+	}
+}
+
+int fuse_daemonize_early_start(unsigned int flags)
+{
+	struct fuse_daemonize *dm = &daemonize;
+	int err = 0;
+
+	dm->flags = flags;
+	dm->signal_pipe_wr = -1;
+	dm->death_pipe_rd = -1;
+	dm->stop_pipe_rd = -1;
+	dm->stop_pipe_wr = -1;
+	dm->active = true;
+
+	if (!(flags & FUSE_DAEMONIZE_NO_CHDIR))
+		(void)chdir("/");
+
+	if (!(flags & FUSE_DAEMONIZE_NO_BACKGROUND))
+		err = do_daemonize(dm);
+
+	return err;
+}
+
+static void close_if_valid(int *fd)
+{
+	if (*fd != -1) {
+		close(*fd);
+		*fd = -1;
+	}
+}
+
+static void fuse_daemonize_early_signal(int status)
+{
+	struct fuse_daemonize *dm = &daemonize;
+	int rc;
+
+	if (!dm->active)
+		errx(EINVAL, "%s: not active and cannot signal status", __func__);
+
+	/* Warn because there might be races */
+	if (status == FUSE_DAEMONIZE_SUCCESS && !dm->mounted)
+		fprintf(stderr, "fuse daemonize success without being mounted\n");
+
+	dm->active = false;
+
+	/* Stop watcher before signaling - parent will exit after this */
+	stop_parent_watcher(dm);
+
+	/* Signal status to parent */
+	if (dm->signal_pipe_wr != -1) {
+		rc = write(dm->signal_pipe_wr, &status, sizeof(status));
+		if (rc != sizeof(status))
+			fprintf(stderr, "%s: write failed\n", __func__);
+	}
+
+	/* Redirect stdout/stderr to /dev/null on success */
+	if (status == FUSE_DAEMONIZE_SUCCESS && dm->daemonized) {
+		int nullfd = open("/dev/null", O_RDWR, 0);
+
+		if (nullfd != -1) {
+			(void)dup2(nullfd, 1);
+			(void)dup2(nullfd, 2);
+			if (nullfd > 2)
+				close(nullfd);
+		}
+	}
+
+	close_if_valid(&dm->signal_pipe_wr);
+	close_if_valid(&dm->death_pipe_rd);
+	close_if_valid(&dm->stop_pipe_rd);
+	close_if_valid(&dm->stop_pipe_wr);
+}
+
+void fuse_daemonize_early_success(void)
+{
+	/*
+	 * Needs to be gracefully handled as automatically called libfuse
+	 * internal from FUSE_INIT handler
+	 */
+	if (!fuse_daemonize_early_is_active())
+		return;
+
+	fuse_daemonize_early_signal(FUSE_DAEMONIZE_SUCCESS);
+}
+
+void fuse_daemonize_early_fail(int err)
+{
+	fuse_daemonize_early_signal(err);
+}
+
+bool fuse_daemonize_early_is_active(void)
+{
+	return daemonize.daemonized || daemonize.active;
+}
+
+void fuse_daemonize_early_set_mounted(void)
+{
+	daemonize.mounted = true;
+}

lib/fuse_daemonize_i.h

@@ -0,0 +1,35 @@
+/*
+ * FUSE: Filesystem in Userspace
+ * Copyright (C) 2026 Bernd Schubert <bsbernd.com>
+ *
+ * This program can be distributed under the terms of the GNU LGPLv2.
+ * See the file COPYING.LIB.
+ *
+ */
+
+#ifndef FUSE_DAEMONIZE_I_H_
+#define FUSE_DAEMONIZE_I_H_
+
+#include <stdint.h>
+#include <stdbool.h>
+
+/**
+ * Set mounted flag.
+ *
+ * Called from fuse_session_mount().
+ */
+void fuse_daemonize_early_set_mounted(void);
+
+/**
+ * Signal daemonization success to parent and cleanup.
+ *
+ * To be called from the child process after successful mount, when
+ * sychronous FUSE_INIT is used (FUSE_INIT as part of the mount)
+ * Automatically called for async FUSE_INIT.
+ *
+ * Not exposed to the ABI yet, as sync FUSE_INIT is not implemented yet.
+ */
+void fuse_daemonize_early_success(void);
+
+#endif /* FUSE_DAEMONIZE_I_H_ */
+

lib/fuse_i.h

@@ -17,7 +17,6 @@
 #include <semaphore.h>
 #include <stdint.h>
 #include <stdbool.h>
-#include <errno.h>
 #include <stdatomic.h>
 
 #define MIN(a, b) \
@@ -110,6 +109,9 @@ struct fuse_session {
 	/* true if reading requests from /dev/fuse are handled internally */
 	bool buf_reallocable;
 
+	/* synchronous FUSE_INIT support */
+	bool want_sync_init;
+
 	/* io_uring */
 	struct fuse_session_uring uring;
 

lib/fuse_lowlevel.c

@@ -19,6 +19,7 @@
 #include "mount_util.h"
 #include "util.h"
 #include "fuse_uring_i.h"
+#include "fuse_daemonize_i.h"
 
 #include <pthread.h>
 #include <stdatomic.h>
@@ -3022,6 +3023,8 @@ _do_init(fuse_req_t req, const fuse_ino_t nodeid, const void *op_in,
 	send_reply_ok(req, &outarg, outargsize);
 	if (enable_io_uring)
 		fuse_uring_wake_ring_threads(se);
+
+	fuse_daemonize_early_success();
 }
 
 static __attribute__((no_sanitize("thread"))) void
@@ -4454,6 +4457,8 @@ out:
 	se->fd = fd;
 	se->mountpoint = mountpoint;
 
+	fuse_daemonize_early_set_mounted();
+
 	return 0;
 
 error_out:

lib/fuse_versionscript

@@ -227,6 +227,11 @@ FUSE_3.19 {
 		fuse_session_start_teardown_watchdog;
 		fuse_session_stop_teardown_watchdog;
 		fuse_lowlevel_notify_prune;
+		fuse_daemonize_early_start;
+		# Not exposed yet for now, as called automatically
+		# fuse_daemonize_early_success;
+		fuse_daemonize_early_fail;
+		fuse_daemonize_early_is_active;
 } FUSE_3.18;
 
 # Local Variables:

lib/helper.c

@@ -15,6 +15,7 @@
 #include "fuse_misc.h"
 #include "fuse_opt.h"
 #include "fuse_lowlevel.h"
+#include "fuse_daemonize.h"
 #include "mount_util.h"
 
 #include <stdio.h>
@@ -252,6 +253,12 @@ int fuse_parse_cmdline_30(struct fuse_args *args,
 
 int fuse_daemonize(int foreground)
 {
+	/* Check if the NEW API is used */
+	if (fuse_daemonize_early_is_active()) {
+		fuse_log(FUSE_LOG_ERR, "fuse_daemonize_start() already used\n");
+		return -1;
+	}
+
 	if (!foreground) {
 		int nullfd;
 		int waiter[2];
@@ -279,7 +286,8 @@ int fuse_daemonize(int foreground)
 			break;
 		default:
 			/* parent */
-			(void) read(waiter[0], &completed, sizeof(completed));
+			(void)read(waiter[0], &completed,
+					sizeof(completed));
 			close(waiter[0]);
 			close(waiter[1]);
 			_exit(0);
@@ -362,17 +370,23 @@ int fuse_main_real_versioned(int argc, char *argv[],
 		goto out1;
 	}
 
+	struct fuse_session *se = fuse_get_session(fuse);
 	if (fuse_mount(fuse,opts.mountpoint) != 0) {
 		res = 4;
 		goto out2;
 	}
 
+	/*
+	 * fuse_daemonize() already checks and fails then, but we need to
+	 * handle it gracefully here, as this is done libfuse internal
+	 * and caller didn't ask to daemonize with old API.
+	 */
+	if (!fuse_daemonize_early_is_active()) {
 		if (fuse_daemonize(opts.foreground) != 0) {
 			res = 5;
 			goto out3;
 		}
-
-	struct fuse_session *se = fuse_get_session(fuse);
+	}
 	if (fuse_set_signal_handlers(se) != 0) {
 		res = 6;
 		goto out3;

lib/meson.build

@@ -2,7 +2,8 @@ libfuse_sources = ['fuse.c', 'fuse_i.h', 'fuse_loop.c', 'fuse_loop_mt.c',
                    'fuse_lowlevel.c', 'fuse_misc.h', 'fuse_opt.c',
                    'fuse_signals.c', 'buffer.c', 'cuse_lowlevel.c',
                    'helper.c', 'modules/subdir.c', 'mount_util.c',
-                   'fuse_log.c', 'compat.c', 'util.c', 'util.h' ]
+                   'fuse_log.c', 'compat.c', 'util.c', 'util.h',
+                   'fuse_daemonize.c' ]
 
 if host_machine.system().startswith('linux')
    libfuse_sources += [ 'mount.c' ]

test/test_want_conversion.c

@@ -8,6 +8,7 @@
 #include <inttypes.h>
 #include <stdbool.h>
 #include <err.h>
+#include <errno.h>
 
 static void print_conn_info(const char *prefix, struct fuse_conn_info *conn)
 {