.\"
.\" SPDX-License-Identifier: BSD-2-Clause
.\"
.\" Copyright 2026 The FreeBSD Foundation
.\"
.\" This documentation was written by Konstantin Belousov <kib@FreeBSD.org>
.\" under sponsorship from the FreeBSD Foundation.
.\"
.Dd June 19, 2026
.Dt NTSYNC 4
.Os
.Sh NAME
.Nm ntsync
.Nd NT-like synchronization operations device driver
.Sh SYNOPSIS
.In dev/ntsync/ntsync.h
.Sh DESCRIPTION
The
.Nm
device driver provides synchronization primitives that mimic the operations
provided by the Windows NT kernel.
The userspace interface is copied from the identically named driver written
for the Linux kernel, with the goal of helping Wine implement Win32 API.
.Pp
The driver provides the
.Pa /dev/ntsync
special device node that handles
.Xr ioctl 2
requests of several groups:
.Bl -tag -width "wait operations:"
.It object creation:
among them semaphores, mutexes, and events
.It wait operations:
waiting for the set of objects to become signaled, either all
or any objects in the set can be waited for
.El
.Pp
An
.Xr open 2
of the
.Pa /dev/ntsync
device returns the file descriptor which represents the synchronization
domain.
Each object created by the
.Xr ioctl 2
on the ntsync file descriptor belongs to it's domain.
Wait requests must not mix the objects belonging to other domains.
.Pp
The synchronization objects are represented by file descriptors, each
implementing type-specific set of ioctl requests.
.Sh SEMAPHORES
The semaphores operations usually take the
.Bd -literal
struct ntsync_sem_args {
	uint32_t count;
	uint32_t max;
}
.Ed
as an argument.
The semaphore requests are:
.Bl -tag -width "NTSYNC_IOC_SEM_RELEASE"
.It Dv NTSYNC_IOC_CREATE_SEM
Creates the semaphore and returns the semaphore file descriptor.
Must be issued on the
.Pa /dev/ntsync
file descriptor.
.Pp
Takes the
.Va struct ntsync_sem_args
argument, where
.Dv count
is the initial semaphore count, and
.Dv max
is the maximum allowed count.
.It Dv NTSYNC_IOC_SEM_RELEASE
Releases the semaphore.
Takes the single
.Va uint32_t
value which is used to decrement the semaphore count.
If the semaphore count reaches zero, the semaphore becomes signaled.
.It Dv NTSYNC_IOC_SEM_READ
Returns the current state of the semaphore in the
.Va struct ntsync_sem_args .
.El
.Sh MUTEXES
The mutexes operations usually take the
.Bd -literal
struct ntsync_mutex_args {
	uint32_t owner;
	uint32_t count;
}
.Ed
as the argument.
.Pp
The mutex requests are:
.Bl -tag -width "NTSYNC_IOC_CREATE_MUTEX"
.It Dv NTSYNC_IOC_CREATE_MUTEX
Creates the mutex and returns the mutex file descriptor.
Must be issued on the
.Pa /dev/ntsync
file descriptor.
.Pp
Takes the
.Va struct ntsync_mutex_args
as the argument.
The
.Dv owner
is an abstract 32bit number that identifies the current mutex owner.
If
.Dv owner
is zero, the mutex is unowned, and
.Dv count
must be zero.
If
.Dv count
is non-zero, indicating the owned mutex, a non-zero
.Dv owner
must be provided.
.It Dv NTSYNC_IOC_MUTEX_UNLOCK
Unlocks the mutex.
Takes the
.Va struct ntsync_mutex_args
argument.
.Pp
The mutex must be owned by the argument's
.Dv owner .
Successful unlock decrements mutex' counter, and makes the mutex
signaled and unowned if the counter reaches zero.
.Pp
The counter value before unlock is returned in the
.Dv count
member of the argument structure.
.It Dv NTSYNC_IOC_MUTEX_KILL
Abandon the mutex.
Takes a single 32bit integer as the argument, indicating the current
mutex owner.
.Pp
The specified owner must be equal to the current mutex owner.
Then, the pending waiters are woken up, and get the
.Ev EOWNERDEAD
result.
The mutex owner and counter are set to zero.
.Pp
The abandoned state is cleared by next successful wait on the mutex.
.It Dv NTSYNC_IOC_MUTEX_READ
Returns the current state of the mutex.
Takes the
.Va struct ntsync_mutex_args
argument where the state is returned.
For abandoned mutexes, the
.Ev EOWNERDEAD
error is returned in addition to the state.
.El
.Sh EVENTS
The events operations usually take the
.Bd -literal
struct ntsync_event_args {
	uint32_t manual;
	uint32_t signaled;
}
.Ed
.Pp
The events requests are:
.Bl -tag -width "NTSYNC_IOC_CREATE_EVENT"
.It Dv NTSYNC_IOC_CREATE_EVENT
Creates the event and returns the event file descriptor.
Must be issued on the
.Pa /dev/ntsync
file descriptor.
Takes the
.Va struct ntsync_event_args
argument.
.Pp
Events can be of two types: manual and automatic.
Manual events needs to be reset by the request after being set.
Automatic events are reset by the system after a wait is satisfied.
.It Dv NTSYNC_IOC_EVENT_SET
Set (arm) the event.
Takes a 32bit integer argument where the state of the event before
the operation is returned.
.It Dv NTSYNC_IOC_EVENT_RESET
Reset (dis-arms) the event.
Takes a 32bit integer argument where the state of the event before
the operation is returned.
.It Dv NTSYNC_IOC_EVENT_PULSE
Atomically sets the event, wakes up eligible waiters,
and then resets the event.
Takes a 32bit integer argument where the state of the event before
the operation is returned.
.Pp
If the manual event is pulsed, it wakes up all waiters,
after which it is reset.
On the other hand, the automatic event is reset after
waking up at most a single waiter.
.It Dv NTSYNC_IOC_EVENT_READ
Returns the current state of the event.
Takes the
.Va struct ntsync_event_args
argument where the current event state is returned.
.El
.Sh WAIT OPERATIONS
Wait operations take the
.Bd -literal
struct ntsync_wait_args {
	uint64_t timeout;
	uint64_t objs;
	uint32_t count;
	uint32_t index;
	uint32_t flags;
	uint32_t owner;
	uint32_t alert;
	uint32_t pad;
}
.Ed
as the argument.
.Pp
The signaled state of the objects which cause the wait to
become satisfied are consumed by the operation,
e.g. the semaphore is acquired by incrementing its counter,
the mutex is locked,
and the manual event becomes not signaled.
.Pp
The
.Dv timeout
is in the nanoseconds.
If
.Dv timeout
is zero, the wait request only returns when either the wait
condition is satisfied, or a signal is queued.
Otherwise, if the wait is not satisfied after the specified time,
it is aborted anyway and the
.Er ETIMEDOUT
error is returned.
.Pp
The
.Dv objs
member points to the array of the synchronization object's file
descriptors.
Its size if passed in the
.Dv count
member.
It might be as large as
.Dv NTSYNC_MAX_WAIT_COUNT ,
which is 64.
.Pp
The
.Dv alert
if non-zero specifies the event file descriptor,
signaling of which finishes the wait regardless the state of
other objects in the
.Dv objs
array.
.Pp
On non-error return from the wait requests, the
.Dv index
member contains the index of the signaled object that caused
the wait request to be satisfied.
If the object at the index is a semaphore, the
.Dv owner
member reports the owner of the signaled semaphore.
If the
.Dv alert
event was signaled to abort the wait,
.Dv index
is set to
.Dv count .
.Pp
The possible values for the
.Va flags
parameter are
.Bl -tag -width "NTSYNC_WAIT_REALTIME"
.It Dv NTSYNC_WAIT_REALTIME
The specified timeout is for
.Dv CLOCK_REALTIME
absolute value, otherwise it is for
.Dv CLOCK_MONOTONIC ,
see
.Xr clock_gettime 2 .
.El
.Pp
The wait requests are:
.Bl -tag -width "NTSYNC_IOC_WAIT_ANY"
.It Dv NTSYNC_IOC_WAIT_ANY
Wait for any of the objects to become signaled.
.It Dv NTSYNC_IOC_WAIT_ALL
Wait for all of the objects to become signaled.
This means that the wait is satisfied only when all objects
can be consumed together, which is done atomically.
.Pp
No duplicate objects are allowed in the
.Dv objs
array.
The
.Dv alert
object is not allowed to be listed in the
.Dv objs
array.
.El
.Sh SEE ALSO
Refer to the file
.Pa Documentation/userspace-api/ntsync.rst
in the Linux kernel sources for the Linux API reference,
that was used for the implementation of the
.Fx
driver.
.Sh HISTORY
The
.Nm
driver and manual page first appeared in
.Fx 15.2 .
.Sh AUTHORS
.An -nosplit
The driver and the manual page were written by
.An Konstantin Belousov Aq Mt kib@FreeBSD.org .