NT synchronization primitive driver

This page documents the user-space API for the ntsync driver.

ntsync is a support driver for emulation of NT synchronization primitives by user-space NT emulators. It exists because implementation in user-space, using existing tools, cannot match Windows performance while offering accurate semantics. It is implemented entirely in software, and does not drive any hardware device.

This interface is meant as a compatibility tool only, and should not be used for general synchronization. Instead use generic, versatile interfaces such as futex(2) and poll(2).

Synchronization primitives

The ntsync driver exposes three types of synchronization primitives: semaphores, mutexes, and events.

A semaphore holds a single volatile 32-bit counter, and a static 32-bit integer denoting the maximum value. It is considered signaled when the counter is nonzero. The counter is decremented by one when a wait is satisfied. Both the initial and maximum count are established when the semaphore is created.

A mutex holds a volatile 32-bit recursion count, and a volatile 32-bit identifier denoting its owner. A mutex is considered signaled when its owner is zero (indicating that it is not owned). The recursion count is incremented when a wait is satisfied, and ownership is set to the given identifier.

A mutex also holds an internal flag denoting whether its previous owner has died; such a mutex is said to be abandoned. Owner death is not tracked automatically based on thread death, but rather must be communicated using NTSYNC_IOC_MUTEX_KILL. An abandoned mutex is inherently considered unowned.

Except for the “unowned” semantics of zero, the actual value of the owner identifier is not interpreted by the ntsync driver at all. The intended use is to store a thread identifier; however, the ntsync driver does not actually validate that a calling thread provides consistent or unique identifiers.

An event holds a volatile boolean state denoting whether it is signaled or not. There are two types of events, auto-reset and manual-reset. An auto-reset event is designaled when a wait is satisfied; a manual-reset event is not. The event type is specified when the event is created.

Unless specified otherwise, all operations on an object are atomic and totally ordered with respect to other operations on the same object.

Objects are represented by files. When all file descriptors to an object are closed, that object is deleted.

Char device

The ntsync driver creates a single char device /dev/ntsync. Each file description opened on the device represents a unique instance intended to back an individual NT virtual machine. Objects created by one ntsync instance may only be used with other objects created by the same instance.

ioctl reference

All operations on the device are done through ioctls. There are four structures used in ioctl calls:

struct ntsync_sem_args {
     __u32 sem;
     __u32 count;
     __u32 max;
};

struct ntsync_mutex_args {
     __u32 mutex;
     __u32 owner;
     __u32 count;
};

struct ntsync_event_args {
     __u32 event;
     __u32 signaled;
     __u32 manual;
};

struct ntsync_wait_args {
     __u64 timeout;
     __u64 objs;
     __u32 count;
     __u32 owner;
     __u32 index;
     __u32 alert;
     __u32 flags;
     __u32 pad;
};

Depending on the ioctl, members of the structure may be used as input, output, or not at all. All ioctls return 0 on success.

The ioctls on the device file are as follows:

NTSYNC_IOC_CREATE_SEM

Create a semaphore object. Takes a pointer to struct ntsync_sem_args, which is used as follows:

sem	On output, contains a file descriptor to the created semaphore.
count	Initial count of the semaphore.
max	Maximum count of the semaphore.

Fails with EINVAL if count is greater than max.

NTSYNC_IOC_CREATE_MUTEX

Create a mutex object. Takes a pointer to struct ntsync_mutex_args, which is used as follows:

mutex	On output, contains a file descriptor to the created mutex.
count	Initial recursion count of the mutex.
owner	Initial owner of the mutex.

If owner is nonzero and count is zero, or if owner is zero and count is nonzero, the function fails with EINVAL.

NTSYNC_IOC_CREATE_EVENT

Create an event object. Takes a pointer to struct ntsync_event_args, which is used as follows:

event	On output, contains a file descriptor to the created event.
signaled	If nonzero, the event is initially signaled, otherwise nonsignaled.
manual	If nonzero, the event is a manual-reset event, otherwise auto-reset.

The ioctls on the individual objects are as follows:

NTSYNC_IOC_SEM_POST

Post to a semaphore object. Takes a pointer to a 32-bit integer, which on input holds the count to be added to the semaphore, and on output contains its previous count.

If adding to the semaphore’s current count would raise the latter past the semaphore’s maximum count, the ioctl fails with EOVERFLOW and the semaphore is not affected. If raising the semaphore’s count causes it to become signaled, eligible threads waiting on this semaphore will be woken and the semaphore’s count decremented appropriately.

NTSYNC_IOC_MUTEX_UNLOCK

Release a mutex object. Takes a pointer to struct ntsync_mutex_args, which is used as follows:

mutex	Ignored.
owner	Specifies the owner trying to release this mutex.
count	On output, contains the previous recursion count.

If owner is zero, the ioctl fails with EINVAL. If owner is not the current owner of the mutex, the ioctl fails with EPERM.

The mutex’s count will be decremented by one. If decrementing the mutex’s count causes it to become zero, the mutex is marked as unowned and signaled, and eligible threads waiting on it will be woken as appropriate.

NTSYNC_IOC_SET_EVENT

Signal an event object. Takes a pointer to a 32-bit integer, which on output contains the previous state of the event.

Eligible threads will be woken, and auto-reset events will be designaled appropriately.

NTSYNC_IOC_RESET_EVENT

Designal an event object. Takes a pointer to a 32-bit integer, which on output contains the previous state of the event.

NTSYNC_IOC_PULSE_EVENT

Wake threads waiting on an event object while leaving it in an unsignaled state. Takes a pointer to a 32-bit integer, which on output contains the previous state of the event.

A pulse operation can be thought of as a set followed by a reset, performed as a single atomic operation. If two threads are waiting on an auto-reset event which is pulsed, only one will be woken. If two threads are waiting a manual-reset event which is pulsed, both will be woken. However, in both cases, the event will be unsignaled afterwards, and a simultaneous read operation will always report the event as unsignaled.

NTSYNC_IOC_READ_SEM

Read the current state of a semaphore object. Takes a pointer to struct ntsync_sem_args, which is used as follows:

sem	Ignored.
count	On output, contains the current count of the semaphore.
max	On output, contains the maximum count of the semaphore.

NTSYNC_IOC_READ_MUTEX

Read the current state of a mutex object. Takes a pointer to struct ntsync_mutex_args, which is used as follows:

mutex	Ignored.
owner	On output, contains the current owner of the mutex, or zero if the mutex is not currently owned.
count	On output, contains the current recursion count of the mutex.

If the mutex is marked as abandoned, the function fails with EOWNERDEAD. In this case, count and owner are set to zero.

NTSYNC_IOC_READ_EVENT

Read the current state of an event object. Takes a pointer to struct ntsync_event_args, which is used as follows:

event	Ignored.
signaled	On output, contains the current state of the event.
manual	On output, contains 1 if the event is a manual-reset event, and 0 otherwise.

NTSYNC_IOC_KILL_OWNER

Mark a mutex as unowned and abandoned if it is owned by the given owner. Takes an input-only pointer to a 32-bit integer denoting the owner. If the owner is zero, the ioctl fails with EINVAL. If the owner does not own the mutex, the function fails with EPERM.

Eligible threads waiting on the mutex will be woken as appropriate (and such waits will fail with EOWNERDEAD, as described below).

NTSYNC_IOC_WAIT_ANY

Poll on any of a list of objects, atomically acquiring at most one. Takes a pointer to struct ntsync_wait_args, which is used as follows:

timeout	Absolute timeout in nanoseconds. If NTSYNC_WAIT_REALTIME is set, the timeout is measured against the REALTIME clock; otherwise it is measured against the MONOTONIC clock. If the timeout is equal to or earlier than the current time, the function returns immediately without sleeping. If timeout is U64_MAX, the function will sleep until an object is signaled, and will not fail with ETIMEDOUT.
objs	Pointer to an array of count file descriptors (specified as an integer so that the structure has the same size regardless of architecture). If any object is invalid, the function fails with EINVAL.
count	Number of objects specified in the objs array. If greater than NTSYNC_MAX_WAIT_COUNT, the function fails with EINVAL.
owner	Mutex owner identifier. If any object in objs is a mutex, the ioctl will attempt to acquire that mutex on behalf of owner. If owner is zero, the ioctl fails with EINVAL.
index	On success, contains the index (into objs) of the object which was signaled. If alert was signaled instead, this contains count.
alert	Optional event object file descriptor. If nonzero, this specifies an “alert” event object which, if signaled, will terminate the wait. If nonzero, the identifier must point to a valid event.
flags	Zero or more flags. Currently the only flag is NTSYNC_WAIT_REALTIME, which causes the timeout to be measured against the REALTIME clock instead of MONOTONIC.
pad	Unused, must be set to zero.

This function attempts to acquire one of the given objects. If unable to do so, it sleeps until an object becomes signaled, subsequently acquiring it, or the timeout expires. In the latter case the ioctl fails with ETIMEDOUT. The function only acquires one object, even if multiple objects are signaled.

A semaphore is considered to be signaled if its count is nonzero, and is acquired by decrementing its count by one. A mutex is considered to be signaled if it is unowned or if its owner matches the owner argument, and is acquired by incrementing its recursion count by one and setting its owner to the owner argument. An auto-reset event is acquired by designaling it; a manual-reset event is not affected by acquisition.

Acquisition is atomic and totally ordered with respect to other operations on the same object. If two wait operations (with different owner identifiers) are queued on the same mutex, only one is signaled. If two wait operations are queued on the same semaphore, and a value of one is posted to it, only one is signaled. The order in which threads are signaled is not specified.

If an abandoned mutex is acquired, the ioctl fails with EOWNERDEAD. Although this is a failure return, the function may otherwise be considered successful. The mutex is marked as owned by the given owner (with a recursion count of 1) and as no longer abandoned, and index is still set to the index of the mutex.

The alert argument is an “extra” event which can terminate the wait, independently of all other objects. If members of objs and alert are both simultaneously signaled, a member of objs will always be given priority and acquired first.

It is valid to pass the same object more than once, including by passing the same event in the objs array and in alert. If a wakeup occurs due to that object being signaled, index is set to the lowest index corresponding to that object.

The function may fail with EINTR if a signal is received.

NTSYNC_IOC_WAIT_ALL

Poll on a list of objects, atomically acquiring all of them. Takes a pointer to struct ntsync_wait_args, which is used identically to NTSYNC_IOC_WAIT_ANY, except that index is always filled with zero on success if not woken via alert.

This function attempts to simultaneously acquire all of the given objects. If unable to do so, it sleeps until all objects become simultaneously signaled, subsequently acquiring them, or the timeout expires. In the latter case the ioctl fails with ETIMEDOUT and no objects are modified.

Objects may become signaled and subsequently designaled (through acquisition by other threads) while this thread is sleeping. Only once all objects are simultaneously signaled does the ioctl acquire them and return. The entire acquisition is atomic and totally ordered with respect to other operations on any of the given objects.

If an abandoned mutex is acquired, the ioctl fails with EOWNERDEAD. Similarly to NTSYNC_IOC_WAIT_ANY, all objects are nevertheless marked as acquired. Note that if multiple mutex objects are specified, there is no way to know which were marked as abandoned.

As with “any” waits, the alert argument is an “extra” event which can terminate the wait. Critically, however, an “all” wait will succeed if all members in objs are signaled, or if alert is signaled. In the latter case index will be set to count. As with “any” waits, if both conditions are filled, the former takes priority, and objects in objs will be acquired.

Unlike NTSYNC_IOC_WAIT_ANY, it is not valid to pass the same object more than once, nor is it valid to pass the same object in objs and in alert. If this is attempted, the function fails with EINVAL.