futex(2) System Calls Manual futex(2)
NAME
futex - fast user-space locking
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <linux/futex.h> /* Definition of FUTEX_* constants */
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
long syscall(SYS_futex, uint32_t *uaddr, int futex_op, uint32_t val,
const struct timespec *timeout, /* or: uint32_t val2 */
uint32_t *uaddr2, uint32_t val3);
Note: glibc provides no wrapper for futex(), necessitating the use of
syscall(2).
DESCRIPTION
The futex() system call provides a method for waiting until a certain
condition becomes true. It is typically used as a blocking construct in
the context of shared-memory synchronization. When using futexes, the
majority of the synchronization operations are performed in user space.
A user-space program employs the futex() system call only when it is
likely that the program has to block for a longer time until the condi-
tion becomes true. Other futex() operations can be used to wake any
processes or threads waiting for a particular condition.
A futex is a 32-bit value—referred to below as a futex word—whose ad-
dress is supplied to the futex() system call. (Futexes are 32 bits in
size on all platforms, including 64-bit systems.) All futex operations
are governed by this value. In order to share a futex between
processes, the futex is placed in a region of shared memory, created us-
ing (for example) mmap(2) or shmat(2). (Thus, the futex word may have
different virtual addresses in different processes, but these addresses
all refer to the same location in physical memory.) In a multithreaded
program, it is sufficient to place the futex word in a global variable
shared by all threads.
When executing a futex operation that requests to block a thread, the
kernel will block only if the futex word has the value that the calling
thread supplied (as one of the arguments of the futex() call) as the ex-
pected value of the futex word. The loading of the futex word's value,
the comparison of that value with the expected value, and the actual
blocking will happen atomically and will be totally ordered with respect
to concurrent operations performed by other threads on the same futex
word. Thus, the futex word is used to connect the synchronization in
user space with the implementation of blocking by the kernel. Analo-
gously to an atomic compare-and-exchange operation that potentially
changes shared memory, blocking via a futex is an atomic compare-and-
block operation.
One use of futexes is for implementing locks. The state of the lock
(i.e., acquired or not acquired) can be represented as an atomically ac-
cessed flag in shared memory. In the uncontended case, a thread can ac-
cess or modify the lock state with atomic instructions, for example
atomically changing it from not acquired to acquired using an atomic
compare-and-exchange instruction. (Such instructions are performed en-
tirely in user mode, and the kernel maintains no information about the
lock state.) On the other hand, a thread may be unable to acquire a
lock because it is already acquired by another thread. It then may pass
the lock's flag as a futex word and the value representing the acquired
state as the expected value to a futex() wait operation. This futex()
operation will block if and only if the lock is still acquired (i.e.,
the value in the futex word still matches the "acquired state"). When
releasing the lock, a thread has to first reset the lock state to not
acquired and then execute a futex operation that wakes threads blocked
on the lock flag used as a futex word (this can be further optimized to
avoid unnecessary wake-ups). See futex(7) for more detail on how to use
futexes.
Besides the basic wait and wake-up futex functionality, there are fur-
ther futex operations aimed at supporting more complex use cases.
Note that no explicit initialization or destruction is necessary to use
futexes; the kernel maintains a futex (i.e., the kernel-internal imple-
mentation artifact) only while operations such as FUTEX_WAIT, described
below, are being performed on a particular futex word.
Arguments
The uaddr argument points to the futex word. On all platforms, futexes
are four-byte integers that must be aligned on a four-byte boundary.
The operation to perform on the futex is specified in the futex_op argu-
ment; val is a value whose meaning and purpose depends on futex_op.
The remaining arguments (timeout, uaddr2, and val3) are required only
for certain of the futex operations described below. Where one of these
arguments is not required, it is ignored.
For several blocking operations, the timeout argument is a pointer to a
timespec structure that specifies a timeout for the operation. However,
notwithstanding the prototype shown above, for some operations, the
least significant four bytes of this argument are instead used as an in-
teger whose meaning is determined by the operation. For these opera-
tions, the kernel casts the timeout value first to unsigned long, then
to uint32_t, and in the remainder of this page, this argument is re-
ferred to as val2 when interpreted in this fashion.
Where it is required, the uaddr2 argument is a pointer to a second futex
word that is employed by the operation.
The interpretation of the final integer argument, val3, depends on the
operation.
Futex operations
The futex_op argument consists of two parts: a command that specifies
the operation to be performed, bitwise ORed with zero or more options
that modify the behaviour of the operation. The options that may be in-
cluded in futex_op are as follows:
FUTEX_PRIVATE_FLAG (since Linux 2.6.22)
This option bit can be employed with all futex operations. It
tells the kernel that the futex is process-private and not shared
with another process (i.e., it is being used for synchronization
only between threads of the same process). This allows the ker-
nel to make some additional performance optimizations.
As a convenience, <linux/futex.h> defines a set of constants with
the suffix _PRIVATE that are equivalents of all of the operations
listed below, but with the FUTEX_PRIVATE_FLAG ORed into the con-
stant value. Thus, there are FUTEX_WAIT_PRIVATE, FUTEX_WAKE_PRI-
VATE, and so on.
FUTEX_CLOCK_REALTIME (since Linux 2.6.28)
This option bit can be employed only with the FUTEX_WAIT_BITSET,
FUTEX_WAIT_REQUEUE_PI, (since Linux 4.5) FUTEX_WAIT, and (since
Linux 5.14) FUTEX_LOCK_PI2 operations.
If this option is set, the kernel measures the timeout against
the CLOCK_REALTIME clock.
If this option is not set, the kernel measures the timeout
against the CLOCK_MONOTONIC clock.
The operation specified in futex_op is one of the following:
FUTEX_WAIT (since Linux 2.6.0)
This operation tests that the value at the futex word pointed to
by the address uaddr still contains the expected value val, and
if so, then sleeps waiting for a FUTEX_WAKE operation on the fu-
tex word. The load of the value of the futex word is an atomic
memory access (i.e., using atomic machine instructions of the re-
spective architecture). This load, the comparison with the ex-
pected value, and starting to sleep are performed atomically and
totally ordered with respect to other futex operations on the
same futex word. If the thread starts to sleep, it is considered
a waiter on this futex word. If the futex value does not match
val, then the call fails immediately with the error EAGAIN.
The purpose of the comparison with the expected value is to pre-
vent lost wake-ups. If another thread changed the value of the
futex word after the calling thread decided to block based on the
prior value, and if the other thread executed a FUTEX_WAKE opera-
tion (or similar wake-up) after the value change and before this
FUTEX_WAIT operation, then the calling thread will observe the
value change and will not start to sleep.
If the timeout is not NULL, the structure it points to specifies
a timeout for the wait. (This interval will be rounded up to the
system clock granularity, and is guaranteed not to expire early.)
The timeout is by default measured according to the CLOCK_MONOTO-
NIC clock, but, since Linux 4.5, the CLOCK_REALTIME clock can be
selected by specifying FUTEX_CLOCK_REALTIME in futex_op. If
timeout is NULL, the call blocks indefinitely.
Note: for FUTEX_WAIT, timeout is interpreted as a relative value.
This differs from other futex operations, where timeout is inter-
preted as an absolute value. To obtain the equivalent of FU-
TEX_WAIT with an absolute timeout, employ FUTEX_WAIT_BITSET with
val3 specified as FUTEX_BITSET_MATCH_ANY.
The arguments uaddr2 and val3 are ignored.
FUTEX_WAKE (since Linux 2.6.0)
This operation wakes at most val of the waiters that are waiting
(e.g., inside FUTEX_WAIT) on the futex word at the address uaddr.
Most commonly, val is specified as either 1 (wake up a single
waiter) or INT_MAX (wake up all waiters). No guarantee is pro-
vided about which waiters are awoken (e.g., a waiter with a
higher scheduling priority is not guaranteed to be awoken in
preference to a waiter with a lower priority).
The arguments timeout, uaddr2, and val3 are ignored.
FUTEX_FD (from Linux 2.6.0 up to and including Linux 2.6.25)
This operation creates a file descriptor that is associated with
the futex at uaddr. The caller must close the returned file de-
scriptor after use. When another process or thread performs a
FUTEX_WAKE on the futex word, the file descriptor indicates as
being readable with select(2), poll(2), and epoll(7)
The file descriptor can be used to obtain asynchronous notifica-
tions: if val is nonzero, then, when another process or thread
executes a FUTEX_WAKE, the caller will receive the signal number
that was passed in val.
The arguments timeout, uaddr2, and val3 are ignored.
Because it was inherently racy, FUTEX_FD has been removed from
Linux 2.6.26 onward.
FUTEX_REQUEUE (since Linux 2.6.0)
This operation performs the same task as FUTEX_CMP_REQUEUE (see
below), except that no check is made using the value in val3.
(The argument val3 is ignored.)
FUTEX_CMP_REQUEUE (since Linux 2.6.7)
This operation first checks whether the location uaddr still con-
tains the value val3. If not, the operation fails with the error
EAGAIN. Otherwise, the operation wakes up a maximum of val wait-
ers that are waiting on the futex at uaddr. If there are more
than val waiters, then the remaining waiters are removed from the
wait queue of the source futex at uaddr and added to the wait
queue of the target futex at uaddr2. The val2 argument specifies
an upper limit on the number of waiters that are requeued to the
futex at uaddr2.
The load from uaddr is an atomic memory access (i.e., using
atomic machine instructions of the respective architecture).
This load, the comparison with val3, and the requeueing of any
waiters are performed atomically and totally ordered with respect
to other operations on the same futex word.
Typical values to specify for val are 0 or 1. (Specifying
INT_MAX is not useful, because it would make the FUTEX_CMP_RE-
QUEUE operation equivalent to FUTEX_WAKE.) The limit value spec-
ified via val2 is typically either 1 or INT_MAX. (Specifying the
argument as 0 is not useful, because it would make the FU-
TEX_CMP_REQUEUE operation equivalent to FUTEX_WAIT.)
The FUTEX_CMP_REQUEUE operation was added as a replacement for
the earlier FUTEX_REQUEUE. The difference is that the check of
the value at uaddr can be used to ensure that requeueing happens
only under certain conditions, which allows race conditions to be
avoided in certain use cases.
Both FUTEX_REQUEUE and FUTEX_CMP_REQUEUE can be used to avoid
"thundering herd" wake-ups that could occur when using FUTEX_WAKE
in cases where all of the waiters that are woken need to acquire
another futex. Consider the following scenario, where multiple
waiter threads are waiting on B, a wait queue implemented using a
futex:
lock(A)
while (!check_value(V)) {
unlock(A);
block_on(B);
lock(A);
};
unlock(A);
If a waker thread used FUTEX_WAKE, then all waiters waiting on B
would be woken up, and they would all try to acquire lock A.
However, waking all of the threads in this manner would be point-
less because all except one of the threads would immediately
block on lock A again. By contrast, a requeue operation wakes
just one waiter and moves the other waiters to lock A, and when
the woken waiter unlocks A then the next waiter can proceed.
FUTEX_WAKE_OP (since Linux 2.6.14)
This operation was added to support some user-space use cases
where more than one futex must be handled at the same time. The
most notable example is the implementation of pthread_cond_sig-
nal(3), which requires operations on two futexes, the one used to
implement the mutex and the one used in the implementation of the
wait queue associated with the condition variable. FUTEX_WAKE_OP
allows such cases to be implemented without leading to high rates
of contention and context switching.
The FUTEX_WAKE_OP operation is equivalent to executing the fol-
lowing code atomically and totally ordered with respect to other
futex operations on any of the two supplied futex words:
uint32_t oldval = *(uint32_t *) uaddr2;
*(uint32_t *) uaddr2 = oldval op oparg;
futex(uaddr, FUTEX_WAKE, val, 0, 0, 0);
if (oldval cmp cmparg)
futex(uaddr2, FUTEX_WAKE, val2, 0, 0, 0);
In other words, FUTEX_WAKE_OP does the following:
• saves the original value of the futex word at uaddr2 and per-
forms an operation to modify the value of the futex at uaddr2;
this is an atomic read-modify-write memory access (i.e., using
atomic machine instructions of the respective architecture)
• wakes up a maximum of val waiters on the futex for the futex
word at uaddr; and
• dependent on the results of a test of the original value of
the futex word at uaddr2, wakes up a maximum of val2 waiters
on the futex for the futex word at uaddr2.
The operation and comparison that are to be performed are encoded
in the bits of the argument val3. Pictorially, the encoding is:
+---+---+-----------+-----------+
|op |cmp| oparg | cmparg |
+---+---+-----------+-----------+
4 4 12 12 <== # of bits
Expressed in code, the encoding is:
#define FUTEX_OP(op, oparg, cmp, cmparg) \
(((op & 0xf) << 28) | \
((cmp & 0xf) << 24) | \
((oparg & 0xfff) << 12) | \
(cmparg & 0xfff))
In the above, op and cmp are each one of the codes listed below.
The oparg and cmparg components are literal numeric values, ex-
cept as noted below.
The op component has one of the following values:
FUTEX_OP_SET 0 /* uaddr2 = oparg; */
FUTEX_OP_ADD 1 /* uaddr2 += oparg; */
FUTEX_OP_OR 2 /* uaddr2 |= oparg; */
FUTEX_OP_ANDN 3 /* uaddr2 &= ~oparg; */
FUTEX_OP_XOR 4 /* uaddr2 ^= oparg; */
In addition, bitwise ORing the following value into op causes
(1 << oparg) to be used as the operand:
FUTEX_OP_ARG_SHIFT 8 /* Use (1 << oparg) as operand */
The cmp field is one of the following:
FUTEX_OP_CMP_EQ 0 /* if (oldval == cmparg) wake */
FUTEX_OP_CMP_NE 1 /* if (oldval != cmparg) wake */
FUTEX_OP_CMP_LT 2 /* if (oldval < cmparg) wake */
FUTEX_OP_CMP_LE 3 /* if (oldval <= cmparg) wake */
FUTEX_OP_CMP_GT 4 /* if (oldval > cmparg) wake */
FUTEX_OP_CMP_GE 5 /* if (oldval >= cmparg) wake */
The return value of FUTEX_WAKE_OP is the sum of the number of
waiters woken on the futex uaddr plus the number of waiters woken
on the futex uaddr2.
FUTEX_WAIT_BITSET (since Linux 2.6.25)
This operation is like FUTEX_WAIT except that val3 is used to
provide a 32-bit bit mask to the kernel. This bit mask, in which
at least one bit must be set, is stored in the kernel-internal
state of the waiter. See the description of FUTEX_WAKE_BITSET
for further details.
If timeout is not NULL, the structure it points to specifies an
absolute timeout for the wait operation. If timeout is NULL, the
operation can block indefinitely.
The uaddr2 argument is ignored.
FUTEX_WAKE_BITSET (since Linux 2.6.25)
This operation is the same as FUTEX_WAKE except that the val3 ar-
gument is used to provide a 32-bit bit mask to the kernel. This
bit mask, in which at least one bit must be set, is used to se-
lect which waiters should be woken up. The selection is done by
a bitwise AND of the "wake" bit mask (i.e., the value in val3)
and the bit mask which is stored in the kernel-internal state of
the waiter (the "wait" bit mask that is set using FUTEX_WAIT_BIT-
SET). All of the waiters for which the result of the AND is
nonzero are woken up; the remaining waiters are left sleeping.
The effect of FUTEX_WAIT_BITSET and FUTEX_WAKE_BITSET is to allow
selective wake-ups among multiple waiters that are blocked on the
same futex. However, note that, depending on the use case, em-
ploying this bit-mask multiplexing feature on a futex can be less
efficient than simply using multiple futexes, because employing
bit-mask multiplexing requires the kernel to check all waiters on
a futex, including those that are not interested in being woken
up (i.e., they do not have the relevant bit set in their "wait"
bit mask).
The constant FUTEX_BITSET_MATCH_ANY, which corresponds to all 32
bits set in the bit mask, can be used as the val3 argument for
FUTEX_WAIT_BITSET and FUTEX_WAKE_BITSET. Other than differences
in the handling of the timeout argument, the FUTEX_WAIT operation
is equivalent to FUTEX_WAIT_BITSET with val3 specified as FU-
TEX_BITSET_MATCH_ANY; that is, allow a wake-up by any waker. The
FUTEX_WAKE operation is equivalent to FUTEX_WAKE_BITSET with val3
specified as FUTEX_BITSET_MATCH_ANY; that is, wake up any
waiter(s).
The uaddr2 and timeout arguments are ignored.
Priority-inheritance futexes
Linux supports priority-inheritance (PI) futexes in order to handle pri-
ority-inversion problems that can be encountered with normal futex
locks. Priority inversion is the problem that occurs when a high-prior-
ity task is blocked waiting to acquire a lock held by a low-priority
task, while tasks at an intermediate priority continuously preempt the
low-priority task from the CPU. Consequently, the low-priority task
makes no progress toward releasing the lock, and the high-priority task
remains blocked.
Priority inheritance is a mechanism for dealing with the priority-inver-
sion problem. With this mechanism, when a high-priority task becomes
blocked by a lock held by a low-priority task, the priority of the low-
priority task is temporarily raised to that of the high-priority task,
so that it is not preempted by any intermediate level tasks, and can
thus make progress toward releasing the lock. To be effective, priority
inheritance must be transitive, meaning that if a high-priority task
blocks on a lock held by a lower-priority task that is itself blocked by
a lock held by another intermediate-priority task (and so on, for chains
of arbitrary length), then both of those tasks (or more generally, all
of the tasks in a lock chain) have their priorities raised to be the
same as the high-priority task.
From a user-space perspective, what makes a futex PI-aware is a policy
agreement (described below) between user space and the kernel about the
value of the futex word, coupled with the use of the PI-futex operations
described below. (Unlike the other futex operations described above,
the PI-futex operations are designed for the implementation of very spe-
cific IPC mechanisms.)
The PI-futex operations described below differ from the other futex op-
erations in that they impose policy on the use of the value of the futex
word:
• If the lock is not acquired, the futex word's value shall be 0.
• If the lock is acquired, the futex word's value shall be the thread
ID (TID; see gettid(2)) of the owning thread.
• If the lock is owned and there are threads contending for the lock,
then the FUTEX_WAITERS bit shall be set in the futex word's value; in
other words, this value is:
FUTEX_WAITERS | TID
(Note that is invalid for a PI futex word to have no owner and FU-
TEX_WAITERS set.)
With this policy in place, a user-space application can acquire an unac-
quired lock or release a lock using atomic instructions executed in user
mode (e.g., a compare-and-swap operation such as cmpxchg on the x86 ar-
chitecture). Acquiring a lock simply consists of using compare-and-swap
to atomically set the futex word's value to the caller's TID if its pre-
vious value was 0. Releasing a lock requires using compare-and-swap to
set the futex word's value to 0 if the previous value was the expected
TID.
If a futex is already acquired (i.e., has a nonzero value), waiters must
employ the FUTEX_LOCK_PI or FUTEX_LOCK_PI2 operations to acquire the
lock. If other threads are waiting for the lock, then the FUTEX_WAITERS
bit is set in the futex value; in this case, the lock owner must employ
the FUTEX_UNLOCK_PI operation to release the lock.
In the cases where callers are forced into the kernel (i.e., required to
perform a futex() call), they then deal directly with a so-called RT-mu-
tex, a kernel locking mechanism which implements the required priority-
inheritance semantics. After the RT-mutex is acquired, the futex value
is updated accordingly, before the calling thread returns to user space.
It is important to note that the kernel will update the futex word's
value prior to returning to user space. (This prevents the possibility
of the futex word's value ending up in an invalid state, such as having
an owner but the value being 0, or having waiters but not having the FU-
TEX_WAITERS bit set.)
If a futex has an associated RT-mutex in the kernel (i.e., there are
blocked waiters) and the owner of the futex/RT-mutex dies unexpectedly,
then the kernel cleans up the RT-mutex and hands it over to the next
waiter. This in turn requires that the user-space value is updated ac-
cordingly. To indicate that this is required, the kernel sets the FU-
TEX_OWNER_DIED bit in the futex word along with the thread ID of the new
owner. User space can detect this situation via the presence of the FU-
TEX_OWNER_DIED bit and is then responsible for cleaning up the stale
state left over by the dead owner.
PI futexes are operated on by specifying one of the values listed below
in futex_op. Note that the PI futex operations must be used as paired
operations and are subject to some additional requirements:
• FUTEX_LOCK_PI, FUTEX_LOCK_PI2, and FUTEX_TRYLOCK_PI pair with FU-
TEX_UNLOCK_PI. FUTEX_UNLOCK_PI must be called only on a futex owned
by the calling thread, as defined by the value policy, otherwise the
error EPERM results.
• FUTEX_WAIT_REQUEUE_PI pairs with FUTEX_CMP_REQUEUE_PI. This must be
performed from a non-PI futex to a distinct PI futex (or the error
EINVAL results). Additionally, val (the number of waiters to be
woken) must be 1 (or the error EINVAL results).
The PI futex operations are as follows:
FUTEX_LOCK_PI (since Linux 2.6.18)
This operation is used after an attempt to acquire the lock via
an atomic user-mode instruction failed because the futex word has
a nonzero value—specifically, because it contained the (PID-name-
space-specific) TID of the lock owner.
The operation checks the value of the futex word at the address
uaddr. If the value is 0, then the kernel tries to atomically
set the futex value to the caller's TID. If the futex word's
value is nonzero, the kernel atomically sets the FUTEX_WAITERS
bit, which signals the futex owner that it cannot unlock the fu-
tex in user space atomically by setting the futex value to 0.
After that, the kernel:
(1) Tries to find the thread which is associated with the owner
TID.
(2) Creates or reuses kernel state on behalf of the owner. (If
this is the first waiter, there is no kernel state for this
futex, so kernel state is created by locking the RT-mutex
and the futex owner is made the owner of the RT-mutex. If
there are existing waiters, then the existing state is
reused.)
(3) Attaches the waiter to the futex (i.e., the waiter is en-
queued on the RT-mutex waiter list).
If more than one waiter exists, the enqueueing of the waiter is
in descending priority order. (For information on priority or-
dering, see the discussion of the SCHED_DEADLINE, SCHED_FIFO, and
SCHED_RR scheduling policies in sched(7).) The owner inherits
either the waiter's CPU bandwidth (if the waiter is scheduled un-
der the SCHED_DEADLINE policy) or the waiter's priority (if the
waiter is scheduled under the SCHED_RR or SCHED_FIFO policy).
This inheritance follows the lock chain in the case of nested
locking and performs deadlock detection.
The timeout argument provides a timeout for the lock attempt. If
timeout is not NULL, the structure it points to specifies an ab-
solute timeout, measured against the CLOCK_REALTIME clock. If
timeout is NULL, the operation will block indefinitely.
The uaddr2, val, and val3 arguments are ignored.
FUTEX_LOCK_PI2 (since Linux 5.14)
This operation is the same as FUTEX_LOCK_PI, except that the
clock against which timeout is measured is selectable. By de-
fault, the (absolute) timeout specified in timeout is measured
against the CLOCK_MONOTONIC clock, but if the FUTEX_CLOCK_REAL-
TIME flag is specified in futex_op, then the timeout is measured
against the CLOCK_REALTIME clock.
FUTEX_TRYLOCK_PI (since Linux 2.6.18)
This operation tries to acquire the lock at uaddr. It is invoked
when a user-space atomic acquire did not succeed because the fu-
tex word was not 0.
Because the kernel has access to more state information than user
space, acquisition of the lock might succeed if performed by the
kernel in cases where the futex word (i.e., the state information
accessible to use-space) contains stale state (FUTEX_WAITERS
and/or FUTEX_OWNER_DIED). This can happen when the owner of the
futex died. User space cannot handle this condition in a race-
free manner, but the kernel can fix this up and acquire the fu-
tex.
The uaddr2, val, timeout, and val3 arguments are ignored.
FUTEX_UNLOCK_PI (since Linux 2.6.18)
This operation wakes the top priority waiter that is waiting in
FUTEX_LOCK_PI or FUTEX_LOCK_PI2 on the futex address provided by
the uaddr argument.
This is called when the user-space value at uaddr cannot be
changed atomically from a TID (of the owner) to 0.
The uaddr2, val, timeout, and val3 arguments are ignored.
FUTEX_CMP_REQUEUE_PI (since Linux 2.6.31)
This operation is a PI-aware variant of FUTEX_CMP_REQUEUE. It
requeues waiters that are blocked via FUTEX_WAIT_REQUEUE_PI on
uaddr from a non-PI source futex (uaddr) to a PI target futex
(uaddr2).
As with FUTEX_CMP_REQUEUE, this operation wakes up a maximum of
val waiters that are waiting on the futex at uaddr. However, for
FUTEX_CMP_REQUEUE_PI, val is required to be 1 (since the main
point is to avoid a thundering herd). The remaining waiters are
removed from the wait queue of the source futex at uaddr and
added to the wait queue of the target futex at uaddr2.
The val2 and val3 arguments serve the same purposes as for FU-
TEX_CMP_REQUEUE.
FUTEX_WAIT_REQUEUE_PI (since Linux 2.6.31)
Wait on a non-PI futex at uaddr and potentially be requeued (via
a FUTEX_CMP_REQUEUE_PI operation in another task) onto a PI futex
at uaddr2. The wait operation on uaddr is the same as for FU-
TEX_WAIT.
The waiter can be removed from the wait on uaddr without requeue-
ing on uaddr2 via a FUTEX_WAKE operation in another task. In
this case, the FUTEX_WAIT_REQUEUE_PI operation fails with the er-
ror EAGAIN.
If timeout is not NULL, the structure it points to specifies an
absolute timeout for the wait operation. If timeout is NULL, the
operation can block indefinitely.
The val3 argument is ignored.
The FUTEX_WAIT_REQUEUE_PI and FUTEX_CMP_REQUEUE_PI were added to
support a fairly specific use case: support for priority-inheri-
tance-aware POSIX threads condition variables. The idea is that
these operations should always be paired, in order to ensure that
user space and the kernel remain in sync. Thus, in the FU-
TEX_WAIT_REQUEUE_PI operation, the user-space application pre-
specifies the target of the requeue that takes place in the FU-
TEX_CMP_REQUEUE_PI operation.
RETURN VALUE
In the event of an error (and assuming that futex() was invoked via
syscall(2)), all operations return -1 and set errno to indicate the er-
ror.
The return value on success depends on the operation, as described in
the following list:
FUTEX_WAIT
Returns 0 if the caller was woken up. Note that a wake-up can
also be caused by common futex usage patterns in unrelated code
that happened to have previously used the futex word's memory lo-
cation (e.g., typical futex-based implementations of Pthreads mu-
texes can cause this under some conditions). Therefore, callers
should always conservatively assume that a return value of 0 can
mean a spurious wake-up, and use the futex word's value (i.e.,
the user-space synchronization scheme) to decide whether to con-
tinue to block or not.
FUTEX_WAKE
Returns the number of waiters that were woken up.
FUTEX_FD
Returns the new file descriptor associated with the futex.
FUTEX_REQUEUE
Returns the number of waiters that were woken up.
FUTEX_CMP_REQUEUE
Returns the total number of waiters that were woken up or re-
queued to the futex for the futex word at uaddr2. If this value
is greater than val, then the difference is the number of waiters
requeued to the futex for the futex word at uaddr2.
FUTEX_WAKE_OP
Returns the total number of waiters that were woken up. This is
the sum of the woken waiters on the two futexes for the futex
words at uaddr and uaddr2.
FUTEX_WAIT_BITSET
Returns 0 if the caller was woken up. See FUTEX_WAIT for how to
interpret this correctly in practice.
FUTEX_WAKE_BITSET
Returns the number of waiters that were woken up.
FUTEX_LOCK_PI
Returns 0 if the futex was successfully locked.
FUTEX_LOCK_PI2
Returns 0 if the futex was successfully locked.
FUTEX_TRYLOCK_PI
Returns 0 if the futex was successfully locked.
FUTEX_UNLOCK_PI
Returns 0 if the futex was successfully unlocked.
FUTEX_CMP_REQUEUE_PI
Returns the total number of waiters that were woken up or re-
queued to the futex for the futex word at uaddr2. If this value
is greater than val, then difference is the number of waiters re-
queued to the futex for the futex word at uaddr2.
FUTEX_WAIT_REQUEUE_PI
Returns 0 if the caller was successfully requeued to the futex
for the futex word at uaddr2.
ERRORS
EACCES No read access to the memory of a futex word.
EAGAIN (FUTEX_WAIT, FUTEX_WAIT_BITSET, FUTEX_WAIT_REQUEUE_PI) The value
pointed to by uaddr was not equal to the expected value val at
the time of the call.
Note: on Linux, the symbolic names EAGAIN and EWOULDBLOCK (both
of which appear in different parts of the kernel futex code) have
the same value.
EAGAIN (FUTEX_CMP_REQUEUE, FUTEX_CMP_REQUEUE_PI) The value pointed to by
uaddr is not equal to the expected value val3.
EAGAIN (FUTEX_LOCK_PI, FUTEX_LOCK_PI2, FUTEX_TRYLOCK_PI, FUTEX_CMP_RE-
QUEUE_PI) The futex owner thread ID of uaddr (for FUTEX_CMP_RE-
QUEUE_PI: uaddr2) is about to exit, but has not yet handled the
internal state cleanup. Try again.
EDEADLK
(FUTEX_LOCK_PI, FUTEX_LOCK_PI2, FUTEX_TRYLOCK_PI, FUTEX_CMP_RE-
QUEUE_PI) The futex word at uaddr is already locked by the
caller.
EDEADLK
(FUTEX_CMP_REQUEUE_PI) While requeueing a waiter to the PI futex
for the futex word at uaddr2, the kernel detected a deadlock.
EFAULT A required pointer argument (i.e., uaddr, uaddr2, or timeout) did
not point to a valid user-space address.
EINTR A FUTEX_WAIT or FUTEX_WAIT_BITSET operation was interrupted by a
signal (see signal(7)). Before Linux 2.6.22, this error could
also be returned for a spurious wakeup; since Linux 2.6.22, this
no longer happens.
EINVAL The operation in futex_op is one of those that employs a timeout,
but the supplied timeout argument was invalid (tv_sec was less
than zero, or tv_nsec was not less than 1,000,000,000).
EINVAL The operation specified in futex_op employs one or both of the
pointers uaddr and uaddr2, but one of these does not point to a
valid object—that is, the address is not four-byte-aligned.
EINVAL (FUTEX_WAIT_BITSET, FUTEX_WAKE_BITSET) The bit mask supplied in
val3 is zero.
EINVAL (FUTEX_CMP_REQUEUE_PI) uaddr equals uaddr2 (i.e., an attempt was
made to requeue to the same futex).
EINVAL (FUTEX_FD) The signal number supplied in val is invalid.
EINVAL (FUTEX_WAKE, FUTEX_WAKE_OP, FUTEX_WAKE_BITSET, FUTEX_REQUEUE, FU-
TEX_CMP_REQUEUE) The kernel detected an inconsistency between the
user-space state at uaddr and the kernel state—that is, it de-
tected a waiter which waits in FUTEX_LOCK_PI or FUTEX_LOCK_PI2 on
uaddr.
EINVAL (FUTEX_LOCK_PI, FUTEX_LOCK_PI2, FUTEX_TRYLOCK_PI, FUTEX_UN-
LOCK_PI) The kernel detected an inconsistency between the user-
space state at uaddr and the kernel state. This indicates either
state corruption or that the kernel found a waiter on uaddr which
is waiting via FUTEX_WAIT or FUTEX_WAIT_BITSET.
EINVAL (FUTEX_CMP_REQUEUE_PI) The kernel detected an inconsistency be-
tween the user-space state at uaddr2 and the kernel state; that
is, the kernel detected a waiter which waits via FUTEX_WAIT or
FUTEX_WAIT_BITSET on uaddr2.
EINVAL (FUTEX_CMP_REQUEUE_PI) The kernel detected an inconsistency be-
tween the user-space state at uaddr and the kernel state; that
is, the kernel detected a waiter which waits via FUTEX_WAIT or
FUTEX_WAIT_BITSET on uaddr.
EINVAL (FUTEX_CMP_REQUEUE_PI) The kernel detected an inconsistency be-
tween the user-space state at uaddr and the kernel state; that
is, the kernel detected a waiter which waits on uaddr via FU-
TEX_LOCK_PI or FUTEX_LOCK_PI2 (instead of FUTEX_WAIT_REQUEUE_PI).
EINVAL (FUTEX_CMP_REQUEUE_PI) An attempt was made to requeue a waiter to
a futex other than that specified by the matching FUTEX_WAIT_RE-
QUEUE_PI call for that waiter.
EINVAL (FUTEX_CMP_REQUEUE_PI) The val argument is not 1.
EINVAL Invalid argument.
ENFILE (FUTEX_FD) The system-wide limit on the total number of open
files has been reached.
ENOMEM (FUTEX_LOCK_PI, FUTEX_LOCK_PI2, FUTEX_TRYLOCK_PI, FUTEX_CMP_RE-
QUEUE_PI) The kernel could not allocate memory to hold state in-
formation.
ENOSYS Invalid operation specified in futex_op.
ENOSYS The FUTEX_CLOCK_REALTIME option was specified in futex_op, but
the accompanying operation was neither FUTEX_WAIT, FU-
TEX_WAIT_BITSET, FUTEX_WAIT_REQUEUE_PI, nor FUTEX_LOCK_PI2.
ENOSYS (FUTEX_LOCK_PI, FUTEX_LOCK_PI2, FUTEX_TRYLOCK_PI, FUTEX_UN-
LOCK_PI, FUTEX_CMP_REQUEUE_PI, FUTEX_WAIT_REQUEUE_PI) A run-time
check determined that the operation is not available. The PI-fu-
tex operations are not implemented on all architectures and are
not supported on some CPU variants.
EPERM (FUTEX_LOCK_PI, FUTEX_LOCK_PI2, FUTEX_TRYLOCK_PI, FUTEX_CMP_RE-
QUEUE_PI) The caller is not allowed to attach itself to the futex
at uaddr (for FUTEX_CMP_REQUEUE_PI: the futex at uaddr2). (This
may be caused by a state corruption in user space.)
EPERM (FUTEX_UNLOCK_PI) The caller does not own the lock represented by
the futex word.
ESRCH (FUTEX_LOCK_PI, FUTEX_LOCK_PI2, FUTEX_TRYLOCK_PI, FUTEX_CMP_RE-
QUEUE_PI) The thread ID in the futex word at uaddr does not ex-
ist.
ESRCH (FUTEX_CMP_REQUEUE_PI) The thread ID in the futex word at uaddr2
does not exist.
ETIMEDOUT
The operation in futex_op employed the timeout specified in time-
out, and the timeout expired before the operation completed.
STANDARDS
Linux.
HISTORY
Linux 2.6.0.
Initial futex support was merged in Linux 2.5.7 but with different se-
mantics from what was described above. A four-argument system call with
the semantics described in this page was introduced in Linux 2.5.40. A
fifth argument was added in Linux 2.5.70, and a sixth argument was added
in Linux 2.6.7.
EXAMPLES
The program below demonstrates use of futexes in a program where a par-
ent process and a child process use a pair of futexes located inside a
shared anonymous mapping to synchronize access to a shared resource: the
terminal. The two processes each write nloops (a command-line argument
that defaults to 5 if omitted) messages to the terminal and employ a
synchronization protocol that ensures that they alternate in writing
messages. Upon running this program we see output such as the follow-
ing:
$ ./futex_demo
Parent (18534) 0
Child (18535) 0
Parent (18534) 1
Child (18535) 1
Parent (18534) 2
Child (18535) 2
Parent (18534) 3
Child (18535) 3
Parent (18534) 4
Child (18535) 4
Program source
/* futex_demo.c
Usage: futex_demo [nloops]
(Default: 5)
Demonstrate the use of futexes in a program where parent and child
use a pair of futexes located inside a shared anonymous mapping to
synchronize access to a shared resource: the terminal. The two
processes each write 'num-loops' messages to the terminal and employ
a synchronization protocol that ensures that they alternate in
writing messages.
*/
#define _GNU_SOURCE
#include <err.h>
#include <errno.h>
#include <linux/futex.h>
#include <stdatomic.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <sys/time.h>
#include <sys/wait.h>
#include <unistd.h>
static uint32_t *futex1, *futex2, *iaddr;
static int
futex(uint32_t *uaddr, int futex_op, uint32_t val,
const struct timespec *timeout, uint32_t *uaddr2, uint32_t val3)
{
return syscall(SYS_futex, uaddr, futex_op, val,
timeout, uaddr2, val3);
}
/* Acquire the futex pointed to by 'futexp': wait for its value to
become 1, and then set the value to 0. */
static void
fwait(uint32_t *futexp)
{
long s;
const uint32_t one = 1;
/* atomic_compare_exchange_strong(ptr, oldval, newval)
atomically performs the equivalent of:
if (*ptr == *oldval)
*ptr = newval;
It returns true if the test yielded true and *ptr was updated. */
while (1) {
/* Is the futex available? */
if (atomic_compare_exchange_strong(futexp, &one, 0))
break; /* Yes */
/* Futex is not available; wait. */
s = futex(futexp, FUTEX_WAIT, 0, NULL, NULL, 0);
if (s == -1 && errno != EAGAIN)
err(EXIT_FAILURE, "futex-FUTEX_WAIT");
}
}
/* Release the futex pointed to by 'futexp': if the futex currently
has the value 0, set its value to 1 and then wake any futex waiters,
so that if the peer is blocked in fwait(), it can proceed. */
static void
fpost(uint32_t *futexp)
{
long s;
const uint32_t zero = 0;
/* atomic_compare_exchange_strong() was described
in comments above. */
if (atomic_compare_exchange_strong(futexp, &zero, 1)) {
s = futex(futexp, FUTEX_WAKE, 1, NULL, NULL, 0);
if (s == -1)
err(EXIT_FAILURE, "futex-FUTEX_WAKE");
}
}
int
main(int argc, char *argv[])
{
pid_t childPid;
unsigned int nloops;
setbuf(stdout, NULL);
nloops = (argc > 1) ? atoi(argv[1]) : 5;
/* Create a shared anonymous mapping that will hold the futexes.
Since the futexes are being shared between processes, we
subsequently use the "shared" futex operations (i.e., not the
ones suffixed "_PRIVATE"). */
iaddr = mmap(NULL, sizeof(*iaddr) * 2, PROT_READ | PROT_WRITE,
MAP_ANONYMOUS | MAP_SHARED, -1, 0);
if (iaddr == MAP_FAILED)
err(EXIT_FAILURE, "mmap");
futex1 = &iaddr[0];
futex2 = &iaddr[1];
*futex1 = 0; /* State: unavailable */
*futex2 = 1; /* State: available */
/* Create a child process that inherits the shared anonymous
mapping. */
childPid = fork();
if (childPid == -1)
err(EXIT_FAILURE, "fork");
if (childPid == 0) { /* Child */
for (unsigned int j = 0; j < nloops; j++) {
fwait(futex1);
printf("Child (%jd) %u\n", (intmax_t) getpid(), j);
fpost(futex2);
}
exit(EXIT_SUCCESS);
}
/* Parent falls through to here. */
for (unsigned int j = 0; j < nloops; j++) {
fwait(futex2);
printf("Parent (%jd) %u\n", (intmax_t) getpid(), j);
fpost(futex1);
}
wait(NULL);
exit(EXIT_SUCCESS);
}
SEE ALSO
get_robust_list(2), restart_syscall(2), pthread_mutexattr_getproto-
col(3), futex(7), sched(7)
The following kernel source files:
• Documentation/pi-futex.txt
• Documentation/futex-requeue-pi.txt
• Documentation/locking/rt-mutex.txt
• Documentation/locking/rt-mutex-design.txt
• Documentation/robust-futex-ABI.txt
Franke, H., Russell, R., and Kirwood, M., 2002. Fuss, Futexes and Fur-
wocks: Fast Userlevel Locking in Linux (from proceedings of the Ottawa
Linux Symposium 2002),
]8;;http://kernel.org/doc/ols/2002/ols2002-pages-479-495.pdf\http://kernel.org/doc/ols/2002/ols2002-pages-479-495.pdf]8;;\
Hart, D., 2009. A futex overview and update, ]8;;http://lwn.net/Articles/360699/\http://lwn.net/Arti-
cles/360699/]8;;\
Hart, D. and Guniguntala, D., 2009. Requeue-PI: Making glibc Condvars
PI-Aware (from proceedings of the 2009 Real-Time Linux Workshop),
]8;;http://lwn.net/images/conf/rtlws11/papers/proc/p10.pdf\http://lwn.net/images/conf/rtlws11/papers/proc/p10.pdf]8;;\
Drepper, U., 2011. Futexes Are Tricky, ]8;;http://www.akkadia.org/drepper/futex.pdf\http://www.akkadia.org/drep-
per/futex.pdf]8;;\
Futex example library, futex-*.tar.bz2 at
]8;;https://mirrors.kernel.org/pub/linux/kernel/people/rusty/\https://mirrors.kernel.org/pub/linux/kernel/people/rusty/]8;;\
Linux man-pages 6.9.1 2024-06-15 futex(2)
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