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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | EXAMPLES | SEE ALSO | COLOPHON |
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futex(2) System Calls Manual futex(2)
futex - fast user-space locking
Standard C library (libc, -lc)
#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).
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
condition 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
address 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 using (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 expected 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.
Analogously 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 accessed flag in shared memory. In the uncontended
case, a thread can access 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 entirely 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
further 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 implementation 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 argument; 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 integer whose meaning is
determined by the operation. For these operations, the kernel
casts the timeout value first to unsigned long, then to uint32_t,
and in the remainder of this page, this argument is referred 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 included 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 kernel 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 constant value. Thus,
there are FUTEX_WAIT_PRIVATE, FUTEX_WAKE_PRIVATE, 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 futex word. The load of the
value of the futex word is an atomic memory access (i.e.,
using atomic machine instructions of the respective
architecture). This load, the comparison with the
expected 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 prevent 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 operation (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_MONOTONIC 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 interpreted as an absolute value. To obtain
the equivalent of FUTEX_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 provided 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 descriptor 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
notifications: 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 contains the value val3. If not, the operation
fails with the error EAGAIN. Otherwise, the operation
wakes up a maximum of val waiters 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_REQUEUE operation equivalent to FUTEX_WAKE.)
The limit value specified via val2 is typically either 1
or INT_MAX. (Specifying the argument as 0 is not useful,
because it would make the FUTEX_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 pointless 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_signal(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
following 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 performs 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, except 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 argument 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 select 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_BITSET). 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, employing 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
FUTEX_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 priority-inversion problems that can be encountered with
normal futex locks. Priority inversion is the problem that
occurs when a high-priority 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-inversion 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 specific IPC mechanisms.)
The PI-futex operations described below differ from the other
futex operations 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
FUTEX_WAITERS set.)
With this policy in place, a user-space application can acquire
an unacquired 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 architecture). 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 previous 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-mutex, 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 FUTEX_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 accordingly. To indicate that this is
required, the kernel sets the FUTEX_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 FUTEX_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
FUTEX_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-namespace-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 futex 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
enqueued 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 ordering, 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 under
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 absolute 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 default, the (absolute) timeout specified in timeout is
measured against the CLOCK_MONOTONIC clock, but if the
FUTEX_CLOCK_REALTIME 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 futex 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 futex.
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
FUTEX_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 FUTEX_WAIT.
The waiter can be removed from the wait on uaddr without
requeueing on uaddr2 via a FUTEX_WAKE operation in another
task. In this case, the FUTEX_WAIT_REQUEUE_PI operation
fails with the error 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-inheritance-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
FUTEX_WAIT_REQUEUE_PI operation, the user-space
application pre-specifies the target of the requeue that
takes place in the FUTEX_CMP_REQUEUE_PI operation.
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 error.
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 location (e.g., typical futex-based
implementations of Pthreads mutexes 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 continue 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
requeued 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
requeued to the futex for the futex word at uaddr2. If
this value is greater than val, then difference is the
number of waiters requeued 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.
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_REQUEUE_PI) The futex owner thread ID of uaddr
(for FUTEX_CMP_REQUEUE_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_REQUEUE_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, FUTEX_CMP_REQUEUE) The kernel detected an
inconsistency between the user-space state at uaddr and
the kernel state—that is, it detected 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_UNLOCK_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 between 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 between 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 between the user-space state at uaddr and
the kernel state; that is, the kernel detected a waiter
which waits on uaddr via FUTEX_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_REQUEUE_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_REQUEUE_PI) The kernel could not allocate memory
to hold state information.
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,
FUTEX_WAIT_BITSET, FUTEX_WAIT_REQUEUE_PI, nor
FUTEX_LOCK_PI2.
ENOSYS (FUTEX_LOCK_PI, FUTEX_LOCK_PI2, FUTEX_TRYLOCK_PI,
FUTEX_UNLOCK_PI, FUTEX_CMP_REQUEUE_PI,
FUTEX_WAIT_REQUEUE_PI) A run-time check determined that
the operation is not available. The PI-futex 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_REQUEUE_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_REQUEUE_PI) The thread ID in the futex word at
uaddr does not exist.
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 timeout, and the timeout expired before the operation
completed.
Linux.
Linux 2.6.0.
Initial futex support was merged in Linux 2.5.7 but with
different semantics 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.
The program below demonstrates use of futexes in a program where
a parent 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 following:
$ ./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);
}
get_robust_list(2), restart_syscall(2),
pthread_mutexattr_getprotocol(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 Furwocks: Fast Userlevel Locking in Linux (from proceedings
of the Ottawa Linux Symposium 2002),
⟨http://kernel.org/doc/ols/2002/ols2002-pages-479-495.pdf⟩
Hart, D., 2009. A futex overview and update,
⟨http://lwn.net/Articles/360699/⟩
Hart, D. and Guniguntala, D., 2009. Requeue-PI: Making glibc
Condvars PI-Aware (from proceedings of the 2009 Real-Time Linux
Workshop),
⟨http://lwn.net/images/conf/rtlws11/papers/proc/p10.pdf⟩
Drepper, U., 2011. Futexes Are Tricky,
⟨http://www.akkadia.org/drepper/futex.pdf⟩
Futex example library, futex-*.tar.bz2 at
⟨https://mirrors.kernel.org/pub/linux/kernel/people/rusty/⟩
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
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version of the page, or you believe there is a better or more up-
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Linux man-pages 6.9.1 2024-06-15 futex(2)
Pages that refer to this page: clone(2), eventfd(2), get_robust_list(2), mprotect(2), PR_SET_TIMERSLACK(2const), restart_syscall(2), set_tid_address(2), syscalls(2), io_uring_prep_futex_wait(3), io_uring_prep_futex_waitv(3), io_uring_prep_futex_wake(3), futex(7), pthreads(7), signal(7)
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