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FUTEX(2) Linux Programmer's Manual FUTEX(2)
futex - fast user-space locking
#include <linux/futex.h>
#include <sys/time.h>
int futex(int *uaddr, int futex_op, int val,
const struct timespec *timeout, /* or: uint32_t val2 */
int *uaddr2, int val3);
Note: There is no glibc wrapper for this system call; see NOTES.
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, bit-wise 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, and (since Linux
4.5) FUTEX_WAIT 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 immedi‐
ately 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:
int oldval = *(int *) uaddr2;
*(int *) 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 wait‐
ers 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, bit-wise 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 bit-wise AND of the "wake" bit mask (i.e., the value
in val3) and the bit mask which is stored in the kernel-inter‐
nal 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 dif‐
ferences 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_BIT‐
SET_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-pri‐
ority task, while tasks at an intermediate priority continuously pre‐
empt the low-priority task from the CPU. Consequently, the low-pri‐
ority 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-prior‐
ity 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 pol‐
icy 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 imple‐
mentation 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 pre‐
vious value was the expected TID.
If a futex is already acquired (i.e., has a nonzero value), waiters
must employ the FUTEX_LOCK_PI operation 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 possibil‐
ity 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 hav‐
ing 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 unexpect‐
edly, 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 and FUTEX_TRYLOCK_PI pair with FUTEX_UNLOCK_PI.
FUTEX_UNLOCK_PI must be called only on a futex owned by the call‐
ing 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 can‐
not 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 indefi‐
nitely.
The uaddr2, val, and val3 arguments are ignored.
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 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. How‐
ever, 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 appli‐
cation 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
cause of 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_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_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_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)). In kernels 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 on
uaddr.
EINVAL (FUTEX_LOCK_PI, 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_BITESET 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 (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_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, nor FUTEX_WAIT_REQUEUE_PI.
ENOSYS (FUTEX_LOCK_PI, 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_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_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.
Futexes were first made available in a stable kernel release with
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.
This system call is Linux-specific.
Glibc does not provide a wrapper for this system call; call it using
syscall(2).
Several higher-level programming abstractions are implemented via
futexes, including POSIX semaphores and various POSIX threads
synchronization mechanisms (mutexes, condition variables, read-write
locks, and barriers).
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 <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <linux/futex.h>
#include <sys/time.h>
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)
static int *futex1, *futex2, *iaddr;
static int
futex(int *uaddr, int futex_op, int val,
const struct timespec *timeout, int *uaddr2, int val3)
{
return syscall(SYS_futex, uaddr, futex_op, val,
timeout, uaddr, 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(int *futexp)
{
int s;
/* __sync_bool_compare_and_swap(ptr, oldval, newval) is a gcc
built-in function. It atomically performs the equivalent of:
if (*ptr == oldval)
*ptr = newval;
It returns true if the test yielded true and *ptr was updated.
The alternative here would be to employ the equivalent atomic
machine-language instructions. For further information, see
the GCC Manual. */
while (1) {
/* Is the futex available? */
if (__sync_bool_compare_and_swap(futexp, 1, 0))
break; /* Yes */
/* Futex is not available; wait */
s = futex(futexp, FUTEX_WAIT, 0, NULL, NULL, 0);
if (s == -1 && errno != EAGAIN)
errExit("futex-FUTEX_WAIT");
}
}
/* Release the futex pointed to by 'futexp': if the futex currently
has the value 0, set its value to 1 and the wake any futex waiters,
so that if the peer is blocked in fpost(), it can proceed. */
static void
fpost(int *futexp)
{
int s;
/* __sync_bool_compare_and_swap() was described in comments above */
if (__sync_bool_compare_and_swap(futexp, 0, 1)) {
s = futex(futexp, FUTEX_WAKE, 1, NULL, NULL, 0);
if (s == -1)
errExit("futex-FUTEX_WAKE");
}
}
int
main(int argc, char *argv[])
{
pid_t childPid;
int j, 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(int) * 2, PROT_READ | PROT_WRITE,
MAP_ANONYMOUS | MAP_SHARED, -1, 0);
if (iaddr == MAP_FAILED)
errExit("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)
errExit("fork");
if (childPid == 0) { /* Child */
for (j = 0; j < nloops; j++) {
fwait(futex1);
printf("Child (%ld) %d\n", (long) getpid(), j);
fpost(futex2);
}
exit(EXIT_SUCCESS);
}
/* Parent falls through to here */
for (j = 0; j < nloops; j++) {
fwait(futex2);
printf("Parent (%ld) %d\n", (long) 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 Cond‐
vars PI-Aware (from proceedings of the 2009 Real-Time Linux Work‐
shop), ⟨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
⟨ftp://ftp.kernel.org/pub/linux/kernel/people/rusty/⟩
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 FUTEX(2)
Pages that refer to this page: clone(2), eventfd(2), get_robust_list(2), mprotect(2), prctl(2), restart_syscall(2), set_tid_address(2), syscalls(2), futex(7), pthreads(7), signal(7)
Copyright and license for this manual page
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