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FUTEX(2)                  Linux Programmer's Manual                 FUTEX(2)

NAME         top

       futex - fast user-space locking

SYNOPSIS         top

       #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.

DESCRIPTION         top

       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 "acquiired 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 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 are 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 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 and FUTEX_WAIT_REQUEUE_PI operations.

              If this option is set, the kernel treats timeout as an
              absolute time based on CLOCK_REALTIME.

              If this option is not set, the kernel treats timeout as a
              relative time, measured 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 argument is non-NULL, its contents specify a
              relative timeout for the wait, measured according to the
              CLOCK_MONOTONIC clock.  (This interval will be rounded up to
              the system clock granularity, and is guaranteed not to expire
              early.)  If timeout is NULL, the call blocks indefinitely.

              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 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 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:

                  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
                 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, 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 mask to the kernel.  This bit mask is stored
              in the kernel-internal state of the waiter.  See the
              description of FUTEX_WAKE_BITSET for further details.

              The FUTEX_WAIT_BITSET operation also interprets the timeout
              argument differently from FUTEX_WAIT.  See the discussion of
              FUTEX_CLOCK_REALTIME, above.

              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 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-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 uaddr2 and timeout arguments are ignored.

              The FUTEX_WAIT and FUTEX_WAKE operations correspond to
              FUTEX_WAIT_BITSET and FUTEX_WAKE_BITSET operations where the
              bit masks are all ones.

   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 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
       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 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.
              It is interpreted as an absolute time, measured against the
              CLOCK_REALTIME clock.  If timeout is NULL, the operation will
              block indefinitely.

              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.
              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, it specifies a timeout for the wait
              operation; this timeout is interpreted as outlined above in
              the description of the FUTEX_CLOCK_REALTIME option.  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.

RETURN VALUE         top

       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.

ERRORS         top

       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 on 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.

       ENOMEM (FUTEX_LOCK_PI, FUTEX_TRYLOCK_PI, FUTEX_CMP_REQUEUE_PI) The
              kernel could not allocate memory to hold state information.

       ENFILE (FUTEX_FD) The system-wide limit on the total number of open
              files has been reached.

       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_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.

VERSIONS         top

       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.

CONFORMING TO         top

       This system call is Linux-specific.

NOTES         top

       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).

EXAMPLE         top

       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);
       }

SEE ALSO         top

       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/⟩

COLOPHON         top

       This page is part of release 4.04 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
       http://www.kernel.org/doc/man-pages/.

Linux                            2015-12-28                         FUTEX(2)