futex

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



NAME
       futex - fast user-space locking

SYNOPSIS
       #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
       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 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 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-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 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.  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_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, 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.

RETURN VALUE
       In the event of an error (and assuming that futex() was invoked via
       syscall(2)), all operations return -1 and set errno to indicate the 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
       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.

VERSIONS
       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
       This system call is Linux-specific.

NOTES
       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).

EXAMPLES
       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 <stdatomic.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, 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(int *futexp)
       {
           int s;

           /* 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? */
               const int one = 1;
               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)
                   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;

           /* atomic_compare_exchange_strong() was described in comments above */

           const int zero = 0;
           if (atomic_compare_exchange_strong(futexp, &zero, 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
       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
       ⟨ftp://ftp.kernel.org/pub/linux/kernel/people/rusty/⟩

COLOPHON
       This page is part of release 5.08 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                              2020-06-09                           FUTEX(2)