futex(2) — Linux manual page

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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 <stdint.h>
       #include <sys/time.h>

       long futex(uint32_t *uaddr, int futex_op, uint32_t val,
                 const struct timespec *timeout,   /* or: uint32_t val2 */
                 uint32_t *uaddr2, uint32_t val3);

       Note: 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 "acquired state").  When releasing the
       lock, a thread has to first reset the lock state to not acquired
       and then execute a futex operation that wakes threads blocked on
       the lock flag used as a futex word (this can be further optimized
       to avoid unnecessary wake-ups).  See futex(7) for more detail on
       how to use futexes.

       Besides the basic wait and wake-up futex functionality, there are
       further futex operations aimed at supporting more complex use
       cases.

       Note that no explicit initialization or destruction is necessary
       to use futexes; the kernel maintains a futex (i.e., the kernel-
       internal implementation artifact) only while operations such as
       FUTEX_WAIT, described below, are being performed on a particular
       futex word.

   Arguments
       The uaddr argument points to the futex word.  On all platforms,
       futexes are four-byte integers that must be aligned on a four-
       byte boundary.  The operation to perform on the futex is
       specified in the futex_op argument; val is a value whose meaning
       and purpose depends on futex_op.

       The remaining arguments (timeout, uaddr2, and val3) are required
       only for certain of the futex operations described below.  Where
       one of these arguments is not required, it is ignored.

       For several blocking operations, the timeout argument is a
       pointer to a timespec structure that specifies a timeout for the
       operation.  However,  notwithstanding the prototype shown above,
       for some operations, the least significant four bytes of this
       argument are instead used as an integer whose meaning is
       determined by the operation.  For these operations, the kernel
       casts the timeout value first to unsigned long, then to uint32_t,
       and in the remainder of this page, this argument is referred to
       as val2 when interpreted in this fashion.

       Where it is required, the uaddr2 argument is a pointer to a
       second futex word that is employed by the operation.

       The interpretation of the final integer argument, val3, depends
       on the operation.

   Futex operations
       The futex_op argument consists of two parts: a command that
       specifies the operation to be performed, bitwise ORed with zero
       or more options that modify the behaviour of the operation.  The
       options that may be included in futex_op are as follows:

       FUTEX_PRIVATE_FLAG (since Linux 2.6.22)
              This option bit can be employed with all futex operations.
              It tells the kernel that the futex is process-private and
              not shared with another process (i.e., it is being used
              for synchronization only between threads of the same
              process).  This allows the kernel to make some additional
              performance optimizations.

              As a convenience, <linux/futex.h> defines a set of
              constants with the suffix _PRIVATE that are equivalents of
              all of the operations listed below, but with the
              FUTEX_PRIVATE_FLAG ORed into the constant value.  Thus,
              there are FUTEX_WAIT_PRIVATE, FUTEX_WAKE_PRIVATE, and so
              on.

       FUTEX_CLOCK_REALTIME (since Linux 2.6.28)
              This option bit can be employed only with the
              FUTEX_WAIT_BITSET, FUTEX_WAIT_REQUEUE_PI, 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:

                  uint32_t oldval = *(uint32_t *) uaddr2;
                  *(uint32_t *) uaddr2 = oldval op oparg;
                  futex(uaddr, FUTEX_WAKE, val, 0, 0, 0);
                  if (oldval cmp cmparg)
                      futex(uaddr2, FUTEX_WAKE, val2, 0, 0, 0);

              In other words, FUTEX_WAKE_OP does the following:

              *  saves the original value of the futex word at uaddr2
                 and performs an operation to modify the value of the
                 futex at uaddr2; this is an atomic read-modify-write
                 memory access (i.e., using atomic machine instructions
                 of the respective architecture)

              *  wakes up a maximum of val waiters on the futex for the
                 futex word at uaddr; and

              *  dependent on the results of a test of the original
                 value of the futex word at uaddr2, wakes up a maximum
                 of val2 waiters on the futex for the futex word at
                 uaddr2.

              The operation and comparison that are to be performed are
              encoded in the bits of the argument val3.  Pictorially,
              the encoding is:

                  +---+---+-----------+-----------+
                  |op |cmp|   oparg   |  cmparg   |
                  +---+---+-----------+-----------+
                    4   4       12          12    <== # of bits

              Expressed in code, the encoding is:

                  #define FUTEX_OP(op, oparg, cmp, cmparg) \
                                  (((op & 0xf) << 28) | \
                                  ((cmp & 0xf) << 24) | \
                                  ((oparg & 0xfff) << 12) | \
                                  (cmparg & 0xfff))

              In the above, op and cmp are each one of the codes listed
              below.  The oparg and cmparg components are literal
              numeric values, except as noted below.

              The op component has one of the following values:

                  FUTEX_OP_SET        0  /* uaddr2 = oparg; */
                  FUTEX_OP_ADD        1  /* uaddr2 += oparg; */
                  FUTEX_OP_OR         2  /* uaddr2 |= oparg; */
                  FUTEX_OP_ANDN       3  /* uaddr2 &= ~oparg; */
                  FUTEX_OP_XOR        4  /* uaddr2 ^= oparg; */

              In addition, bitwise ORing the following value into op
              causes (1 << oparg) to be used as the operand:

                  FUTEX_OP_ARG_SHIFT  8  /* Use (1 << oparg) as operand */

              The cmp field is one of the following:

                  FUTEX_OP_CMP_EQ     0  /* if (oldval == cmparg) wake */
                  FUTEX_OP_CMP_NE     1  /* if (oldval != cmparg) wake */
                  FUTEX_OP_CMP_LT     2  /* if (oldval < cmparg) wake */
                  FUTEX_OP_CMP_LE     3  /* if (oldval <= cmparg) wake */
                  FUTEX_OP_CMP_GT     4  /* if (oldval > cmparg) wake */
                  FUTEX_OP_CMP_GE     5  /* if (oldval >= cmparg) wake */

              The return value of FUTEX_WAKE_OP is the sum of the number
              of waiters woken on the futex uaddr plus the number of
              waiters woken on the futex uaddr2.

       FUTEX_WAIT_BITSET (since Linux 2.6.25)
              This operation is like FUTEX_WAIT except that val3 is used
              to provide a 32-bit bit mask to the kernel.  This bit
              mask, in which at least one bit must be set, is stored in
              the kernel-internal state of the waiter.  See the
              description of FUTEX_WAKE_BITSET for further details.

              If timeout is not NULL, the structure it points to
              specifies an absolute timeout for the wait operation.  If
              timeout is NULL, the operation can block indefinitely.

              The uaddr2 argument is ignored.

       FUTEX_WAKE_BITSET (since Linux 2.6.25)
              This operation is the same as FUTEX_WAKE except that the
              val3 argument is used to provide a 32-bit bit mask to the
              kernel.  This bit mask, in which at least one bit must be
              set, is used to select which waiters should be woken up.
              The selection is done by a bitwise AND of the "wake" bit
              mask (i.e., the value in val3) and the bit mask which is
              stored in the kernel-internal state of the waiter (the
              "wait" bit mask that is set using FUTEX_WAIT_BITSET).  All
              of the waiters for which the result of the AND is nonzero
              are woken up; the remaining waiters are left sleeping.

              The effect of FUTEX_WAIT_BITSET and FUTEX_WAKE_BITSET is
              to allow selective wake-ups among multiple waiters that
              are blocked on the same futex.  However, note that,
              depending on the use case, employing this bit-mask
              multiplexing feature on a futex can be less efficient than
              simply using multiple futexes, because employing bit-mask
              multiplexing requires the kernel to check all waiters on a
              futex, including those that are not interested in being
              woken up (i.e., they do not have the relevant bit set in
              their "wait" bit mask).

              The constant FUTEX_BITSET_MATCH_ANY, which corresponds to
              all 32 bits set in the bit mask, can be used as the val3
              argument for FUTEX_WAIT_BITSET and FUTEX_WAKE_BITSET.
              Other than differences in the handling of the timeout
              argument, the FUTEX_WAIT operation is equivalent to
              FUTEX_WAIT_BITSET with val3 specified as
              FUTEX_BITSET_MATCH_ANY; that is, allow a wake-up by any
              waker.  The FUTEX_WAKE operation is equivalent to
              FUTEX_WAKE_BITSET with val3 specified as
              FUTEX_BITSET_MATCH_ANY; that is, wake up any waiter(s).

              The uaddr2 and timeout arguments are ignored.

   Priority-inheritance futexes
       Linux supports priority-inheritance (PI) futexes in order to
       handle priority-inversion problems that can be encountered with
       normal futex locks.  Priority inversion is the problem that
       occurs when a high-priority task is blocked waiting to acquire a
       lock held by a low-priority task, while tasks at an intermediate
       priority continuously preempt the low-priority task from the CPU.
       Consequently, the low-priority task makes no progress toward
       releasing the lock, and the high-priority task remains blocked.

       Priority inheritance is a mechanism for dealing with the
       priority-inversion problem.  With this mechanism, when a high-
       priority task becomes blocked by a lock held by a low-priority
       task, the priority of the low-priority task is temporarily raised
       to that of the high-priority task, so that it is not preempted by
       any intermediate level tasks, and can thus make progress toward
       releasing the lock.  To be effective, priority inheritance must
       be transitive, meaning that if a high-priority task blocks on a
       lock held by a lower-priority task that is itself blocked by a
       lock held by another intermediate-priority task (and so on, for
       chains of arbitrary length), then both of those tasks (or more
       generally, all of the tasks in a lock chain) have their
       priorities raised to be the same as the high-priority task.

       From a user-space perspective, what makes a futex PI-aware is a
       policy agreement (described below) between user space and the
       kernel about the value of the futex word, coupled with the use of
       the PI-futex operations described below.  (Unlike the other futex
       operations described above, the PI-futex operations are designed
       for the implementation of very specific IPC mechanisms.)

       The PI-futex operations described below differ from the other
       futex operations in that they impose policy on the use of the
       value of the futex word:

       *  If the lock is not acquired, the futex word's value shall be
          0.

       *  If the lock is acquired, the futex word's value shall be the
          thread ID (TID; see gettid(2)) of the owning thread.

       *  If the lock is owned and there are threads contending for the
          lock, then the FUTEX_WAITERS bit shall be set in the futex
          word's value; in other words, this value is:

              FUTEX_WAITERS | TID

          (Note that is invalid for a PI futex word to have no owner and
          FUTEX_WAITERS set.)

       With this policy in place, a user-space application can acquire
       an unacquired lock or release a lock using atomic instructions
       executed in user mode (e.g., a compare-and-swap operation such as
       cmpxchg on the x86 architecture).  Acquiring a lock simply
       consists of using compare-and-swap to atomically set the futex
       word's value to the caller's TID if its previous value was 0.
       Releasing a lock requires using compare-and-swap to set the futex
       word's value to 0 if the previous value was the expected TID.

       If a futex is already acquired (i.e., has a nonzero value),
       waiters must employ the FUTEX_LOCK_PI 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         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 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
              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_BITSET on uaddr.

       EINVAL (FUTEX_CMP_REQUEUE_PI) The kernel detected an
              inconsistency between the user-space state at uaddr and
              the kernel state; that is, the kernel detected a waiter
              which waits on uaddr via FUTEX_LOCK_PI (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         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).

EXAMPLES         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 <stdatomic.h>
       #include <stdint.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 uint32_t *futex1, *futex2, *iaddr;

       static int
       futex(uint32_t *uaddr, int futex_op, uint32_t val,
             const struct timespec *timeout, uint32_t *uaddr2, uint32_t val3)
       {
           return syscall(SYS_futex, uaddr, futex_op, val,
                          timeout, uaddr2, val3);
       }

       /* Acquire the futex pointed to by 'futexp': wait for its value to
          become 1, and then set the value to 0. */

       static void
       fwait(uint32_t *futexp)
       {
           long s;

           /* 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 uint32_t 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 fwait(), it can proceed. */

       static void
       fpost(uint32_t *futexp)
       {
           long s;

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

           const uint32_t 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 nloops;

           setbuf(stdout, NULL);

           nloops = (argc > 1) ? atoi(argv[1]) : 5;

           /* Create a shared anonymous mapping that will hold the futexes.
              Since the futexes are being shared between processes, we
              subsequently use the "shared" futex operations (i.e., not the
              ones suffixed "_PRIVATE"). */

           iaddr = mmap(NULL, sizeof(*iaddr) * 2, PROT_READ | PROT_WRITE,
                       MAP_ANONYMOUS | MAP_SHARED, -1, 0);
           if (iaddr == MAP_FAILED)
               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 (int j = 0; j < nloops; j++) {
                   fwait(futex1);
                   printf("Child  (%jd) %d\n", (intmax_t) getpid(), j);
                   fpost(futex2);
               }

               exit(EXIT_SUCCESS);
           }

           /* Parent falls through to here. */

           for (int j = 0; j < nloops; j++) {
               fwait(futex2);
               printf("Parent (%jd) %d\n", (intmax_t) 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
       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         top

       This page is part of release 5.11 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                          2021-03-22                       FUTEX(2)

Pages that refer to this page: clone(2)eventfd(2)get_robust_list(2)mprotect(2)prctl(2)restart_syscall(2)set_tid_address(2)syscalls(2)futex(7)pthreads(7)signal(7)