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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).

EXAMPLE

       The program below demonstrates use of futexes in a program where a parent  process  and  a
       child  process  use  a  pair  of  futexes  located  inside  a  shared anonymous mapping to
       synchronize access to a shared resource: the  terminal.   The  two  processes  each  write
       nloops  (a  command-line  argument that defaults to 5 if omitted) messages to the terminal
       and employ a  synchronization  protocol  that  ensures  that  they  alternate  in  writing
       messages.  Upon running this program we see output such as the following:

           $ ./futex_demo
           Parent (18534) 0
           Child  (18535) 0
           Parent (18534) 1
           Child  (18535) 1
           Parent (18534) 2
           Child  (18535) 2
           Parent (18534) 3
           Child  (18535) 3
           Parent (18534) 4
           Child  (18535) 4

   Program source

       /* futex_demo.c

          Usage: futex_demo [nloops]
                           (Default: 5)

          Demonstrate the use of futexes in a program where parent and child
          use a pair of futexes located inside a shared anonymous mapping to
          synchronize access to a shared resource: the terminal. The two
          processes each write 'num-loops' messages to the terminal and employ
          a synchronization protocol that ensures that they alternate in
          writing messages.
       */
       #define _GNU_SOURCE
       #include <stdio.h>
       #include <errno.h>
       #include <stdlib.h>
       #include <unistd.h>
       #include <sys/wait.h>
       #include <sys/mman.h>
       #include <sys/syscall.h>
       #include <linux/futex.h>
       #include <sys/time.h>

       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                               } while (0)

       static int *futex1, *futex2, *iaddr;

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

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

       static void
       fwait(int *futexp)
       {
           int s;

           /* __sync_bool_compare_and_swap(ptr, oldval, newval) is a gcc
              built-in function.  It atomically performs the equivalent of:

                  if (*ptr == oldval)
                      *ptr = newval;

              It returns true if the test yielded true and *ptr was updated.
              The alternative here would be to employ the equivalent atomic
              machine-language instructions.  For further information, see
              the GCC Manual. */

           while (1) {

               /* Is the futex available? */

               if (__sync_bool_compare_and_swap(futexp, 1, 0))
                   break;      /* Yes */

               /* Futex is not available; wait */

               s = futex(futexp, FUTEX_WAIT, 0, NULL, NULL, 0);
               if (s == -1 && errno != EAGAIN)
                   errExit("futex-FUTEX_WAIT");
           }
       }

       /* Release the futex pointed to by 'futexp': if the futex currently
          has the value 0, set its value to 1 and the wake any futex waiters,
          so that if the peer is blocked in fpost(), it can proceed. */

       static void
       fpost(int *futexp)
       {
           int s;

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

           if (__sync_bool_compare_and_swap(futexp, 0, 1)) {

               s = futex(futexp, FUTEX_WAKE, 1, NULL, NULL, 0);
               if (s  == -1)
                   errExit("futex-FUTEX_WAKE");
           }
       }

       int
       main(int argc, char *argv[])
       {
           pid_t childPid;
           int j, nloops;

           setbuf(stdout, NULL);

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

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

           iaddr = mmap(NULL, sizeof(int) * 2, PROT_READ | PROT_WRITE,
                       MAP_ANONYMOUS | MAP_SHARED, -1, 0);
           if (iaddr == MAP_FAILED)
               errExit("mmap");

           futex1 = &iaddr[0];
           futex2 = &iaddr[1];

           *futex1 = 0;        /* State: unavailable */
           *futex2 = 1;        /* State: available */

           /* Create a child process that inherits the shared anonymous
              mapping */

           childPid = fork();
           if (childPid == -1)
               errExit("fork");

           if (childPid == 0) {        /* Child */
               for (j = 0; j < nloops; j++) {
                   fwait(futex1);
                   printf("Child  (%ld) %d\n", (long) getpid(), j);
                   fpost(futex2);
               }

               exit(EXIT_SUCCESS);
           }

           /* Parent falls through to here */

           for (j = 0; j < nloops; j++) {
               fwait(futex2);
               printf("Parent (%ld) %d\n", (long) getpid(), j);
               fpost(futex1);
           }

           wait(NULL);

           exit(EXIT_SUCCESS);
       }

SEE ALSO

       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 4.16 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/.