futex

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

NAME
       futex - fast user-space locking

SYNOPSIS
       #include <linux/futex.h>
       #include <sys/time.h>

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

       Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION
       The  futex()  system call provides a method for waiting until a certain
       condition becomes true.  It is typically used as a  blocking  construct
       in  the  context of shared-memory synchronization.  When using futexes,
       the majority of the synchronization operations are  performed  in  user
       space.   A user-space program employs the futex() system call only when
       it is likely that the program has to block for a longer time until  the
       condition  becomes  true.  Other futex() operations can be used to wake
       any processes or threads waiting for a particular condition.

       A futex is a 32-bit value--referred to below  as  a  futex  word--whose
       address  is  supplied to the futex() system call.  (Futexes are 32 bits
       in size on all platforms, including 64-bit systems.)  All futex  opera-
       tions  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  synchroniza-
       tion  in  user space with the implementation of blocking by the kernel.
       Analogously to an atomic  compare-and-exchange  operation  that  poten-
       tially  changes  shared  memory, blocking via a futex is an atomic com-
       pare-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 fur-
       ther 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 imple-
       mentation 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.  How-
       ever,  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 synchro-
              nization 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 opera-
              tions.

              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  atomi-
              cally 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 pre-
              vent 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 notifica-
              tions: if val is nonzero, then, when another process  or  thread
              executes a FUTEX_WAKE, the caller will receive the signal number
              that was passed in val.

              The arguments timeout, uaddr2 and val3 are ignored.

              Because it was inherently racy, FUTEX_FD has been  removed  from
              Linux 2.6.26 onward.

       FUTEX_REQUEUE (since Linux 2.6.0)
              This  operation performs the same task as FUTEX_CMP_REQUEUE (see
              below), except that no check is made using the  value  in  val3.
              (The argument val3 is ignored.)

       FUTEX_CMP_REQUEUE (since Linux 2.6.7)
              This  operation  first  checks  whether the location uaddr still
              contains the value val3.  If not, the operation fails  with  the
              error  EAGAIN.   Otherwise,  the operation wakes up a maximum of
              val waiters that are waiting on the futex at  uaddr.   If  there
              are  more  than  val  waiters,  then  the  remaining waiters are
              removed from the wait queue of the source  futex  at  uaddr  and
              added to the wait queue of the target futex at uaddr2.  The val2
              argument specifies an upper limit on the number of waiters  that
              are requeued to the futex at uaddr2.

              The  load  from  uaddr  is  an atomic memory access (i.e., using
              atomic machine instructions  of  the  respective  architecture).
              This  load,  the comparison with val3, and the requeueing of any
              waiters  are  performed  atomically  and  totally  ordered  with
              respect to other operations on the same futex word.

              Typical  values  to  specify  for  val  are 0 or 1.  (Specifying
              INT_MAX   is   not   useful,   because   it   would   make   the
              FUTEX_CMP_REQUEUE  operation  equivalent  to  FUTEX_WAKE.)   The
              limit value specified via val2 is typically either 1 or INT_MAX.
              (Specifying  the  argument  as 0 is not useful, because it would
              make the FUTEX_CMP_REQUEUE operation equivalent to FUTEX_WAIT.)

              The FUTEX_CMP_REQUEUE operation was added as a  replacement  for
              the  earlier FUTEX_REQUEUE.  The difference is that the check of
              the value at uaddr can be used to ensure that requeueing happens
              only  under  certain conditions, which allows race conditions to
              be avoided in certain use cases.

              Both FUTEX_REQUEUE and FUTEX_CMP_REQUEUE can be  used  to  avoid
              "thundering   herd"   wake-ups   that  could  occur  when  using
              FUTEX_WAKE in cases where all of the waiters that are woken need
              to  acquire  another  futex.   Consider  the following scenario,
              where multiple waiter threads are waiting on  B,  a  wait  queue
              implemented using a futex:

                  lock(A)
                  while (!check_value(V)) {
                      unlock(A);
                      block_on(B);
                      lock(A);
                  };
                  unlock(A);

              If a waker thread used FUTEX_WAKE, then all waiters waiting on B
              would be woken up, and they would all try  to  acquire  lock  A.
              However,  waking  all  of  the  threads  in this manner would be
              pointless because all except one of the  threads  would  immedi-
              ately  block  on lock A again.  By contrast, a requeue operation
              wakes just one waiter and moves the other waiters to lock A, and
              when  the  woken  waiter unlocks A then the next waiter can pro-
              ceed.

       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_sig-
              nal(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  lead-
              ing to high rates of contention and context switching.

              The  FUTEX_WAKE_OP operation is equivalent to executing the fol-
              lowing 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 per-
                 forms 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 rele-
              vant 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 opera-
              tion 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-pri-
       ority  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-prior-
       ity  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-prior-
       ity 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  opera-
       tions  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 pri-
       ority-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 inher-
              its either the waiter's CPU bandwidth (if the waiter  is  sched-
              uled  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 con-
              dition 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 argu-
              ment.

              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 wait-
              ers 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-inheri-
              tance-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, call-
              ers 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  time-
              out,  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 ker-
              nel 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 argu-
       ment 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  synchro-
       nization  mechanisms  (mutexes,  condition variables, read-write locks,
       and barriers).

EXAMPLE
       The program below demonstrates use of futexes in a program where a par-
       ent  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_getproto-
       col(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 Fur-
       wocks: 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 4.15 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

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