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    Command:

    futex

    
    
    

    SYNOPSIS

           #include <linux/futex.h>
           #include <sys/time.h>
    
           int futex(int *uaddr, int op, int val, const struct timespec *timeout,
                     int *uaddr2, int val3);
    
    
    

    DESCRIPTION

           The  futex()  system call provides a method for a program to wait for a
           value at a given address to change, and a  method  to  wake  up  anyone
           waiting  on a particular address (while the addresses for the same mem-
           ory in separate processes may not be equal, the kernel maps them inter-
           nally  so the same memory mapped in different locations will correspond
           for futex() calls).  This system call is typically  used  to  implement
           the  contended  case  of  a  lock  in  shared  memory,  as described in
           futex(7).
    
           When a futex(7) operation did not finish uncontended in user  space,  a
           call  needs  to  be  made  to the kernel to arbitrate.  Arbitration can
           either mean putting the calling process to sleep or, conversely, waking
           a waiting process.
    
           Callers of this function are expected to adhere to the semantics as set
           out in futex(7).  As these semantics involve writing nonportable assem-
           bly  instructions,  this in turn probably means that most users will in
           fact be library authors and not general application developers.
    
           The uaddr argument needs to point to an aligned  integer  which  stores
           the  counter.   The operation to execute is passed via the op argument,
           along with a value val.
    
           Five operations are currently defined:
    
           FUTEX_WAIT
                  This operation atomically verifies that the futex address  uaddr
                  still  contains the value val, and sleeps awaiting FUTEX_WAKE on
                  this futex address.  If the timeout argument  is  non-NULL,  its
                  contents  specify the duration of the wait.  (This interval will
                  be rounded up  to  the  system  clock  granularity,  and  kernel
                  scheduling delays mean that the blocking interval may overrun by
                  a small amount.)  If timeout is NULL, the  call  blocks  indefi-
                  nitely.  The arguments uaddr2 and val3 are ignored.
    
                  For  futex(7),  this  call is executed if decrementing the count
                  gave a negative value (indicating contention),  and  will  sleep
                  until  another  process  releases  the  futex  and  executes the
                  FUTEX_WAKE operation.
    
           FUTEX_WAKE
                  This operation wakes at most val processes waiting on this futex
                  address  (i.e.,  inside  FUTEX_WAIT).   The  arguments  timeout,
                  uaddr2 and val3 are ignored.
                  has been upped after FUTEX_FD returns.
    
                  Because it was inherently racy, FUTEX_FD has been  removed  from
                  Linux 2.6.26 onward.
    
           FUTEX_REQUEUE (since Linux 2.5.70)
                  This  operation  was  introduced in order to avoid a "thundering
                  herd" effect when FUTEX_WAKE is used and all processes woken  up
                  need  to  acquire  another  futex.   This call wakes up val pro-
                  cesses, and requeues all other waiters on the futex  at  address
                  uaddr2.  The arguments timeout and val3 are ignored.
    
           FUTEX_CMP_REQUEUE (since Linux 2.6.7)
                  There  was  a  race  in  the  intended  use of FUTEX_REQUEUE, so
                  FUTEX_CMP_REQUEUE  was   introduced.    This   is   similar   to
                  FUTEX_REQUEUE, but first checks whether the location uaddr still
                  contains the value val3.  If not, the operation fails  with  the
                  error EAGAIN.  The argument timeout is ignored.
    
    
    

    RETURN VALUE

           In  the  event  of an error, all operations return -1, and set errno to
           indicate the error.  The return value on success depends on the  opera-
           tion, as described in the following list:
    
           FUTEX_WAIT
                  Returns  0  if  the process was woken by a FUTEX_WAKE call.  See
                  ERRORS for the various possible error returns.
    
           FUTEX_WAKE
                  Returns the number of processes woken up.
    
           FUTEX_FD
                  Returns the new file descriptor associated with the futex.
    
           FUTEX_REQUEUE
                  Returns the number of processes woken up.
    
           FUTEX_CMP_REQUEUE
                  Returns the number of processes woken up.
    
    
    

    ERRORS

           EACCES No read access to futex memory.
    
           EAGAIN FUTEX_CMP_REQUEUE detected that the value pointed to by uaddr is
                  not  equal to the expected value val3.  (This probably indicates
                  a race; use the safe FUTEX_WAKE now.)
    
           EFAULT Error retrieving timeout information from user space.
    
           EINTR  A FUTEX_WAIT operation was interrupted by  a  signal  (see  sig-
                  nal(7)) or a spurious wakeup.
    
    
    

    VERSIONS

           Initial  futex  support  was  merged  in Linux 2.5.7 but with different
           semantics from what was described above.  A 4-argument system call with
           the  semantics  described  in this page was introduced in Linux 2.5.40.
           In Linux 2.5.70 one argument was added.  In Linux 2.6.7 a  sixth  argu-
           ment was added--messy, especially on the s390 architecture.
    
    
    

    CONFORMING TO

           This system call is Linux-specific.
    
    
    

    NOTES

           To  reiterate, bare futexes are not intended as an easy-to-use abstrac-
           tion for end-users.  (There is no wrapper function for this system call
           in  glibc.)   Implementors  are expected to be assembly literate and to
           have read the sources of the futex user-space library referenced below.
    
    
    

    SEE ALSO

           restart_syscall(2), futex(7)
    
           Fuss,  Futexes  and Furwocks: Fast Userlevel Locking in Linux (proceed-
           ings of the Ottawa Linux Symposium 2002), online at
    
           Futex example library, futex-*.tar.bz2 at
    
    
    

    Linux 2013-12-12 FUTEX(2)

    
    
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