futex(2)

1FUTEX(2)                   Linux Programmer's Manual                  FUTEX(2)
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NAME

6       futex - fast user-space locking
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SYNOPSIS

9       #include <linux/futex.h>
10       #include <sys/time.h>
11
12       int futex(int *uaddr, int op, int val, const struct timespec *timeout,
13                 int *uaddr2, int val3);
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DESCRIPTION

16       The  futex()  system call provides a method for a program to wait for a
17       value at a given address to change, and a  method  to  wake  up  anyone
18       waiting  on a particular address (while the addresses for the same mem‐
19       ory in separate processes may not be equal, the kernel maps them inter‐
20       nally  so the same memory mapped in different locations will correspond
21       for futex() calls).  This system call is typically  used  to  implement
22       the  contended  case  of  a  lock  in  shared  memory,  as described in
23       futex(7).
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25       When a futex(7) operation did not finish uncontended in user  space,  a
26       call  needs  to  be  made  to the kernel to arbitrate.  Arbitration can
27       either mean putting the calling process to sleep or, conversely, waking
28       a waiting process.
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30       Callers of this function are expected to adhere to the semantics as set
31       out in futex(7).  As these semantics involve writing nonportable assem‐
32       bly  instructions,  this in turn probably means that most users will in
33       fact be library authors and not general application developers.
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35       The uaddr argument needs to point to an aligned  integer  which  stores
36       the  counter.   The operation to execute is passed via the op argument,
37       along with a value val.
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39       Five operations are currently defined:
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41       FUTEX_WAIT
42              This operation atomically verifies that the futex address  uaddr
43              still  contains the value val, and sleeps awaiting FUTEX_WAKE on
44              this futex address.  If the timeout argument  is  non-NULL,  its
45              contents  describe  the  minimum  duration of the wait, which is
46              infinite otherwise.  The arguments uaddr2 and val3 are ignored.
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48              For futex(7), this call is executed if  decrementing  the  count
49              gave  a  negative  value (indicating contention), and will sleep
50              until another  process  releases  the  futex  and  executes  the
51              FUTEX_WAKE operation.
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53       FUTEX_WAKE
54              This operation wakes at most val processes waiting on this futex
55              address  (i.e.,  inside  FUTEX_WAIT).   The  arguments  timeout,
56              uaddr2 and val3 are ignored.
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58              For  futex(7), this is executed if incrementing the count showed
59              that there were waiters, once the futex value has been set to  1
60              (indicating that it is available).
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62       FUTEX_FD (present up to and including Linux 2.6.25)
63              To  support  asynchronous  wakeups,  this operation associates a
64              file descriptor with a futex.  If  another  process  executes  a
65              FUTEX_WAKE,  the process will receive the signal number that was
66              passed in val.  The calling process must close the returned file
67              descriptor  after  use.   The arguments timeout, uaddr2 and val3
68              are ignored.
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70              To prevent race conditions, the caller should test if the  futex
71              has been upped after FUTEX_FD returns.
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73              Because  it  was inherently racy, FUTEX_FD has been removed from
74              Linux 2.6.26 onward.
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76       FUTEX_REQUEUE (since Linux 2.5.70)
77              This operation was introduced in order to  avoid  a  "thundering
78              herd"  effect when FUTEX_WAKE is used and all processes woken up
79              need to acquire another futex.  This  call  wakes  up  val  pro‐
80              cesses,  and  requeues all other waiters on the futex at address
81              uaddr2.  The arguments timeout and val3 are ignored.
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83       FUTEX_CMP_REQUEUE (since Linux 2.6.7)
84              There was a race  in  the  intended  use  of  FUTEX_REQUEUE,  so
85              FUTEX_CMP_REQUEUE   was   introduced.    This   is   similar  to
86              FUTEX_REQUEUE, but first checks whether the location uaddr still
87              contains  the  value val3.  If not, the operation fails with the
88              error EAGAIN.  The argument timeout is ignored.
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RETURN VALUE

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

112       EACCES No read access to futex memory.
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114       EAGAIN FUTEX_CMP_REQUEUE detected that the value pointed to by uaddr is
115              not equal to the expected value val3.  (This probably  indicates
116              a race; use the safe FUTEX_WAKE now.)
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118       EFAULT Error retrieving timeout information from user space.
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120       EINTR  A  FUTEX_WAIT  operation  was  interrupted by a signal (see sig‐
121              nal(7)) or a spurious wakeup.
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123       EINVAL Invalid argument.
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125       ENFILE The system limit on the total number  of  open  files  has  been
126              reached.
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128       ENOSYS Invalid operation specified in op.
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130       ETIMEDOUT
131              Timeout during the FUTEX_WAIT operation.
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133       EWOULDBLOCK
134              op  was  FUTEX_WAIT  and  the  value pointed to by uaddr was not
135              equal to the expected value val at the time of the call.
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VERSIONS

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

145       This system call is Linux-specific.
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NOTES

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

164       This page is part of release 3.53 of the Linux  man-pages  project.   A
165       description  of  the project, and information about reporting bugs, can
166       be found at http://www.kernel.org/doc/man-pages/.
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170Linux                             2013-07-30                          FUTEX(2)