2004-11-03 13:51:07 +00:00
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.\" Page by b.hubert - may be freely modified and distributed
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.\"
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.\" Niki A. Rahimi (LTC Security Development, narahimi@us.ibm.com)
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.\" added ERRORS section.
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.\"
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.\" Modified 2004-06-17 mtk
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.\" Modified 2004-10-07 aeb, added FUTEX_REQUEUE, FUTEX_CMP_REQUEUE
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.\"
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2010-08-31 04:50:46 +00:00
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.\" FIXME See also https://bugzilla.kernel.org/show_bug.cgi?id=14303
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2008-07-04 21:10:31 +00:00
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.\" 2.6.14 adds FUTEX_WAKE_OP
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2007-04-12 22:42:49 +00:00
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.\" 2.6.18 adds (Ingo Molnar) priority inheritance support:
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.\" FUTEX_LOCK_PI, FUTEX_UNLOCK_PI, and FUTEX_TRYLOCK_PI. These need
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2006-09-26 06:38:06 +00:00
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.\" to be documented in the manual page. Probably there is sufficient
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.\" material in the kernel source file Documentation/pi-futex.txt.
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2008-07-04 21:10:31 +00:00
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.\" 2.6.25 adds FUTEX_WAKE_BITSET, FUTEX_WAIT_BITSET
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2006-09-26 06:38:06 +00:00
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.\"
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2012-08-13 12:50:26 +00:00
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.TH FUTEX 2 2012-08-13 "Linux" "Linux Programmer's Manual"
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2004-11-03 13:51:07 +00:00
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.SH NAME
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futex \- Fast Userspace Locking system call
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.SH SYNOPSIS
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2006-08-04 09:41:28 +00:00
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.nf
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2004-11-03 13:51:07 +00:00
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.sp
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.B "#include <linux/futex.h>"
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.B "#include <sys/time.h>"
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.sp
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2006-08-04 09:41:28 +00:00
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.BI "int futex(int *" uaddr ", int " op ", int " val \
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", const struct timespec *" timeout ,
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.br
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.BI " int *" uaddr2 ", int " val3 );
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2004-11-03 13:51:07 +00:00
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.\" int *? void *? u32 *?
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2006-08-04 09:41:28 +00:00
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.fi
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2004-11-03 13:51:07 +00:00
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.SH "DESCRIPTION"
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.PP
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The
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2005-10-19 06:54:38 +00:00
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.BR futex ()
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2004-11-03 13:51:07 +00:00
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system call provides a method for
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a program to wait for a value at a given address to change, and a
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method to wake up anyone waiting on a particular address (while the
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addresses for the same memory in separate processes may not be
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equal, the kernel maps them internally so the same memory mapped in
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different locations will correspond for
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2005-10-19 06:54:38 +00:00
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.BR futex ()
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2007-04-12 22:42:49 +00:00
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calls).
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2010-08-29 04:21:27 +00:00
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This system call is typically used to
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2004-11-03 13:51:07 +00:00
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implement the contended case of a lock in shared memory, as
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described in
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2006-04-21 01:24:06 +00:00
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.BR futex (7).
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2004-11-03 13:51:07 +00:00
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.PP
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2007-04-12 22:42:49 +00:00
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When a
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2006-04-21 01:24:06 +00:00
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.BR futex (7)
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2004-11-03 13:51:07 +00:00
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operation did not finish uncontended in userspace, a call needs to be made
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2007-04-12 22:42:49 +00:00
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to the kernel to arbitrate.
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Arbitration can either mean putting the calling
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2004-11-03 13:51:07 +00:00
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process to sleep or, conversely, waking a waiting process.
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.PP
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Callers of this function are expected to adhere to the semantics as set out in
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2006-04-21 01:24:06 +00:00
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.BR futex (7).
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2004-11-03 13:51:07 +00:00
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As these
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execve.2, fallocate.2, futex.2, sched_rr_get_interval.2, select_tut.2, shmget.2, timer_getoverrun.2, times.2, pthread_attr_init.3, pthread_attr_setaffinity_np.3, pthread_cleanup_push_defer_np.3, pthread_getattr_np.3, pthread_self.3, pthread_setaffinity_np.3, pthread_tryjoin_np.3, sem_open.3, stdin.3, rtc.4, tty_ioctl.4, unix.7: Global fix: s/non-portable/nonportable/
The tendency in English, as prescribed in style guides like
Chicago MoS, is towards removing hyphens after prefixes
like "non-" etc.
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
2010-01-16 16:46:09 +00:00
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semantics involve writing nonportable assembly instructions, this in turn
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2004-11-03 13:51:07 +00:00
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probably means that most users will in fact be library authors and not
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general application developers.
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.PP
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The
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.I uaddr
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argument needs to point to an aligned integer which stores the counter.
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The operation to execute is passed via the
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.I op
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2008-07-10 20:53:08 +00:00
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argument, along with a value
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2004-11-03 13:51:07 +00:00
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.IR val .
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.PP
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Five operations are currently defined:
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.TP
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.B FUTEX_WAIT
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This operation atomically verifies that the futex address
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.I uaddr
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still contains the value
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.IR val ,
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2007-06-22 17:16:20 +00:00
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and sleeps awaiting
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.B FUTEX_WAKE
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on this futex address.
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2007-04-12 22:42:49 +00:00
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If the
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2004-11-03 13:51:07 +00:00
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.I timeout
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argument is non-NULL, its contents describe the maximum
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2007-04-12 22:42:49 +00:00
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duration of the wait, which is infinite otherwise.
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The arguments
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2004-11-03 13:51:07 +00:00
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.I uaddr2
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and
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.I val3
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are ignored.
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For
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2006-04-21 01:24:06 +00:00
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.BR futex (7),
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2004-11-03 13:51:07 +00:00
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this call is executed if decrementing the count gave a negative value
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(indicating contention), and will sleep until another process releases
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2007-06-22 17:16:20 +00:00
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the futex and executes the
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.B FUTEX_WAKE
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operation.
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2004-11-03 13:51:07 +00:00
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.TP
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.B FUTEX_WAKE
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2007-07-18 20:24:30 +00:00
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This operation wakes at most \fIval\fP
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2007-10-03 19:56:24 +00:00
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processes waiting on this futex address (i.e., inside
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2007-06-22 17:16:20 +00:00
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.BR FUTEX_WAIT ).
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2004-11-03 13:51:07 +00:00
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The arguments
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.IR timeout ,
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.I uaddr2
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and
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.I val3
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are ignored.
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For
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2006-04-21 01:24:06 +00:00
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.BR futex (7),
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2004-11-03 13:51:07 +00:00
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this is executed if incrementing
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the count showed that there were waiters, once the futex value has been set
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to 1 (indicating that it is available).
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.TP
|
2008-05-21 13:48:54 +00:00
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.BR FUTEX_FD " (present up to and including Linux 2.6.25)"
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2004-11-03 13:51:07 +00:00
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To support asynchronous wakeups, this operation associates a file descriptor
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with a futex.
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.\" , suitable for .BR poll (2).
|
2007-06-22 17:16:20 +00:00
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If another process executes a
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.BR FUTEX_WAKE ,
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the process will receive the signal number that was passed in
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2004-11-03 13:51:07 +00:00
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.IR val .
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The calling process must close the returned file descriptor after use.
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The arguments
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.IR timeout ,
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.I uaddr2
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and
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.I val3
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are ignored.
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2007-04-12 22:42:49 +00:00
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To prevent race conditions, the caller should test if the futex has
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2007-06-22 17:16:20 +00:00
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been upped after
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.B FUTEX_FD
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returns.
|
2006-11-25 01:57:28 +00:00
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|
2008-05-21 13:48:54 +00:00
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Because it was inherently racy,
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2007-06-22 17:16:20 +00:00
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.B FUTEX_FD
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Changes.old, clone.2, execve.2, fcntl.2, futex.2, getitimer.2, getpriority.2, mmap.2, mount.2, mprotect.2, sched_setscheduler.2, select_tut.2, setuid.2, sigaltstack.2, vfork.2, div.3, fenv.3, fmod.3, memchr.3, pthread_attr_setstackaddr.3, pthread_attr_setstacksize.3, pthread_getattr_np.3, queue.3, scanf.3, trunc.3, st.4, proc.5, services.5, utmp.5, bootparam.7, capabilities.7, feature_test_macros.7, futex.7, glob.7, man.7, netlink.7, unicode.7: Switch to American usage: "-wards" ==> "-ward"
American English uses "afterward" in preference to "afterwards",
and so on
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
2010-09-26 05:34:47 +00:00
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has been removed from Linux 2.6.26 onward.
|
2004-11-03 13:51:07 +00:00
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.TP
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.BR FUTEX_REQUEUE " (since Linux 2.5.70)"
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This operation was introduced in order to avoid a "thundering herd" effect
|
2007-06-22 17:16:20 +00:00
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when
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.B FUTEX_WAKE
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is used and all processes woken up need to acquire another futex.
|
2007-04-12 22:42:49 +00:00
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This call wakes up
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2004-11-03 13:51:07 +00:00
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.I val
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processes, and requeues all other waiters on the futex at address
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.IR uaddr2 .
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The arguments
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.I timeout
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and
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.I val3
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are ignored.
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.TP
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.BR FUTEX_CMP_REQUEUE " (since Linux 2.6.7)"
|
2007-06-22 17:16:20 +00:00
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There was a race in the intended use of
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.BR FUTEX_REQUEUE ,
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so
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.B FUTEX_CMP_REQUEUE
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was introduced.
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This is similar to
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.BR FUTEX_REQUEUE ,
|
2004-11-03 13:51:07 +00:00
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but first checks whether the location
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.I uaddr
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still contains the value
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.IR val3 .
|
2012-08-13 12:50:26 +00:00
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If not, the operation fails with the error
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.BR EAGAIN .
|
2004-11-03 13:51:07 +00:00
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The argument
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.I timeout
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is ignored.
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.SH "RETURN VALUE"
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.PP
|
2012-08-13 12:50:26 +00:00
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In the event of an error, all operations return \-1, and set
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.I errno
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to indicate the error.
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The return value on success depends on the operation,
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as described in the following list:
|
2004-11-03 13:51:07 +00:00
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.TP
|
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.B FUTEX_WAIT
|
2007-06-22 17:16:20 +00:00
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Returns 0 if the process was woken by a
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.B FUTEX_WAKE
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call.
|
2012-08-13 12:50:26 +00:00
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See ERRORS for the various possible error returns.
|
2004-11-03 13:51:07 +00:00
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.TP
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.B FUTEX_WAKE
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Returns the number of processes woken up.
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.TP
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.B FUTEX_FD
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Returns the new file descriptor associated with the futex.
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.TP
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.B FUTEX_REQUEUE
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Returns the number of processes woken up.
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.TP
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.B FUTEX_CMP_REQUEUE
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Returns the number of processes woken up.
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.SH ERRORS
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.TP
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.B EACCES
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No read access to futex memory.
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.TP
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.B EAGAIN
|
2007-06-22 17:16:20 +00:00
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.B FUTEX_CMP_REQUEUE
|
2012-08-13 12:50:26 +00:00
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detected that the value pointed to by
|
2012-08-13 12:25:13 +00:00
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.I uaddr
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is not equal to the expected value
|
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.IR val3 .
|
2004-11-03 13:51:07 +00:00
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(This probably indicates a race;
|
2007-06-22 17:16:20 +00:00
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use the safe
|
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.B FUTEX_WAKE
|
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now.)
|
2004-11-03 13:51:07 +00:00
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.TP
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.B EFAULT
|
2012-08-13 12:50:26 +00:00
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Error retrieving
|
2004-11-03 13:51:07 +00:00
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.I timeout
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information from userspace.
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.TP
|
2012-08-13 12:25:13 +00:00
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.B EINTR
|
2012-08-13 12:50:26 +00:00
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A
|
2012-08-13 12:25:13 +00:00
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.B FUTEX_WAIT
|
2012-08-13 12:50:26 +00:00
|
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operation was interrupted by a signal (see
|
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.BR signal (7))
|
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or a spurious wakeup.
|
2012-08-13 12:25:13 +00:00
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.TP
|
2004-11-03 13:51:07 +00:00
|
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.B EINVAL
|
2012-08-13 12:51:22 +00:00
|
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Invalid argument.
|
2004-11-03 13:51:07 +00:00
|
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.TP
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.B ENFILE
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The system limit on the total number of open files has been reached.
|
2008-01-31 10:35:05 +00:00
|
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.TP
|
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.B ENOSYS
|
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Invalid operation specified in
|
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.IR op .
|
2012-08-13 12:25:13 +00:00
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.TP
|
|
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.B ETIMEDOUT
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Timeout during the
|
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.B FUTEX_WAIT
|
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operation.
|
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.TP
|
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.B EWOULDBLOCK
|
2012-08-13 12:50:26 +00:00
|
|
|
.I op
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was
|
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.BR FUTEX_WAIT
|
|
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|
and the value pointed to by
|
2012-08-13 12:25:13 +00:00
|
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|
.I uaddr
|
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was not equal to the expected value
|
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.I val
|
2012-08-13 12:50:26 +00:00
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at the time of the call.
|
2007-05-18 16:06:42 +00:00
|
|
|
.SH "VERSIONS"
|
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.PP
|
|
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|
Initial futex support was merged in Linux 2.5.7 but with different semantics
|
|
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|
from what was described above.
|
2008-07-10 20:53:08 +00:00
|
|
|
A 4-argument system call with the semantics
|
2010-08-29 04:21:27 +00:00
|
|
|
described in this page was introduced in Linux 2.5.40.
|
2008-07-10 20:53:08 +00:00
|
|
|
In Linux 2.5.70 one argument
|
2007-05-18 16:06:42 +00:00
|
|
|
was added.
|
intro.1, fork.2, futex.2, open.2, rename.2, select_tut.2, semop.2, spu_create.2, stat.2, netlink.3, random.3, scanf.3, shm_open.3, strftime.3, console.4, console_codes.4, sk98lin.4, st.4, bootparam.7, cpuset.7, credentials.7, man-pages.7, path_resolution.7, uri.7: Global fix: remove spaces around em-dash
Normal English typographical convention is not to have
spaces around em dashes.
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
2011-10-03 06:47:35 +00:00
|
|
|
In Linux 2.6.7 a sixth argument was added\(emmessy, especially
|
2007-05-18 16:06:42 +00:00
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on the s390 architecture.
|
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|
.SH "CONFORMING TO"
|
2007-12-25 21:28:09 +00:00
|
|
|
This system call is Linux-specific.
|
2004-11-03 13:51:07 +00:00
|
|
|
.SH "NOTES"
|
|
|
|
.PP
|
2008-11-28 16:31:35 +00:00
|
|
|
To reiterate, bare futexes are not intended as an easy-to-use abstraction
|
2007-04-12 22:42:49 +00:00
|
|
|
for end-users.
|
2008-11-28 16:31:35 +00:00
|
|
|
(There is no wrapper function for this system call in glibc.)
|
2007-04-12 22:42:49 +00:00
|
|
|
Implementors are expected to be assembly literate and to have
|
2004-11-03 13:51:07 +00:00
|
|
|
read the sources of the futex userspace library referenced below.
|
|
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|
.\" .SH "AUTHORS"
|
|
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|
.\" .PP
|
|
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|
.\" Futexes were designed and worked on by
|
|
|
|
.\" Hubertus Franke (IBM Thomas J. Watson Research Center),
|
|
|
|
.\" Matthew Kirkwood, Ingo Molnar (Red Hat)
|
|
|
|
.\" and Rusty Russell (IBM Linux Technology Center).
|
|
|
|
.\" This page written by bert hubert.
|
|
|
|
.SH "SEE ALSO"
|
2007-12-17 11:28:40 +00:00
|
|
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.BR futex (7)
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2004-11-03 13:51:07 +00:00
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.PP
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2007-06-21 21:58:51 +00:00
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\fIFuss, Futexes and Furwocks: Fast Userlevel Locking in Linux\fP
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2010-05-22 03:23:14 +00:00
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(proceedings of the Ottawa Linux Symposium 2002), online at
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.br
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capget.2, futex.2, sched_setscheduler.2, spu_create.2, spu_run.2, vfork.2, getnameinfo.3, netlink.3, cciss.4, console_codes.4, dsp56k.4, hpsa.4, sk98lin.4, filesystems.5, protocols.5, aio.7, capabilities.7, charsets.7, feature_test_macros.7, futex.7, hier.7, iso_8859-16.7, iso_8859-2.7, koi8-r.7, libc.7, man.7, netlink.7, numa.7, standards.7, unicode.7, units.7, uri.7, x25.7: Global fix: use UR macro where applicable
The syntax .UR http://example.com paired with .UE will create
links which one can interact, if the pager allows that. One
way to see the effect is ask the man(1) command to use browser
display, e.g.:
man -H man7/uri.7
("\:" is optional groff syntax to permit hyphenless line breaks.)
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
2012-08-13 08:02:36 +00:00
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.UR http://kernel.org\:/doc\:/ols\:/2002\:/ols2002-pages-479-495.pdf
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.UE
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2010-05-22 03:23:14 +00:00
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.PP
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Futex example library, futex-*.tar.bz2 at
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.br
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capget.2, futex.2, sched_setscheduler.2, spu_create.2, spu_run.2, vfork.2, getnameinfo.3, netlink.3, cciss.4, console_codes.4, dsp56k.4, hpsa.4, sk98lin.4, filesystems.5, protocols.5, aio.7, capabilities.7, charsets.7, feature_test_macros.7, futex.7, hier.7, iso_8859-16.7, iso_8859-2.7, koi8-r.7, libc.7, man.7, netlink.7, numa.7, standards.7, unicode.7, units.7, uri.7, x25.7: Global fix: use UR macro where applicable
The syntax .UR http://example.com paired with .UE will create
links which one can interact, if the pager allows that. One
way to see the effect is ask the man(1) command to use browser
display, e.g.:
man -H man7/uri.7
("\:" is optional groff syntax to permit hyphenless line breaks.)
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
2012-08-13 08:02:36 +00:00
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.UR ftp://ftp.nl.kernel.org\:/pub\:/linux\:/kernel\:/people\:/rusty/
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.UE
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