.\" Page by b.hubert .\" .\" %%%LICENSE_START(FREELY_REDISTRIBUTABLE) .\" may be freely modified and distributed .\" %%%LICENSE_END .\" .\" Niki A. Rahimi (LTC Security Development, narahimi@us.ibm.com) .\" added ERRORS section. .\" .\" Modified 2004-06-17 mtk .\" Modified 2004-10-07 aeb, added FUTEX_REQUEUE, FUTEX_CMP_REQUEUE .\" .\" FIXME . .\" See also https://bugzilla.kernel.org/show_bug.cgi?id=14303 .\" 2.6.14 adds FUTEX_WAKE_OP .\" commit 4732efbeb997189d9f9b04708dc26bf8613ed721 .\" Author: Jakub Jelinek .\" Date: Tue Sep 6 15:16:25 2005 -0700 .\" .\" FIXME . .\" 2.6.18 adds (Ingo Molnar) priority inheritance support: .\" FUTEX_LOCK_PI, FUTEX_UNLOCK_PI, and FUTEX_TRYLOCK_PI. These need .\" to be documented in the manual page. Probably there is sufficient .\" material in the kernel source file Documentation/pi-futex.txt. .\" commit c87e2837be82df479a6bae9f155c43516d2feebc .\" Author: Ingo Molnar .\" Date: Tue Jun 27 02:54:58 2006 -0700 .\" .\" commit e2970f2fb6950183a34e8545faa093eb49d186e1 .\" Author: Ingo Molnar .\" Date: Tue Jun 27 02:54:47 2006 -0700 .\" .\" See Documentation/pi-futex.txt .\" .\" FIXME . .\" 2.6.25 adds FUTEX_WAKE_BITSET, FUTEX_WAIT_BITSET .\" commit cd689985cf49f6ff5c8eddc48d98b9d581d9475d .\" Author: Thomas Gleixner .\" Date: Fri Feb 1 17:45:14 2008 +0100 .\" .\" FIXME . .\" 2.6.31 adds FUTEX_WAIT_REQUEUE_PI, FUTEX_CMP_REQUEUE_PI .\" commit 52400ba946759af28442dee6265c5c0180ac7122 .\" Author: Darren Hart .\" Date: Fri Apr 3 13:40:49 2009 -0700 .\" .\" commit ba9c22f2c01cf5c88beed5a6b9e07d42e10bd358 .\" Author: Darren Hart .\" Date: Mon Apr 20 22:22:22 2009 -0700 .\" .\" See Documentation/futex-requeue-pi.txt .\" .TH FUTEX 2 2014-05-21 "Linux" "Linux Programmer's Manual" .SH NAME futex \- fast user-space locking .SH SYNOPSIS .nf .sp .B "#include " .B "#include " .sp .BI "int futex(int *" uaddr ", int " op ", int " val \ ", const struct timespec *" timeout , .br .BI " int *" uaddr2 ", int " val3 ); .\" int *? void *? u32 *? .fi .IR Note : There is no glibc wrapper for this system call; see NOTES. .SH DESCRIPTION .PP The .BR 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 memory in separate processes may not be equal, the kernel maps them internally so the same memory mapped in different locations will correspond for .BR futex () calls). This system call is typically used to implement the contended case of a lock in shared memory, as described in .BR futex (7). .PP When a .BR 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. .PP Callers of this function are expected to adhere to the semantics as set out in .BR futex (7). As these semantics involve writing nonportable assembly instructions, this in turn probably means that most users will in fact be library authors and not general application developers. .PP The .I uaddr argument needs to point to an aligned integer which stores the counter. The operation to execute is passed via the .I op argument, along with a value .IR val . .PP Five operations are currently defined: .TP .B FUTEX_WAIT This operation atomically verifies that the futex address .I uaddr still contains the value .IR val , and sleeps awaiting .B FUTEX_WAKE on this futex address. If the .I 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 .I timeout is NULL, the call blocks indefinitely. The arguments .I uaddr2 and .I val3 are ignored. For .BR 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 .B FUTEX_WAKE operation. .TP .B FUTEX_WAKE This operation wakes at most \fIval\fP processes waiting on this futex address (i.e., inside .BR FUTEX_WAIT ). The arguments .IR timeout , .I uaddr2 and .I val3 are ignored. For .BR futex (7), this is executed if incrementing the count showed that there were waiters, once the futex value has been set to 1 (indicating that it is available). .TP .BR FUTEX_FD " (present up to and including Linux 2.6.25)" To support asynchronous wakeups, this operation associates a file descriptor with a futex. .\" , suitable for .BR poll (2). If another process executes a .BR FUTEX_WAKE , the process will receive the signal number that was passed in .IR val . The calling process must close the returned file descriptor after use. The arguments .IR timeout , .I uaddr2 and .I val3 are ignored. To prevent race conditions, the caller should test if the futex has been upped after .B FUTEX_FD returns. Because it was inherently racy, .B FUTEX_FD has been removed from Linux 2.6.26 onward. .TP .BR FUTEX_REQUEUE " (since Linux 2.5.70)" This operation was introduced in order to avoid a "thundering herd" effect when .B FUTEX_WAKE is used and all processes woken up need to acquire another futex. This call wakes up .I val processes, and requeues all other waiters on the futex at address .IR uaddr2 . The arguments .I timeout and .I val3 are ignored. .TP .BR FUTEX_CMP_REQUEUE " (since Linux 2.6.7)" There was a race in the intended use of .BR FUTEX_REQUEUE , so .B FUTEX_CMP_REQUEUE was introduced. This is similar to .BR FUTEX_REQUEUE , but first checks whether the location .I uaddr still contains the value .IR val3 . If not, the operation fails with the error .BR EAGAIN . The argument .I timeout is ignored. .SH RETURN VALUE .PP In the event of an error, all operations return \-1, and set .I errno to indicate the error. The return value on success depends on the operation, as described in the following list: .TP .B FUTEX_WAIT Returns 0 if the process was woken by a .B FUTEX_WAKE call. See ERRORS for the various possible error returns. .TP .B FUTEX_WAKE Returns the number of processes woken up. .TP .B FUTEX_FD Returns the new file descriptor associated with the futex. .TP .B FUTEX_REQUEUE Returns the number of processes woken up. .TP .B FUTEX_CMP_REQUEUE Returns the number of processes woken up. .SH ERRORS .TP .B EACCES No read access to futex memory. .TP .B EAGAIN .B FUTEX_CMP_REQUEUE detected that the value pointed to by .I uaddr is not equal to the expected value .IR val3 . (This probably indicates a race; use the safe .B FUTEX_WAKE now.) .TP .B EFAULT Error retrieving .I timeout information from user space. .TP .B EINTR A .B FUTEX_WAIT operation was interrupted by a signal (see .BR signal (7)) or a spurious wakeup. .TP .B EINVAL Invalid argument. .TP .B ENFILE The system limit on the total number of open files has been reached. .TP .B ENOSYS Invalid operation specified in .IR op . .TP .B ETIMEDOUT Timeout during the .B FUTEX_WAIT operation. .TP .B EWOULDBLOCK .I op was .BR FUTEX_WAIT and the value pointed to by .I uaddr was not equal to the expected value .I val at the time of the call. .SH VERSIONS .PP 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 argument was added\(emmessy, especially on the s390 architecture. .SH CONFORMING TO This system call is Linux-specific. .SH NOTES .PP To reiterate, bare futexes are not intended as an easy-to-use abstraction 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. .\" .SH "AUTHORS" .\" .PP .\" 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 .BR restart_syscall (2), .BR futex (7) .PP \fIFuss, Futexes and Furwocks: Fast Userlevel Locking in Linux\fP (proceedings of the Ottawa Linux Symposium 2002), online at .br .UR http://kernel.org\:/doc\:/ols\:/2002\:/ols2002-pages-479-495.pdf .UE .PP Futex example library, futex-*.tar.bz2 at .br .UR ftp://ftp.kernel.org\:/pub\:/linux\:/kernel\:/people\:/rusty/ .UE .SH COLOPHON This page is part of release 3.74 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%http://www.kernel.org/doc/man\-pages/.