3 .\" %%%LICENSE_START(FREELY_REDISTRIBUTABLE)
4 .\" may be freely modified and distributed
7 .\" Niki A. Rahimi (LTC Security Development, narahimi@us.ibm.com)
8 .\" added ERRORS section.
10 .\" Modified 2004-06-17 mtk
11 .\" Modified 2004-10-07 aeb, added FUTEX_REQUEUE, FUTEX_CMP_REQUEUE
14 .\" See also https://bugzilla.kernel.org/show_bug.cgi?id=14303
15 .\" 2.6.14 adds FUTEX_WAKE_OP
16 .\" commit 4732efbeb997189d9f9b04708dc26bf8613ed721
17 .\" Author: Jakub Jelinek <jakub@redhat.com>
18 .\" Date: Tue Sep 6 15:16:25 2005 -0700
21 .\" 2.6.18 adds (Ingo Molnar) priority inheritance support:
22 .\" FUTEX_LOCK_PI, FUTEX_UNLOCK_PI, and FUTEX_TRYLOCK_PI. These need
23 .\" to be documented in the manual page. Probably there is sufficient
24 .\" material in the kernel source file Documentation/pi-futex.txt.
25 .\" commit c87e2837be82df479a6bae9f155c43516d2feebc
26 .\" Author: Ingo Molnar <mingo@elte.hu>
27 .\" Date: Tue Jun 27 02:54:58 2006 -0700
29 .\" commit e2970f2fb6950183a34e8545faa093eb49d186e1
30 .\" Author: Ingo Molnar <mingo@elte.hu>
31 .\" Date: Tue Jun 27 02:54:47 2006 -0700
33 .\" See Documentation/futex-requeue-pi.txt
36 .\" 2.6.25 adds FUTEX_WAKE_BITSET, FUTEX_WAIT_BITSET
37 .\" commit cd689985cf49f6ff5c8eddc48d98b9d581d9475d
38 .\" Author: Thomas Gleixner <tglx@linutronix.de>
39 .\" Date: Fri Feb 1 17:45:14 2008 +0100
42 .\" 2.6.31 adds FUTEX_WAIT_REQUEUE_PI, FUTEX_CMP_REQUEUE_PI
43 .\" commit 52400ba946759af28442dee6265c5c0180ac7122
44 .\" Author: Darren Hart <dvhltc@us.ibm.com>
45 .\" Date: Fri Apr 3 13:40:49 2009 -0700
47 .\" commit ba9c22f2c01cf5c88beed5a6b9e07d42e10bd358
48 .\" Author: Darren Hart <dvhltc@us.ibm.com>
49 .\" Date: Mon Apr 20 22:22:22 2009 -0700
51 .\" See Documentation/futex-requeue-pi.txt
53 .TH FUTEX 2 2013-12-12 "Linux" "Linux Programmer's Manual"
55 futex \- fast user-space locking
59 .B "#include <linux/futex.h>"
60 .B "#include <sys/time.h>"
62 .BI "int futex(int *" uaddr ", int " op ", int " val \
63 ", const struct timespec *" timeout ,
65 .BI " int *" uaddr2 ", int " val3 );
66 .\" int *? void *? u32 *?
72 system call provides a method for
73 a program to wait for a value at a given address to change, and a
74 method to wake up anyone waiting on a particular address (while the
75 addresses for the same memory in separate processes may not be
76 equal, the kernel maps them internally so the same memory mapped in
77 different locations will correspond for
80 This system call is typically used to
81 implement the contended case of a lock in shared memory, as
87 operation did not finish uncontended in user space, a call needs to be made
88 to the kernel to arbitrate.
89 Arbitration can either mean putting the calling
90 process to sleep or, conversely, waking a waiting process.
92 Callers of this function are expected to adhere to the semantics as set out in
95 semantics involve writing nonportable assembly instructions, this in turn
96 probably means that most users will in fact be library authors and not
97 general application developers.
101 argument needs to point to an aligned integer which stores the counter.
102 The operation to execute is passed via the
104 argument, along with a value
107 Five operations are currently defined:
110 This operation atomically verifies that the futex address
112 still contains the value
116 on this futex address.
119 argument is non-NULL, its contents specify the duration of the wait.
120 (This interval will be rounded up to the system clock granularity,
121 and kernel scheduling delays mean that the
122 blocking interval may overrun by a small amount.)
125 is NULL, the call blocks indefinitely.
134 this call is executed if decrementing the count gave a negative value
135 (indicating contention), and will sleep until another process releases
136 the futex and executes the
141 This operation wakes at most \fIval\fP
142 processes waiting on this futex address (i.e., inside
153 this is executed if incrementing
154 the count showed that there were waiters, once the futex value has been set
155 to 1 (indicating that it is available).
157 .BR FUTEX_FD " (present up to and including Linux 2.6.25)"
158 To support asynchronous wakeups, this operation associates a file descriptor
160 .\" , suitable for .BR poll (2).
161 If another process executes a
163 the process will receive the signal number that was passed in
165 The calling process must close the returned file descriptor after use.
173 To prevent race conditions, the caller should test if the futex has
178 Because it was inherently racy,
180 has been removed from Linux 2.6.26 onward.
182 .BR FUTEX_REQUEUE " (since Linux 2.5.70)"
183 This operation was introduced in order to avoid a "thundering herd" effect
186 is used and all processes woken up need to acquire another futex.
189 processes, and requeues all other waiters on the futex at address
197 .BR FUTEX_CMP_REQUEUE " (since Linux 2.6.7)"
198 There was a race in the intended use of
205 but first checks whether the location
207 still contains the value
209 If not, the operation fails with the error
216 In the event of an error, all operations return \-1, and set
218 to indicate the error.
219 The return value on success depends on the operation,
220 as described in the following list:
223 Returns 0 if the process was woken by a
226 See ERRORS for the various possible error returns.
229 Returns the number of processes woken up.
232 Returns the new file descriptor associated with the futex.
235 Returns the number of processes woken up.
238 Returns the number of processes woken up.
242 No read access to futex memory.
246 detected that the value pointed to by
248 is not equal to the expected value
250 (This probably indicates a race;
258 information from user space.
263 operation was interrupted by a signal (see
265 or a spurious wakeup.
271 The system limit on the total number of open files has been reached.
274 Invalid operation specified in
286 and the value pointed to by
288 was not equal to the expected value
290 at the time of the call.
293 Initial futex support was merged in Linux 2.5.7 but with different semantics
294 from what was described above.
295 A 4-argument system call with the semantics
296 described in this page was introduced in Linux 2.5.40.
297 In Linux 2.5.70 one argument
299 In Linux 2.6.7 a sixth argument was added\(emmessy, especially
300 on the s390 architecture.
302 This system call is Linux-specific.
305 To reiterate, bare futexes are not intended as an easy-to-use abstraction
307 (There is no wrapper function for this system call in glibc.)
308 Implementors are expected to be assembly literate and to have
309 read the sources of the futex user-space library referenced below.
312 .\" Futexes were designed and worked on by
313 .\" Hubertus Franke (IBM Thomas J. Watson Research Center),
314 .\" Matthew Kirkwood, Ingo Molnar (Red Hat)
315 .\" and Rusty Russell (IBM Linux Technology Center).
316 .\" This page written by bert hubert.
318 .BR restart_syscall (2),
321 \fIFuss, Futexes and Furwocks: Fast Userlevel Locking in Linux\fP
322 (proceedings of the Ottawa Linux Symposium 2002), online at
324 .UR http://kernel.org\:/doc\:/ols\:/2002\:/ols2002-pages-479-495.pdf
327 Futex example library, futex-*.tar.bz2 at
329 .UR ftp://ftp.kernel.org\:/pub\:/linux\:/kernel\:/people\:/rusty/
332 This page is part of release 3.64 of the Linux
335 A description of the project,
336 and information about reporting bugs,
338 \%http://www.kernel.org/doc/man\-pages/.