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8f0aff2a | 1 | .\" Page by b.hubert |
2297bf0e | 2 | .\" |
2e46a6e7 | 3 | .\" %%%LICENSE_START(FREELY_REDISTRIBUTABLE) |
8f0aff2a | 4 | .\" may be freely modified and distributed |
8ff7380d | 5 | .\" %%%LICENSE_END |
fea681da MK |
6 | .\" |
7 | .\" Niki A. Rahimi (LTC Security Development, narahimi@us.ibm.com) | |
8 | .\" added ERRORS section. | |
9 | .\" | |
10 | .\" Modified 2004-06-17 mtk | |
11 | .\" Modified 2004-10-07 aeb, added FUTEX_REQUEUE, FUTEX_CMP_REQUEUE | |
12 | .\" | |
4f58b197 MK |
13 | .\" FIXME |
14 | .\" See also https://bugzilla.kernel.org/show_bug.cgi?id=14303 | |
40d5cf23 | 15 | .\" 2.6.14 adds FUTEX_WAKE_OP |
4f58b197 MK |
16 | .\" commit 4732efbeb997189d9f9b04708dc26bf8613ed721 |
17 | .\" Author: Jakub Jelinek <jakub@redhat.com> | |
18 | .\" Date: Tue Sep 6 15:16:25 2005 -0700 | |
19 | .\" | |
20 | .\" FIXME | |
c13182ef MK |
21 | .\" 2.6.18 adds (Ingo Molnar) priority inheritance support: |
22 | .\" FUTEX_LOCK_PI, FUTEX_UNLOCK_PI, and FUTEX_TRYLOCK_PI. These need | |
34f7665a MK |
23 | .\" to be documented in the manual page. Probably there is sufficient |
24 | .\" material in the kernel source file Documentation/pi-futex.txt. | |
4f58b197 MK |
25 | .\" commit c87e2837be82df479a6bae9f155c43516d2feebc |
26 | .\" Author: Ingo Molnar <mingo@elte.hu> | |
27 | .\" Date: Tue Jun 27 02:54:58 2006 -0700 | |
28 | .\" | |
29 | .\" commit e2970f2fb6950183a34e8545faa093eb49d186e1 | |
30 | .\" Author: Ingo Molnar <mingo@elte.hu> | |
31 | .\" Date: Tue Jun 27 02:54:47 2006 -0700 | |
32 | .\" | |
33 | .\" See Documentation/futex-requeue-pi.txt | |
34 | .\" | |
35 | .\" FIXME | |
40d5cf23 | 36 | .\" 2.6.25 adds FUTEX_WAKE_BITSET, FUTEX_WAIT_BITSET |
4f58b197 MK |
37 | .\" commit cd689985cf49f6ff5c8eddc48d98b9d581d9475d |
38 | .\" Author: Thomas Gleixner <tglx@linutronix.de> | |
39 | .\" Date: Fri Feb 1 17:45:14 2008 +0100 | |
40 | .\" | |
41 | .\" FIXME | |
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 | |
46 | .\" | |
47 | .\" commit ba9c22f2c01cf5c88beed5a6b9e07d42e10bd358 | |
48 | .\" Author: Darren Hart <dvhltc@us.ibm.com> | |
49 | .\" Date: Mon Apr 20 22:22:22 2009 -0700 | |
50 | .\" | |
51 | .\" See Documentation/futex-requeue-pi.txt | |
34f7665a | 52 | .\" |
42f05d64 | 53 | .TH FUTEX 2 2013-12-12 "Linux" "Linux Programmer's Manual" |
fea681da | 54 | .SH NAME |
ce154705 | 55 | futex \- fast user-space locking |
fea681da | 56 | .SH SYNOPSIS |
9d9dc1e8 | 57 | .nf |
fea681da MK |
58 | .sp |
59 | .B "#include <linux/futex.h>" | |
fea681da MK |
60 | .B "#include <sys/time.h>" |
61 | .sp | |
9d9dc1e8 MK |
62 | .BI "int futex(int *" uaddr ", int " op ", int " val \ |
63 | ", const struct timespec *" timeout , | |
64 | .br | |
65 | .BI " int *" uaddr2 ", int " val3 ); | |
fea681da | 66 | .\" int *? void *? u32 *? |
9d9dc1e8 | 67 | .fi |
47297adb | 68 | .SH DESCRIPTION |
fea681da MK |
69 | .PP |
70 | The | |
e511ffb6 | 71 | .BR futex () |
fea681da MK |
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 | |
e511ffb6 | 78 | .BR futex () |
c13182ef | 79 | calls). |
fd3fa7ef | 80 | This system call is typically used to |
fea681da MK |
81 | implement the contended case of a lock in shared memory, as |
82 | described in | |
a8bda636 | 83 | .BR futex (7). |
fea681da | 84 | .PP |
c13182ef | 85 | When a |
a8bda636 | 86 | .BR futex (7) |
7fac88a9 | 87 | operation did not finish uncontended in user space, a call needs to be made |
c13182ef MK |
88 | to the kernel to arbitrate. |
89 | Arbitration can either mean putting the calling | |
fea681da MK |
90 | process to sleep or, conversely, waking a waiting process. |
91 | .PP | |
92 | Callers of this function are expected to adhere to the semantics as set out in | |
a8bda636 | 93 | .BR futex (7). |
fea681da | 94 | As these |
d603cc27 | 95 | semantics involve writing nonportable assembly instructions, this in turn |
fea681da MK |
96 | probably means that most users will in fact be library authors and not |
97 | general application developers. | |
98 | .PP | |
99 | The | |
100 | .I uaddr | |
101 | argument needs to point to an aligned integer which stores the counter. | |
102 | The operation to execute is passed via the | |
103 | .I op | |
c4bb193f | 104 | argument, along with a value |
fea681da MK |
105 | .IR val . |
106 | .PP | |
107 | Five operations are currently defined: | |
108 | .TP | |
109 | .B FUTEX_WAIT | |
110 | This operation atomically verifies that the futex address | |
111 | .I uaddr | |
112 | still contains the value | |
113 | .IR val , | |
682edefb MK |
114 | and sleeps awaiting |
115 | .B FUTEX_WAKE | |
116 | on this futex address. | |
c13182ef | 117 | If the |
fea681da | 118 | .I timeout |
82a6092b MK |
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.) | |
123 | If | |
124 | .I timeout | |
125 | is NULL, the call blocks indefinitely. | |
c13182ef | 126 | The arguments |
fea681da MK |
127 | .I uaddr2 |
128 | and | |
129 | .I val3 | |
130 | are ignored. | |
131 | ||
132 | For | |
a8bda636 | 133 | .BR futex (7), |
fea681da MK |
134 | this call is executed if decrementing the count gave a negative value |
135 | (indicating contention), and will sleep until another process releases | |
682edefb MK |
136 | the futex and executes the |
137 | .B FUTEX_WAKE | |
138 | operation. | |
fea681da MK |
139 | .TP |
140 | .B FUTEX_WAKE | |
a8d55537 | 141 | This operation wakes at most \fIval\fP |
b87dcfb9 | 142 | processes waiting on this futex address (i.e., inside |
682edefb | 143 | .BR FUTEX_WAIT ). |
fea681da MK |
144 | The arguments |
145 | .IR timeout , | |
146 | .I uaddr2 | |
147 | and | |
148 | .I val3 | |
149 | are ignored. | |
150 | ||
151 | For | |
a8bda636 | 152 | .BR futex (7), |
fea681da MK |
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). | |
156 | .TP | |
da36351e | 157 | .BR FUTEX_FD " (present up to and including Linux 2.6.25)" |
fea681da MK |
158 | To support asynchronous wakeups, this operation associates a file descriptor |
159 | with a futex. | |
160 | .\" , suitable for .BR poll (2). | |
682edefb MK |
161 | If another process executes a |
162 | .BR FUTEX_WAKE , | |
163 | the process will receive the signal number that was passed in | |
fea681da MK |
164 | .IR val . |
165 | The calling process must close the returned file descriptor after use. | |
166 | The arguments | |
167 | .IR timeout , | |
168 | .I uaddr2 | |
169 | and | |
170 | .I val3 | |
171 | are ignored. | |
172 | ||
c13182ef | 173 | To prevent race conditions, the caller should test if the futex has |
682edefb MK |
174 | been upped after |
175 | .B FUTEX_FD | |
176 | returns. | |
266a5e91 | 177 | |
da36351e | 178 | Because it was inherently racy, |
682edefb | 179 | .B FUTEX_FD |
5fab2e7c | 180 | has been removed from Linux 2.6.26 onward. |
fea681da MK |
181 | .TP |
182 | .BR FUTEX_REQUEUE " (since Linux 2.5.70)" | |
183 | This operation was introduced in order to avoid a "thundering herd" effect | |
682edefb MK |
184 | when |
185 | .B FUTEX_WAKE | |
186 | is used and all processes woken up need to acquire another futex. | |
c13182ef | 187 | This call wakes up |
fea681da MK |
188 | .I val |
189 | processes, and requeues all other waiters on the futex at address | |
190 | .IR uaddr2 . | |
191 | The arguments | |
192 | .I timeout | |
193 | and | |
194 | .I val3 | |
195 | are ignored. | |
196 | .TP | |
197 | .BR FUTEX_CMP_REQUEUE " (since Linux 2.6.7)" | |
682edefb MK |
198 | There was a race in the intended use of |
199 | .BR FUTEX_REQUEUE , | |
200 | so | |
201 | .B FUTEX_CMP_REQUEUE | |
202 | was introduced. | |
203 | This is similar to | |
204 | .BR FUTEX_REQUEUE , | |
fea681da MK |
205 | but first checks whether the location |
206 | .I uaddr | |
207 | still contains the value | |
208 | .IR val3 . | |
e808bba0 MK |
209 | If not, the operation fails with the error |
210 | .BR EAGAIN . | |
fea681da MK |
211 | The argument |
212 | .I timeout | |
213 | is ignored. | |
47297adb | 214 | .SH RETURN VALUE |
fea681da | 215 | .PP |
e808bba0 MK |
216 | In the event of an error, all operations return \-1, and set |
217 | .I errno | |
218 | to indicate the error. | |
219 | The return value on success depends on the operation, | |
220 | as described in the following list: | |
fea681da MK |
221 | .TP |
222 | .B FUTEX_WAIT | |
682edefb MK |
223 | Returns 0 if the process was woken by a |
224 | .B FUTEX_WAKE | |
225 | call. | |
e808bba0 | 226 | See ERRORS for the various possible error returns. |
fea681da MK |
227 | .TP |
228 | .B FUTEX_WAKE | |
229 | Returns the number of processes woken up. | |
230 | .TP | |
231 | .B FUTEX_FD | |
232 | Returns the new file descriptor associated with the futex. | |
233 | .TP | |
234 | .B FUTEX_REQUEUE | |
235 | Returns the number of processes woken up. | |
236 | .TP | |
237 | .B FUTEX_CMP_REQUEUE | |
238 | Returns the number of processes woken up. | |
239 | .SH ERRORS | |
240 | .TP | |
241 | .B EACCES | |
242 | No read access to futex memory. | |
243 | .TP | |
244 | .B EAGAIN | |
682edefb | 245 | .B FUTEX_CMP_REQUEUE |
e808bba0 | 246 | detected that the value pointed to by |
9f6c40c0 МК |
247 | .I uaddr |
248 | is not equal to the expected value | |
249 | .IR val3 . | |
fea681da | 250 | (This probably indicates a race; |
682edefb MK |
251 | use the safe |
252 | .B FUTEX_WAKE | |
253 | now.) | |
fea681da MK |
254 | .TP |
255 | .B EFAULT | |
e808bba0 | 256 | Error retrieving |
fea681da | 257 | .I timeout |
7fac88a9 | 258 | information from user space. |
fea681da | 259 | .TP |
9f6c40c0 | 260 | .B EINTR |
e808bba0 | 261 | A |
9f6c40c0 | 262 | .B FUTEX_WAIT |
e808bba0 MK |
263 | operation was interrupted by a signal (see |
264 | .BR signal (7)) | |
265 | or a spurious wakeup. | |
9f6c40c0 | 266 | .TP |
fea681da | 267 | .B EINVAL |
4832b48a | 268 | Invalid argument. |
fea681da MK |
269 | .TP |
270 | .B ENFILE | |
271 | The system limit on the total number of open files has been reached. | |
4701fc28 MK |
272 | .TP |
273 | .B ENOSYS | |
274 | Invalid operation specified in | |
275 | .IR op . | |
9f6c40c0 МК |
276 | .TP |
277 | .B ETIMEDOUT | |
278 | Timeout during the | |
279 | .B FUTEX_WAIT | |
280 | operation. | |
281 | .TP | |
282 | .B EWOULDBLOCK | |
e808bba0 MK |
283 | .I op |
284 | was | |
285 | .BR FUTEX_WAIT | |
286 | and the value pointed to by | |
9f6c40c0 МК |
287 | .I uaddr |
288 | was not equal to the expected value | |
289 | .I val | |
e808bba0 | 290 | at the time of the call. |
47297adb | 291 | .SH VERSIONS |
a1d5f77c MK |
292 | .PP |
293 | Initial futex support was merged in Linux 2.5.7 but with different semantics | |
294 | from what was described above. | |
c4bb193f | 295 | A 4-argument system call with the semantics |
fd3fa7ef | 296 | described in this page was introduced in Linux 2.5.40. |
c4bb193f | 297 | In Linux 2.5.70 one argument |
a1d5f77c | 298 | was added. |
5503c85e | 299 | In Linux 2.6.7 a sixth argument was added\(emmessy, especially |
a1d5f77c | 300 | on the s390 architecture. |
47297adb | 301 | .SH CONFORMING TO |
8382f16d | 302 | This system call is Linux-specific. |
47297adb | 303 | .SH NOTES |
fea681da | 304 | .PP |
fcdad7d6 | 305 | To reiterate, bare futexes are not intended as an easy-to-use abstraction |
c13182ef | 306 | for end-users. |
fcdad7d6 | 307 | (There is no wrapper function for this system call in glibc.) |
c13182ef | 308 | Implementors are expected to be assembly literate and to have |
7fac88a9 | 309 | read the sources of the futex user-space library referenced below. |
fea681da MK |
310 | .\" .SH "AUTHORS" |
311 | .\" .PP | |
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. | |
47297adb | 317 | .SH SEE ALSO |
d806bc05 | 318 | .BR restart_syscall (2), |
14d8dd3b | 319 | .BR futex (7) |
fea681da | 320 | .PP |
52087dd3 | 321 | \fIFuss, Futexes and Furwocks: Fast Userlevel Locking in Linux\fP |
9b936e9e MK |
322 | (proceedings of the Ottawa Linux Symposium 2002), online at |
323 | .br | |
608bf950 SK |
324 | .UR http://kernel.org\:/doc\:/ols\:/2002\:/ols2002-pages-479-495.pdf |
325 | .UE | |
9b936e9e MK |
326 | .PP |
327 | Futex example library, futex-*.tar.bz2 at | |
328 | .br | |
a605264d | 329 | .UR ftp://ftp.kernel.org\:/pub\:/linux\:/kernel\:/people\:/rusty/ |
608bf950 | 330 | .UE |