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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 | .\" | |
bea08fec | 13 | .\" FIXME . |
4f58b197 | 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 | .\" | |
bea08fec | 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 | .\" | |
27b38e1c | 33 | .\" See Documentation/pi-futex.txt |
4f58b197 | 34 | .\" |
bea08fec | 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 | .\" | |
bea08fec | 41 | .\" FIXME . |
4f58b197 MK |
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 | .\" |
3d155313 | 53 | .TH FUTEX 2 2014-05-21 "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 |
409f08b0 | 68 | |
b939d6e4 MK |
69 | .IR Note : |
70 | There is no glibc wrapper for this system call; see NOTES. | |
47297adb | 71 | .SH DESCRIPTION |
fea681da MK |
72 | .PP |
73 | The | |
e511ffb6 | 74 | .BR futex () |
fea681da MK |
75 | system call provides a method for |
76 | a program to wait for a value at a given address to change, and a | |
77 | method to wake up anyone waiting on a particular address (while the | |
78 | addresses for the same memory in separate processes may not be | |
79 | equal, the kernel maps them internally so the same memory mapped in | |
80 | different locations will correspond for | |
e511ffb6 | 81 | .BR futex () |
c13182ef | 82 | calls). |
fd3fa7ef | 83 | This system call is typically used to |
fea681da MK |
84 | implement the contended case of a lock in shared memory, as |
85 | described in | |
a8bda636 | 86 | .BR futex (7). |
fea681da | 87 | .PP |
c13182ef | 88 | When a |
a8bda636 | 89 | .BR futex (7) |
7fac88a9 | 90 | operation did not finish uncontended in user space, a call needs to be made |
c13182ef MK |
91 | to the kernel to arbitrate. |
92 | Arbitration can either mean putting the calling | |
fea681da MK |
93 | process to sleep or, conversely, waking a waiting process. |
94 | .PP | |
95 | Callers of this function are expected to adhere to the semantics as set out in | |
a8bda636 | 96 | .BR futex (7). |
fea681da | 97 | As these |
d603cc27 | 98 | semantics involve writing nonportable assembly instructions, this in turn |
fea681da MK |
99 | probably means that most users will in fact be library authors and not |
100 | general application developers. | |
101 | .PP | |
102 | The | |
103 | .I uaddr | |
104 | argument needs to point to an aligned integer which stores the counter. | |
105 | The operation to execute is passed via the | |
106 | .I op | |
c4bb193f | 107 | argument, along with a value |
fea681da MK |
108 | .IR val . |
109 | .PP | |
6be4bad7 MK |
110 | The |
111 | .I op | |
112 | argument consists of two parts: | |
113 | a command that specifies the operation to be performed, | |
114 | bit-wise ORed with zero or or more options that | |
115 | modify the behaviour of the operation. | |
fc30eb79 TG |
116 | The options that may be included in |
117 | .I op | |
118 | are as follows: | |
119 | .TP | |
120 | .BR FUTEX_PRIVATE_FLAG " (since Linux 2.6.22)" | |
121 | .\" commit 34f01cc1f512fa783302982776895c73714ebbc2 | |
122 | This option bit can be employed with all futex operations. | |
123 | It tells the kernel that the futex is process private and not shared | |
124 | with another process. | |
125 | This allows the kernel to choose the fast path for validating | |
126 | the user-space address and avoids expensive VMA lookups, | |
127 | taking reference counts on file backing store, and so on. | |
6be4bad7 MK |
128 | .PP |
129 | The operation specified in | |
130 | .I op | |
131 | is one of the following: | |
fea681da MK |
132 | .TP |
133 | .B FUTEX_WAIT | |
134 | This operation atomically verifies that the futex address | |
135 | .I uaddr | |
136 | still contains the value | |
137 | .IR val , | |
682edefb MK |
138 | and sleeps awaiting |
139 | .B FUTEX_WAKE | |
140 | on this futex address. | |
c13182ef | 141 | If the |
fea681da | 142 | .I timeout |
82a6092b MK |
143 | argument is non-NULL, its contents specify the duration of the wait. |
144 | (This interval will be rounded up to the system clock granularity, | |
145 | and kernel scheduling delays mean that the | |
146 | blocking interval may overrun by a small amount.) | |
147 | If | |
148 | .I timeout | |
149 | is NULL, the call blocks indefinitely. | |
4798a7f3 | 150 | |
c13182ef | 151 | The arguments |
fea681da MK |
152 | .I uaddr2 |
153 | and | |
154 | .I val3 | |
155 | are ignored. | |
156 | ||
157 | For | |
a8bda636 | 158 | .BR futex (7), |
fea681da MK |
159 | this call is executed if decrementing the count gave a negative value |
160 | (indicating contention), and will sleep until another process releases | |
682edefb MK |
161 | the futex and executes the |
162 | .B FUTEX_WAKE | |
163 | operation. | |
fea681da MK |
164 | .TP |
165 | .B FUTEX_WAKE | |
a8d55537 | 166 | This operation wakes at most \fIval\fP |
b87dcfb9 | 167 | processes waiting on this futex address (i.e., inside |
682edefb | 168 | .BR FUTEX_WAIT ). |
4798a7f3 | 169 | |
fea681da MK |
170 | The arguments |
171 | .IR timeout , | |
172 | .I uaddr2 | |
173 | and | |
174 | .I val3 | |
175 | are ignored. | |
176 | ||
177 | For | |
a8bda636 | 178 | .BR futex (7), |
fea681da MK |
179 | this is executed if incrementing |
180 | the count showed that there were waiters, once the futex value has been set | |
181 | to 1 (indicating that it is available). | |
182 | .TP | |
da36351e | 183 | .BR FUTEX_FD " (present up to and including Linux 2.6.25)" |
fea681da MK |
184 | To support asynchronous wakeups, this operation associates a file descriptor |
185 | with a futex. | |
186 | .\" , suitable for .BR poll (2). | |
682edefb MK |
187 | If another process executes a |
188 | .BR FUTEX_WAKE , | |
189 | the process will receive the signal number that was passed in | |
fea681da MK |
190 | .IR val . |
191 | The calling process must close the returned file descriptor after use. | |
4798a7f3 | 192 | |
fea681da MK |
193 | The arguments |
194 | .IR timeout , | |
195 | .I uaddr2 | |
196 | and | |
197 | .I val3 | |
198 | are ignored. | |
199 | ||
c13182ef | 200 | To prevent race conditions, the caller should test if the futex has |
682edefb MK |
201 | been upped after |
202 | .B FUTEX_FD | |
203 | returns. | |
266a5e91 | 204 | |
da36351e | 205 | Because it was inherently racy, |
682edefb | 206 | .B FUTEX_FD |
5fab2e7c | 207 | has been removed from Linux 2.6.26 onward. |
fea681da MK |
208 | .TP |
209 | .BR FUTEX_REQUEUE " (since Linux 2.5.70)" | |
210 | This operation was introduced in order to avoid a "thundering herd" effect | |
682edefb MK |
211 | when |
212 | .B FUTEX_WAKE | |
213 | is used and all processes woken up need to acquire another futex. | |
2abb73b9 | 214 | The argument |
fea681da | 215 | .I val |
2abb73b9 TG |
216 | contains the number of waiters on |
217 | .I uaddr | |
218 | that are immediately woken up. | |
219 | The | |
fea681da | 220 | .I timeout |
2abb73b9 TG |
221 | argument is (ab)used to specify the number of waiters |
222 | that are requeued to the futex at | |
223 | .IR uaddr2 ; | |
224 | the kernel casts the | |
225 | .I timeout | |
226 | value to | |
227 | .IR u32 . | |
228 | .\" FIXME What are the constraints (if any) on the values of 'val' vs | |
229 | .\" 'timeout' vs [the number of waites on 'uaddr']? | |
230 | ||
231 | The argument | |
fea681da | 232 | .I val3 |
2abb73b9 | 233 | is ignored. |
fea681da MK |
234 | .TP |
235 | .BR FUTEX_CMP_REQUEUE " (since Linux 2.6.7)" | |
682edefb MK |
236 | There was a race in the intended use of |
237 | .BR FUTEX_REQUEUE , | |
238 | so | |
239 | .B FUTEX_CMP_REQUEUE | |
240 | was introduced. | |
a72a3aeb | 241 | .\" FIXME should there be a statement in the description of FUTEX_REQUEUE |
a1f47699 | 242 | .\" to say that it should be avoided in favor of FUTEX_CMP_REQUEUE? |
03433acb | 243 | This operation is similar to |
682edefb | 244 | .BR FUTEX_REQUEUE , |
fea681da MK |
245 | but first checks whether the location |
246 | .I uaddr | |
247 | still contains the value | |
248 | .IR val3 . | |
e808bba0 MK |
249 | If not, the operation fails with the error |
250 | .BR EAGAIN . | |
4798a7f3 | 251 | |
03433acb MK |
252 | The arguments |
253 | .IR val , | |
254 | .IR uaddr , | |
255 | .IR uaddr2 , | |
256 | and | |
fea681da | 257 | .I timeout |
03433acb MK |
258 | are as for |
259 | .BR FUTEX_REQUEUE . | |
47297adb | 260 | .SH RETURN VALUE |
fea681da | 261 | .PP |
e808bba0 MK |
262 | In the event of an error, all operations return \-1, and set |
263 | .I errno | |
264 | to indicate the error. | |
265 | The return value on success depends on the operation, | |
266 | as described in the following list: | |
fea681da MK |
267 | .TP |
268 | .B FUTEX_WAIT | |
682edefb MK |
269 | Returns 0 if the process was woken by a |
270 | .B FUTEX_WAKE | |
271 | call. | |
e808bba0 | 272 | See ERRORS for the various possible error returns. |
fea681da MK |
273 | .TP |
274 | .B FUTEX_WAKE | |
275 | Returns the number of processes woken up. | |
276 | .TP | |
277 | .B FUTEX_FD | |
278 | Returns the new file descriptor associated with the futex. | |
279 | .TP | |
280 | .B FUTEX_REQUEUE | |
281 | Returns the number of processes woken up. | |
282 | .TP | |
283 | .B FUTEX_CMP_REQUEUE | |
284 | Returns the number of processes woken up. | |
285 | .SH ERRORS | |
286 | .TP | |
287 | .B EACCES | |
288 | No read access to futex memory. | |
289 | .TP | |
290 | .B EAGAIN | |
682edefb | 291 | .B FUTEX_CMP_REQUEUE |
e808bba0 | 292 | detected that the value pointed to by |
9f6c40c0 МК |
293 | .I uaddr |
294 | is not equal to the expected value | |
295 | .IR val3 . | |
fd1dc4c2 | 296 | .\" FIXME: Is the following sentence correct? |
fea681da | 297 | (This probably indicates a race; |
682edefb MK |
298 | use the safe |
299 | .B FUTEX_WAKE | |
300 | now.) | |
fea681da MK |
301 | .TP |
302 | .B EFAULT | |
1ea901e8 MK |
303 | A required pointer argument (i.e., |
304 | .IR uaddr , | |
305 | .IR uaddr2 , | |
306 | or | |
307 | .IR timeout ) | |
496df304 | 308 | did not point to a valid user-space address. |
fea681da | 309 | .TP |
9f6c40c0 | 310 | .B EINTR |
e808bba0 | 311 | A |
9f6c40c0 | 312 | .B FUTEX_WAIT |
e808bba0 MK |
313 | operation was interrupted by a signal (see |
314 | .BR signal (7)) | |
315 | or a spurious wakeup. | |
9f6c40c0 | 316 | .TP |
fea681da | 317 | .B EINVAL |
fb2f4c27 MK |
318 | .RB ( FUTEX_WAIT , |
319 | .BR FUTEX_WAIT_REQUEUE_PI ) | |
320 | The supplied | |
321 | .I timeout | |
322 | argument was invalid | |
323 | .RI ( tv_sec | |
324 | was less than zero, or | |
325 | .IR tv_nsec | |
326 | was not less than 1000,000,000). | |
327 | .TP | |
328 | .B EINVAL | |
ea355b7f | 329 | .RB ( FUTEX_WAIT , |
caf1ff25 | 330 | .BR FUTEX_WAKE , |
a1f47699 MK |
331 | .BR FUTEX_REQUEUE , |
332 | .BR FUTEX_CMP_REQUEUE ) | |
51ee94be | 333 | .I uaddr |
caf1ff25 | 334 | or (for |
a1f47699 MK |
335 | .BR FUTEX_REQUEUE |
336 | and | |
337 | .BR FUTEX_CMP_REQUEUE ) | |
caf1ff25 | 338 | .I uaddr2 |
51ee94be MK |
339 | does not point to a valid object\(emthat is, |
340 | the address is not 4-byte-aligned. | |
341 | .TP | |
342 | .B EINVAL | |
bae14b6c | 343 | .RB ( FUTEX_WAKE , |
e169277f MK |
344 | .BR FUTEX_REQUEUE , |
345 | .BR FUTEX_CMP_REQUEUE ) | |
496df304 | 346 | The kernel detected an inconsistency between the user-space state at |
9534086b TG |
347 | .I uaddr |
348 | and the kernel state\(emthat is, it detected a waiter which waits in | |
349 | .BR FUTEX_LOCK_PI . | |
350 | .TP | |
351 | .B EINVAL | |
add875c0 MK |
352 | .RB ( FUTEX_REQUEUE ) |
353 | .\" FIXME tglx suggested adding this, but does this error really | |
354 | .\" occur for FUTEX_REQUEUE? | |
355 | .I uaddr | |
356 | equals | |
357 | .IR uaddr2 | |
358 | (i.e., an attempt was made to requeue to the same futex). | |
359 | .TP | |
360 | .B EINVAL | |
4832b48a | 361 | Invalid argument. |
fea681da MK |
362 | .TP |
363 | .B ENFILE | |
364 | The system limit on the total number of open files has been reached. | |
4701fc28 MK |
365 | .TP |
366 | .B ENOSYS | |
367 | Invalid operation specified in | |
368 | .IR op . | |
9f6c40c0 МК |
369 | .TP |
370 | .B ETIMEDOUT | |
d1926d78 MK |
371 | .RB ( FUTEX_WAIT ) |
372 | The operation timed out. | |
9f6c40c0 МК |
373 | .TP |
374 | .B EWOULDBLOCK | |
6b5025a6 MK |
375 | .RB ( FUTEX_WAIT ) |
376 | The atomic enqueueing failed. | |
377 | .TP | |
378 | .B EWOULDBLOCK | |
e808bba0 MK |
379 | .I op |
380 | was | |
381 | .BR FUTEX_WAIT | |
382 | and the value pointed to by | |
9f6c40c0 МК |
383 | .I uaddr |
384 | was not equal to the expected value | |
385 | .I val | |
e808bba0 | 386 | at the time of the call. |
47297adb | 387 | .SH VERSIONS |
a1d5f77c MK |
388 | .PP |
389 | Initial futex support was merged in Linux 2.5.7 but with different semantics | |
390 | from what was described above. | |
c4bb193f | 391 | A 4-argument system call with the semantics |
fd3fa7ef | 392 | described in this page was introduced in Linux 2.5.40. |
11b520ed | 393 | In Linux 2.5.70, one argument |
a1d5f77c | 394 | was added. |
11b520ed | 395 | In Linux 2.6.7, a sixth argument was added\(emmessy, especially |
a1d5f77c | 396 | on the s390 architecture. |
47297adb | 397 | .SH CONFORMING TO |
8382f16d | 398 | This system call is Linux-specific. |
47297adb | 399 | .SH NOTES |
fea681da | 400 | .PP |
fcdad7d6 | 401 | To reiterate, bare futexes are not intended as an easy-to-use abstraction |
c13182ef | 402 | for end-users. |
fcdad7d6 | 403 | (There is no wrapper function for this system call in glibc.) |
c13182ef | 404 | Implementors are expected to be assembly literate and to have |
7fac88a9 | 405 | read the sources of the futex user-space library referenced below. |
d282bb24 | 406 | .\" .SH AUTHORS |
fea681da MK |
407 | .\" .PP |
408 | .\" Futexes were designed and worked on by | |
409 | .\" Hubertus Franke (IBM Thomas J. Watson Research Center), | |
410 | .\" Matthew Kirkwood, Ingo Molnar (Red Hat) | |
411 | .\" and Rusty Russell (IBM Linux Technology Center). | |
412 | .\" This page written by bert hubert. | |
47297adb | 413 | .SH SEE ALSO |
d806bc05 | 414 | .BR restart_syscall (2), |
14d8dd3b | 415 | .BR futex (7) |
fea681da | 416 | .PP |
52087dd3 | 417 | \fIFuss, Futexes and Furwocks: Fast Userlevel Locking in Linux\fP |
9b936e9e MK |
418 | (proceedings of the Ottawa Linux Symposium 2002), online at |
419 | .br | |
608bf950 SK |
420 | .UR http://kernel.org\:/doc\:/ols\:/2002\:/ols2002-pages-479-495.pdf |
421 | .UE | |
9b936e9e MK |
422 | .PP |
423 | Futex example library, futex-*.tar.bz2 at | |
424 | .br | |
a605264d | 425 | .UR ftp://ftp.kernel.org\:/pub\:/linux\:/kernel\:/people\:/rusty/ |
608bf950 | 426 | .UE |