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47 .TH CANCELLATION 5 "Oct 4, 2005"
48 .SH NAME
49 cancellation \- overview of concepts related to POSIX thread cancellation
50 .SH DESCRIPTION
51 .sp
52
53 .sp
54 .TS
55 box;
56 c | c
57 l | l .
58 FUNCTION ACTION
59 _
60 \fBpthread_cancel()\fR Cancels thread execution.
61 \fBpthread_setcancelstate()\fR Sets the cancellation \fIstate\fR of a thread.
62 \fBpthread_setcanceltype()\fR Sets the cancellation \fItype\fR of a thread.
63 \fBpthread_testcancel()\fR T{
64 Creates a cancellation point in the calling thread.
65 T}
66 \fBpthread_cleanup_push()\fR Pushes a cleanup handler routine.
67 \fBpthread_cleanup_pop()\fR Pops a cleanup handler routine.
68 .TE
69
70 .SS "Cancellation"
71 .sp
72 .LP
73 Thread cancellation allows a thread to terminate the execution of any
74 application thread in the process. Cancellation is useful when further
75 operations of one or more threads are undesirable or unnecessary.
76 .sp
77 .LP
78 An example of a situation that could benefit from using cancellation is an
79 asynchronously-generated cancel condition such as a user requesting to close or
80 exit some running operation. Another example is the completion of a task
81 undertaken by a number of threads, such as solving a maze. While many threads
82 search for the solution, one of the threads might solve the puzzle while the
83 others continue to operate. Since they are serving no purpose at that point,
84 they should all be canceled.
85 .SS "Planning Steps"
86 .sp
87 .LP
88 Planning and programming for most cancellations follow this pattern:
89 .RS +4
90 .TP
91 1.
92 Identify which threads you want to cancel, and insert
93 \fBpthread_cancel\fR(3C) statements.
94 .RE
95 .RS +4
96 .TP
97 2.
98 Identify system-defined cancellation points where a thread that might be
99 canceled could have changed system or program state that should be restored.
100 See the \fBCancellation Points\fR for a list.
101 .RE
102 .RS +4
103 .TP
104 3.
105 When a thread changes the system or program state just before a cancellation
106 point, and should restore that state before the thread is canceled, place a
107 cleanup handler before the cancellation point with
108 \fBpthread_cleanup_push\fR(3C). Wherever a thread restores the changed state,
109 pop the cleanup handler from the cleanup stack with
110 \fBpthread_cleanup_pop\fR(3C).
111 .RE
112 .RS +4
113 .TP
114 4.
115 Know whether the threads you are canceling call into cancel-unsafe
116 libraries, and disable cancellation with \fBpthread_setcancelstate\fR(3C)
117 before the call into the library. See \fBCancellation State\fR and
118 \fBCancel-Safe\fR.
119 .RE
120 .RS +4
121 .TP
122 5.
123 To cancel a thread in a procedure that contains no cancellation points,
124 insert your own cancellation points with \fBpthread_testcancel\fR(3C). This
125 function creates cancellation points by testing for pending cancellations and
126 performing those cancellations if they are found. Push and pop cleanup handlers
127 around the cancellation point, if necessary (see Step 3, above).
128 .RE
129 .SS "Cancellation Points"
130 .sp
131 .LP
132 The system defines certain points at which cancellation can occur (cancellation
133 points), and you can create additional cancellation points in your application
134 with \fBpthread_testcancel()\fR.
135 .sp
136 .LP
137 The following cancellation points are defined by the system (system-defined
138 cancellation points): \fBcreat\fR(2), \fBaio_suspend\fR(3C), \fBclose\fR(2),
139 \fBcreat\fR(2), \fBgetmsg\fR(2), \fBgetpmsg\fR(2), \fBlockf\fR(3C),
140 \fBmq_receive\fR(3C), \fBmq_send\fR(3C), \fBmsgrcv\fR(2), \fBmsgsnd\fR(2),
141 \fBmsync\fR(3C), \fBnanosleep\fR(3C), \fBopen\fR(2), \fBpause\fR(2),
142 \fBpoll\fR(2), \fBpread\fR(2), \fBpthread_cond_timedwait\fR(3C),
143 \fBpthread_cond_wait\fR(3C), \fBpthread_join\fR(3C),
144 \fBpthread_testcancel\fR(3C), \fBputmsg\fR(2), \fBputpmsg\fR(2),
145 \fBpwrite\fR(2), \fBread\fR(2), \fBreadv\fR(2), \fBselect\fR(3C),
146 \fBsem_wait\fR(3C), \fBsigpause\fR(3C), \fBsigwaitinfo\fR(3C),
147 \fBsigsuspend\fR(2), \fBsigtimedwait\fR(3C), \fBsigwait\fR(2), \fBsleep\fR(3C),
148 \fBsync\fR(2), \fBsystem\fR(3C), \fBtcdrain\fR(3C), \fBusleep\fR(3C),
149 \fBwait\fR(3C), \fBwaitid\fR(2), \fBwait3\fR(3C), \fBwaitpid\fR(3C),
150 \fBwrite\fR(2), \fBwritev\fR(2), and \fBfcntl\fR(2), when specifying
151 \fBF_SETLKW\fR as the command.
152 .sp
153 .LP
154 When cancellation is asynchronous, cancellation can occur at any time (before,
155 during, or after the execution of the function defined as the cancellation
156 point). When cancellation is deferred (the default case), cancellation occurs
157 only within the scope of a function defined as a cancellation point (after the
158 function is called and before the function returns). See \fBCancellation
159 Type\fR for more information about deferred and asynchronous cancellation.
160 .sp
161 .LP
162 Choosing where to place cancellation points and understanding how cancellation
163 affects your program depend upon your understanding of both your application
164 and of cancellation mechanics.
165 .sp
166 .LP
167 Typically, any call that might require a long wait should be a cancellation
168 point. Operations need to check for pending cancellation requests when the
169 operation is about to block indefinitely. This includes threads waiting in
170 \fBpthread_cond_wait()\fR and \fBpthread_cond_timedwait()\fR, threads waiting
171 for the termination of another thread in \fBpthread_join()\fR, and threads
172 blocked on \fBsigwait()\fR.
173 .sp
174 .LP
175 A mutex is explicitly not a cancellation point and should be held for only the
176 minimal essential time.
177 .sp
178 .LP
179 Most of the dangers in performing cancellations deal with properly restoring
180 invariants and freeing shared resources. For example, a carelessly canceled
181 thread might leave a mutex in a locked state, leading to a deadlock. Or it
182 might leave a region of memory allocated with no way to identify it and
183 therefore no way to free it.
184 .SS "Cleanup Handlers"
185 .sp
186 .LP
187 When a thread is canceled, it should release resources and clean up the state
188 that is shared with other threads. So, whenever a thread that might be canceled
189 changes the state of the system or of the program, be sure to push a cleanup
190 handler with \fBpthread_cleanup_push\fR(3C) before the cancellation point.
191 .sp
192 .LP
193 When a thread is canceled, all the currently-stacked cleanup handlers are
194 executed in last-in-first-out (LIFO) order. Each handler is run in the scope in
195 which it was pushed. When the last cleanup handler returns, the thread-specific
196 data destructor functions are called. Thread execution terminates when the last
197 destructor function returns.
198 .sp
199 .LP
200 When, in the normal course of the program, an uncanceled thread restores state
201 that it had previously changed, be sure to pop the cleanup handler (that you
202 had set up where the change took place) using \fBpthread_cleanup_pop\fR(3C).
203 That way, if the thread is canceled later, only currently-changed state will be
204 restored by the handlers that are left in the stack.
205 .sp
206 .LP
207 The \fBpthread_cleanup_push()\fR and \fBpthread_cleanup_pop()\fR functions can
208 be implemented as macros. The application must ensure that they appear as
209 statements, and in pairs within the same lexical scope (that is, the
210 \fBpthread_cleanup_push()\fR macro can be thought to expand to a token list
211 whose first token is '{' with \fBpthread_cleanup_pop()\fR expanding to a token
212 list whose last token is the corresponding '}').
213 .sp
214 .LP
215 The effect of the use of \fBreturn\fR, \fBbreak\fR, \fBcontinue\fR, and
216 \fBgoto\fR to prematurely leave a code block described by a pair of
217 \fBpthread_cleanup_push()\fR and \fBpthread_cleanup_pop()\fR function calls is
218 undefined.
219 .SS "Cancellation State"
220 .sp
221 .LP
222 Most programmers will use only the default cancellation state of
223 \fBPTHREAD_CANCEL_ENABLE\fR, but can choose to change the state by using
224 \fBpthread_setcancelstate\fR(3C), which determines whether a thread is
225 cancelable at all. With the default \fIstate\fR of
226 \fBPTHREAD_CANCEL_ENABLE\fR, cancellation is enabled and the thread is
227 cancelable at points determined by its cancellation \fItype\fR. See
228 \fBCancellation Type\fR.
229 .sp
230 .LP
231 If the \fIstate\fR is \fBPTHREAD_CANCEL_DISABLE\fR, cancellation is disabled,
232 the thread is not cancelable at any point, and all cancellation requests to it
233 are held pending.
234 .sp
235 .LP
236 You might want to disable cancellation before a call to a cancel-unsafe
237 library, restoring the old cancel state when the call returns from the library.
238 See \fBCancel-Safe\fR for explanations of cancel safety.
239 .SS "Cancellation Type"
240 .sp
241 .LP
242 A thread's cancellation \fBtype\fR is set with \fBpthread_setcanceltype\fR(3C),
243 and determines whether the thread can be canceled anywhere in its execution or
244 only at cancellation points.
245 .sp
246 .LP
247 With the default \fItype\fR of \fBPTHREAD_CANCEL_DEFERRED\fR, the thread is
248 cancelable only at cancellation points, and then only when cancellation is
249 enabled.
250 .sp
251 .LP
252 If the \fItype\fR is \fBPTHREAD_CANCEL_ASYNCHRONOUS\fR, the thread is
253 cancelable at any point in its execution (assuming, of course, that
254 cancellation is enabled). Try to limit regions of asynchronous cancellation to
255 sequences with no external dependencies that could result in dangling resources
256 or unresolved state conditions. Using asynchronous cancellation is discouraged
257 because of the danger involved in trying to guarantee correct cleanup handling
258 at absolutely every point in the program.
259 .sp
260
261 .sp
262 .TS
263 box;
264 c | c | c
265 l | l | l .
266 Cancellation Type/State Table
267 Type State
268 Enabled (Default) Disabled
269 _
270 Deferred (Default) T{
271 Cancellation occurs when the target thread reaches a cancellation point and a cancel is pending. (Default)
272 T} T{
273 All cancellation requests to the target thread are held pending.
274 T}
275 Asynchronous T{
276 Receipt of a \fBpthread_cancel()\fR call causes immediate cancellation.
277 T} T{
278 All cancellation requests to the target thread are held pending; as soon as cancellation is re-enabled, pending cancellations are executedimmediately.
279 T}
280 .TE
281
282 .SS "Cancel-Safe"
283 .sp
284 .LP
285 With the arrival of POSIX cancellation, the Cancel-Safe level has been added to
286 the list of MT-Safety levels. See \fBattributes\fR(5). An application or
287 library is Cancel-Safe whenever it has arranged for cleanup handlers to restore
288 system or program state wherever cancellation can occur. The application or
289 library is specifically Deferred-Cancel-Safe when it is Cancel-Safe for threads
290 whose cancellation type is \fBPTHREAD_CANCEL_DEFERRED\fR. See \fBCancellation
291 State\fR. It is specifically Asynchronous-Cancel-Safe when it is Cancel-Safe
292 for threads whose cancellation type is \fBPTHREAD_CANCEL_ASYNCHRONOUS\fR.
293 .sp
294 .LP
295 It is easier to arrange for deferred cancel safety, as this requires system and
296 program state protection only around cancellation points. In general, expect
297 that most applications and libraries are not Asynchronous-Cancel-Safe.
298 .SS "POSIX Threads Only"
299 .sp
300 .LP
301 The cancellation functions described in this manual page are available for
302 POSIX threads, only (the Solaris threads interfaces do not provide cancellation
303 functions).
304 .SH EXAMPLES
305 .LP
306 \fBExample 1 \fRCancellation example
307 .sp
308 .LP
309 The following short C++ example shows the pushing/popping of cancellation
310 handlers, the disabling/enabling of cancellation, the use of
311 \fBpthread_testcancel()\fR, and so on. The \fBfree_res()\fR cancellation
312 handler in this example is a dummy function that simply prints a message, but
313 that would free resources in a real application. The function \fBf2()\fR is
314 called from the main thread, and goes deep into its call stack by calling
315 itself recursively.
316
317 .sp
318 .LP
319 Before \fBf2()\fR starts running, the newly created thread has probably posted
320 a cancellation on the main thread since the main thread calls \fBthr_yield()\fR
321 right after creating thread2. Because cancellation was initially disabled in
322 the main thread, through a call to \fBpthread_setcancelstate()\fR, the call to
323 \fBf2()\fR from \fBmain()\fR continues and constructs X at each recursive
324 call, even though the main thread has a pending cancellation.
325
326 .sp
327 .LP
328 When \fBf2()\fR is called for the fifty-first time (when \fB"i == 50"\fR),
329 \fBf2()\fR enables cancellation by calling \fBpthread_setcancelstate()\fR. It
330 then establishes a cancellation point for itself by calling
331 \fBpthread_testcancel()\fR. (Because a cancellation is pending, a call to a
332 cancellation point such as \fBread\fR(2) or \fBwrite\fR(2) would also cancel
333 the caller here.)
334
335 .sp
336 .LP
337 After the \fBmain()\fR thread is canceled at the fifty-first iteration, all the
338 cleanup handlers that were pushed are called in sequence; this is indicated by
339 the calls to \fBfree_res()\fR and the calls to the destructor for \fIX\fR. At
340 each level, the C++ runtime calls the destructor for \fIX\fR and then the
341 cancellation handler, \fBfree_res()\fR. The print messages from
342 \fBfree_res()\fR and \fIX\fR's destructor show the sequence of calls.
343
344 .sp
345 .LP
346 At the end, the main thread is joined by thread2. Because the main thread was
347 canceled, its return status from \fBpthread_join()\fR is
348 \fBPTHREAD_CANCELED\fR. After the status is printed, thread2 returns, killing
349 the process (since it is the last thread in the process).
350
351 .sp
352 .in +2
353 .nf
354 #include <pthread.h>
355 #include <sched.h>
356 extern "C" void thr_yield(void);
357
358 extern "C" void printf(...);
359
360 struct X {
361 int x;
362 X(int i){x = i; printf("X(%d) constructed.\en", i);}
363 ~X(){ printf("X(%d) destroyed.\en", x);}
364 };
365
366 void
367 free_res(void *i)
368 {
369 printf("Freeing `%d`\en",i);
370 }
371
372 char* f2(int i)
373 {
374 try {
375 X dummy(i);
376 pthread_cleanup_push(free_res, (void *)i);
377 if (i == 50) {
378 pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL);
379 pthread_testcancel();
380 }
381 f2(i+1);
382 pthread_cleanup_pop(0);
383 }
384 catch (int) {
385 printf("Error: In handler.\en");
386 }
387 return "f2";
388 }
389
390 void *
391 thread2(void *tid)
392 {
393 void *sts;
394
395 printf("I am new thread :%d\en", pthread_self());
396
397 pthread_cancel((pthread_t)tid);
398
399 pthread_join((pthread_t)tid, &sts);
400
401 printf("main thread cancelled due to %d\en", sts);
402
403 return (sts);
404 }
405
406 main()
407 {
408 pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, NULL);
409 pthread_create(NULL, NULL, thread2, (void *)pthread_self());
410 thr_yield();
411 printf("Returned from %s\en",f2(0));
412 }
413 .fi
414 .in -2
415
416 .SH ATTRIBUTES
417 .sp
418 .LP
419 See \fBattributes\fR(5) for descriptions of the following attributes:
420 .sp
421
422 .sp
423 .TS
424 box;
425 c | c
426 l | l .
427 ATTRIBUTE TYPE ATTRIBUTE VALUE
428 _
429 MT-Level MT-Safe
430 .TE
431
432 .SH SEE ALSO
433 .sp
434 .LP
435 \fBread\fR(2), \fBsigwait\fR(2), \fBwrite\fR(2), \fBIntro\fR(3),
436 \fBcondition\fR(5), \fBpthread_cleanup_pop\fR(3C),
437 \fBpthread_cleanup_push\fR(3C), \fBpthread_exit\fR(3C), \fBpthread_join\fR(3C),
438 \fBpthread_setcancelstate\fR(3C), \fBpthread_setcanceltype\fR(3C),
439 \fBpthread_testcancel\fR(3C), \fBsetjmp\fR(3C), \fBattributes\fR(5),
440 \fBstandards\fR(5)