31 .\" You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
32 .\" or http://www.opensolaris.org/os/licensing.
33 .\" See the License for the specific language governing permissions
34 .\" and limitations under the License.
35 .\"
36 .\" When distributing Covered Code, include this CDDL HEADER in each
37 .\" file and include the License file at usr/src/OPENSOLARIS.LICENSE.
38 .\" If applicable, add the following below this CDDL HEADER, with the
39 .\" fields enclosed by brackets "[]" replaced with your own identifying
40 .\" information: Portions Copyright [yyyy] [name of copyright owner]
41 .\"
42 .\"
43 .\" Portions Copyright (c) 1995 IEEE. All Rights Reserved.
44 .\" Copyright (c) 2001, The IEEE and The Open Group. All Rights Reserved.
45 .\" Copyright (c) 2008, Sun Microsystems, Inc. All Rights Reserved.
46 .\"
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
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
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
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)
|
31 .\" You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
32 .\" or http://www.opensolaris.org/os/licensing.
33 .\" See the License for the specific language governing permissions
34 .\" and limitations under the License.
35 .\"
36 .\" When distributing Covered Code, include this CDDL HEADER in each
37 .\" file and include the License file at usr/src/OPENSOLARIS.LICENSE.
38 .\" If applicable, add the following below this CDDL HEADER, with the
39 .\" fields enclosed by brackets "[]" replaced with your own identifying
40 .\" information: Portions Copyright [yyyy] [name of copyright owner]
41 .\"
42 .\"
43 .\" Portions Copyright (c) 1995 IEEE. All Rights Reserved.
44 .\" Copyright (c) 2001, The IEEE and The Open Group. All Rights Reserved.
45 .\" Copyright (c) 2008, Sun Microsystems, Inc. All Rights Reserved.
46 .\"
47 .TH CANCELLATION 5 "Oct 4, 2005"
48 .SH NAME
49 cancellation \- overview of concepts related to POSIX thread cancellation
50 .SH DESCRIPTION
51 .TS
52 box;
53 c | c
54 l | l .
55 FUNCTION ACTION
56 _
57 \fBpthread_cancel()\fR Cancels thread execution.
58 \fBpthread_setcancelstate()\fR Sets the cancellation \fIstate\fR of a thread.
59 \fBpthread_setcanceltype()\fR Sets the cancellation \fItype\fR of a thread.
60 \fBpthread_testcancel()\fR T{
61 Creates a cancellation point in the calling thread.
62 T}
63 \fBpthread_cleanup_push()\fR Pushes a cleanup handler routine.
64 \fBpthread_cleanup_pop()\fR Pops a cleanup handler routine.
65 .TE
66
67 .SS "Cancellation"
68 .LP
69 Thread cancellation allows a thread to terminate the execution of any
70 application thread in the process. Cancellation is useful when further
71 operations of one or more threads are undesirable or unnecessary.
72 .sp
73 .LP
74 An example of a situation that could benefit from using cancellation is an
75 asynchronously-generated cancel condition such as a user requesting to close or
76 exit some running operation. Another example is the completion of a task
77 undertaken by a number of threads, such as solving a maze. While many threads
78 search for the solution, one of the threads might solve the puzzle while the
79 others continue to operate. Since they are serving no purpose at that point,
80 they should all be canceled.
81 .SS "Planning Steps"
82 .LP
83 Planning and programming for most cancellations follow this pattern:
84 .RS +4
85 .TP
86 1.
87 Identify which threads you want to cancel, and insert
88 \fBpthread_cancel\fR(3C) statements.
89 .RE
90 .RS +4
91 .TP
92 2.
93 Identify system-defined cancellation points where a thread that might be
94 canceled could have changed system or program state that should be restored.
95 See the \fBCancellation Points\fR for a list.
96 .RE
97 .RS +4
98 .TP
99 3.
100 When a thread changes the system or program state just before a cancellation
101 point, and should restore that state before the thread is canceled, place a
105 \fBpthread_cleanup_pop\fR(3C).
106 .RE
107 .RS +4
108 .TP
109 4.
110 Know whether the threads you are canceling call into cancel-unsafe
111 libraries, and disable cancellation with \fBpthread_setcancelstate\fR(3C)
112 before the call into the library. See \fBCancellation State\fR and
113 \fBCancel-Safe\fR.
114 .RE
115 .RS +4
116 .TP
117 5.
118 To cancel a thread in a procedure that contains no cancellation points,
119 insert your own cancellation points with \fBpthread_testcancel\fR(3C). This
120 function creates cancellation points by testing for pending cancellations and
121 performing those cancellations if they are found. Push and pop cleanup handlers
122 around the cancellation point, if necessary (see Step 3, above).
123 .RE
124 .SS "Cancellation Points"
125 .LP
126 The system defines certain points at which cancellation can occur (cancellation
127 points), and you can create additional cancellation points in your application
128 with \fBpthread_testcancel()\fR.
129 .sp
130 .LP
131 The following cancellation points are defined by the system (system-defined
132 cancellation points): \fBcreat\fR(2), \fBaio_suspend\fR(3C), \fBclose\fR(2),
133 \fBcreat\fR(2), \fBgetmsg\fR(2), \fBgetpmsg\fR(2), \fBlockf\fR(3C),
134 \fBmq_receive\fR(3C), \fBmq_send\fR(3C), \fBmsgrcv\fR(2), \fBmsgsnd\fR(2),
135 \fBmsync\fR(3C), \fBnanosleep\fR(3C), \fBopen\fR(2), \fBpause\fR(2),
136 \fBpoll\fR(2), \fBpread\fR(2), \fBpthread_cond_timedwait\fR(3C),
137 \fBpthread_cond_wait\fR(3C), \fBpthread_join\fR(3C),
138 \fBpthread_testcancel\fR(3C), \fBputmsg\fR(2), \fBputpmsg\fR(2),
139 \fBpwrite\fR(2), \fBread\fR(2), \fBreadv\fR(2), \fBselect\fR(3C),
140 \fBsem_wait\fR(3C), \fBsigpause\fR(3C), \fBsigwaitinfo\fR(3C),
141 \fBsigsuspend\fR(2), \fBsigtimedwait\fR(3C), \fBsigwait\fR(2), \fBsleep\fR(3C),
142 \fBsync\fR(2), \fBsystem\fR(3C), \fBtcdrain\fR(3C), \fBusleep\fR(3C),
143 \fBwait\fR(3C), \fBwaitid\fR(2), \fBwait3\fR(3C), \fBwaitpid\fR(3C),
144 \fBwrite\fR(2), \fBwritev\fR(2), and \fBfcntl\fR(2), when specifying
159 .sp
160 .LP
161 Typically, any call that might require a long wait should be a cancellation
162 point. Operations need to check for pending cancellation requests when the
163 operation is about to block indefinitely. This includes threads waiting in
164 \fBpthread_cond_wait()\fR and \fBpthread_cond_timedwait()\fR, threads waiting
165 for the termination of another thread in \fBpthread_join()\fR, and threads
166 blocked on \fBsigwait()\fR.
167 .sp
168 .LP
169 A mutex is explicitly not a cancellation point and should be held for only the
170 minimal essential time.
171 .sp
172 .LP
173 Most of the dangers in performing cancellations deal with properly restoring
174 invariants and freeing shared resources. For example, a carelessly canceled
175 thread might leave a mutex in a locked state, leading to a deadlock. Or it
176 might leave a region of memory allocated with no way to identify it and
177 therefore no way to free it.
178 .SS "Cleanup Handlers"
179 .LP
180 When a thread is canceled, it should release resources and clean up the state
181 that is shared with other threads. So, whenever a thread that might be canceled
182 changes the state of the system or of the program, be sure to push a cleanup
183 handler with \fBpthread_cleanup_push\fR(3C) before the cancellation point.
184 .sp
185 .LP
186 When a thread is canceled, all the currently-stacked cleanup handlers are
187 executed in last-in-first-out (LIFO) order. Each handler is run in the scope in
188 which it was pushed. When the last cleanup handler returns, the thread-specific
189 data destructor functions are called. Thread execution terminates when the last
190 destructor function returns.
191 .sp
192 .LP
193 When, in the normal course of the program, an uncanceled thread restores state
194 that it had previously changed, be sure to pop the cleanup handler (that you
195 had set up where the change took place) using \fBpthread_cleanup_pop\fR(3C).
196 That way, if the thread is canceled later, only currently-changed state will be
197 restored by the handlers that are left in the stack.
198 .sp
199 .LP
200 The \fBpthread_cleanup_push()\fR and \fBpthread_cleanup_pop()\fR functions can
201 be implemented as macros. The application must ensure that they appear as
202 statements, and in pairs within the same lexical scope (that is, the
203 \fBpthread_cleanup_push()\fR macro can be thought to expand to a token list
204 whose first token is '{' with \fBpthread_cleanup_pop()\fR expanding to a token
205 list whose last token is the corresponding '}').
206 .sp
207 .LP
208 The effect of the use of \fBreturn\fR, \fBbreak\fR, \fBcontinue\fR, and
209 \fBgoto\fR to prematurely leave a code block described by a pair of
210 \fBpthread_cleanup_push()\fR and \fBpthread_cleanup_pop()\fR function calls is
211 undefined.
212 .SS "Cancellation State"
213 .LP
214 Most programmers will use only the default cancellation state of
215 \fBPTHREAD_CANCEL_ENABLE\fR, but can choose to change the state by using
216 \fBpthread_setcancelstate\fR(3C), which determines whether a thread is
217 cancelable at all. With the default \fIstate\fR of
218 \fBPTHREAD_CANCEL_ENABLE\fR, cancellation is enabled and the thread is
219 cancelable at points determined by its cancellation \fItype\fR. See
220 \fBCancellation Type\fR.
221 .sp
222 .LP
223 If the \fIstate\fR is \fBPTHREAD_CANCEL_DISABLE\fR, cancellation is disabled,
224 the thread is not cancelable at any point, and all cancellation requests to it
225 are held pending.
226 .sp
227 .LP
228 You might want to disable cancellation before a call to a cancel-unsafe
229 library, restoring the old cancel state when the call returns from the library.
230 See \fBCancel-Safe\fR for explanations of cancel safety.
231 .SS "Cancellation Type"
232 .LP
233 A thread's cancellation \fBtype\fR is set with \fBpthread_setcanceltype\fR(3C),
234 and determines whether the thread can be canceled anywhere in its execution or
235 only at cancellation points.
236 .sp
237 .LP
238 With the default \fItype\fR of \fBPTHREAD_CANCEL_DEFERRED\fR, the thread is
239 cancelable only at cancellation points, and then only when cancellation is
240 enabled.
241 .sp
242 .LP
243 If the \fItype\fR is \fBPTHREAD_CANCEL_ASYNCHRONOUS\fR, the thread is
244 cancelable at any point in its execution (assuming, of course, that
245 cancellation is enabled). Try to limit regions of asynchronous cancellation to
246 sequences with no external dependencies that could result in dangling resources
247 or unresolved state conditions. Using asynchronous cancellation is discouraged
248 because of the danger involved in trying to guarantee correct cleanup handling
249 at absolutely every point in the program.
250 .sp
251
252 .sp
253 .TS
254 box;
255 c | c | c
256 l | l | l .
257 Cancellation Type/State Table
258 Type State
259 Enabled (Default) Disabled
260 _
261 Deferred (Default) T{
262 Cancellation occurs when the target thread reaches a cancellation point and a cancel is pending. (Default)
263 T} T{
264 All cancellation requests to the target thread are held pending.
265 T}
266 Asynchronous T{
267 Receipt of a \fBpthread_cancel()\fR call causes immediate cancellation.
268 T} T{
269 All cancellation requests to the target thread are held pending; as
270 soon as cancellation is re-enabled, pending cancellations are executed
271 immediately.
272 T}
273 .TE
274
275 .SS "Cancel-Safe"
276 .LP
277 With the arrival of POSIX cancellation, the Cancel-Safe level has been added to
278 the list of MT-Safety levels. See \fBattributes\fR(5). An application or
279 library is Cancel-Safe whenever it has arranged for cleanup handlers to restore
280 system or program state wherever cancellation can occur. The application or
281 library is specifically Deferred-Cancel-Safe when it is Cancel-Safe for threads
282 whose cancellation type is \fBPTHREAD_CANCEL_DEFERRED\fR. See \fBCancellation
283 State\fR. It is specifically Asynchronous-Cancel-Safe when it is Cancel-Safe
284 for threads whose cancellation type is \fBPTHREAD_CANCEL_ASYNCHRONOUS\fR.
285 .sp
286 .LP
287 It is easier to arrange for deferred cancel safety, as this requires system and
288 program state protection only around cancellation points. In general, expect
289 that most applications and libraries are not Asynchronous-Cancel-Safe.
290 .SS "POSIX Threads Only"
291 .LP
292 The cancellation functions described in this manual page are available for
293 POSIX threads, only (the Solaris threads interfaces do not provide cancellation
294 functions).
295 .SH EXAMPLES
296 .LP
297 \fBExample 1 \fRCancellation example
298 .sp
299 .LP
300 The following short C++ example shows the pushing/popping of cancellation
301 handlers, the disabling/enabling of cancellation, the use of
302 \fBpthread_testcancel()\fR, and so on. The \fBfree_res()\fR cancellation
303 handler in this example is a dummy function that simply prints a message, but
304 that would free resources in a real application. The function \fBf2()\fR is
305 called from the main thread, and goes deep into its call stack by calling
306 itself recursively.
307
308 .sp
309 .LP
310 Before \fBf2()\fR starts running, the newly created thread has probably posted
388 pthread_cancel((pthread_t)tid);
389
390 pthread_join((pthread_t)tid, &sts);
391
392 printf("main thread cancelled due to %d\en", sts);
393
394 return (sts);
395 }
396
397 main()
398 {
399 pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, NULL);
400 pthread_create(NULL, NULL, thread2, (void *)pthread_self());
401 thr_yield();
402 printf("Returned from %s\en",f2(0));
403 }
404 .fi
405 .in -2
406
407 .SH ATTRIBUTES
408 .LP
409 See \fBattributes\fR(5) for descriptions of the following attributes:
410 .sp
411
412 .sp
413 .TS
414 box;
415 c | c
416 l | l .
417 ATTRIBUTE TYPE ATTRIBUTE VALUE
418 _
419 MT-Level MT-Safe
420 .TE
421
422 .SH SEE ALSO
423 .LP
424 \fBread\fR(2), \fBsigwait\fR(2), \fBwrite\fR(2), \fBIntro\fR(3),
425 \fBcondition\fR(5), \fBpthread_cleanup_pop\fR(3C),
426 \fBpthread_cleanup_push\fR(3C), \fBpthread_exit\fR(3C), \fBpthread_join\fR(3C),
427 \fBpthread_setcancelstate\fR(3C), \fBpthread_setcanceltype\fR(3C),
428 \fBpthread_testcancel\fR(3C), \fBsetjmp\fR(3C), \fBattributes\fR(5),
429 \fBstandards\fR(5)
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