9842 man page typos and spelling
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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 102 cleanup handler before the cancellation point with 103 \fBpthread_cleanup_push\fR(3C). Wherever a thread restores the changed state, 104 pop the cleanup handler from the cleanup stack with 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 145 \fBF_SETLKW\fR as the command. 146 .sp 147 .LP 148 When cancellation is asynchronous, cancellation can occur at any time (before, 149 during, or after the execution of the function defined as the cancellation 150 point). When cancellation is deferred (the default case), cancellation occurs 151 only within the scope of a function defined as a cancellation point (after the 152 function is called and before the function returns). See \fBCancellation 153 Type\fR for more information about deferred and asynchronous cancellation. 154 .sp 155 .LP 156 Choosing where to place cancellation points and understanding how cancellation 157 affects your program depend upon your understanding of both your application 158 and of cancellation mechanics. 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 311 a cancellation on the main thread since the main thread calls \fBthr_yield()\fR 312 right after creating thread2. Because cancellation was initially disabled in 313 the main thread, through a call to \fBpthread_setcancelstate()\fR, the call to 314 \fBf2()\fR from \fBmain()\fR continues and constructs X at each recursive 315 call, even though the main thread has a pending cancellation. 316 317 .sp 318 .LP 319 When \fBf2()\fR is called for the fifty-first time (when \fB"i == 50"\fR), 320 \fBf2()\fR enables cancellation by calling \fBpthread_setcancelstate()\fR. It 321 then establishes a cancellation point for itself by calling 322 \fBpthread_testcancel()\fR. (Because a cancellation is pending, a call to a 323 cancellation point such as \fBread\fR(2) or \fBwrite\fR(2) would also cancel 324 the caller here.) 325 326 .sp 327 .LP 328 After the \fBmain()\fR thread is canceled at the fifty-first iteration, all the 329 cleanup handlers that were pushed are called in sequence; this is indicated by 330 the calls to \fBfree_res()\fR and the calls to the destructor for \fIX\fR. At 331 each level, the C++ runtime calls the destructor for \fIX\fR and then the 332 cancellation handler, \fBfree_res()\fR. The print messages from 333 \fBfree_res()\fR and \fIX\fR's destructor show the sequence of calls. 334 335 .sp 336 .LP 337 At the end, the main thread is joined by thread2. Because the main thread was 338 canceled, its return status from \fBpthread_join()\fR is 339 \fBPTHREAD_CANCELED\fR. After the status is printed, thread2 returns, killing 340 the process (since it is the last thread in the process). 341 342 .sp 343 .in +2 344 .nf 345 #include <pthread.h> 346 #include <sched.h> 347 extern "C" void thr_yield(void); 348 349 extern "C" void printf(...); 350 351 struct X { 352 int x; 353 X(int i){x = i; printf("X(%d) constructed.\en", i);} 354 ~X(){ printf("X(%d) destroyed.\en", x);} 355 }; 356 357 void 358 free_res(void *i) 359 { 360 printf("Freeing `%d`\en",i); 361 } 362 363 char* f2(int i) 364 { 365 try { 366 X dummy(i); 367 pthread_cleanup_push(free_res, (void *)i); 368 if (i == 50) { 369 pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL); 370 pthread_testcancel(); 371 } 372 f2(i+1); 373 pthread_cleanup_pop(0); 374 } 375 catch (int) { 376 printf("Error: In handler.\en"); 377 } 378 return "f2"; 379 } 380 381 void * 382 thread2(void *tid) 383 { 384 void *sts; 385 386 printf("I am new thread :%d\en", pthread_self()); 387 388 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) --- EOF ---