'\" te .\" Copyright (c) 2008, Sun Microsystems, Inc. All Rights Reserved. .\" Portions Copyright (c) 2001, the Institute of Electrical and Electronics Engineers, Inc. and The Open Group. All Rights Reserved. .\" Portions Copyright (c) 1995 IEEE. All Rights Reserved. .\" Sun Microsystems, Inc. gratefully acknowledges The Open Group for permission to reproduce portions of its copyrighted documentation. Original documentation from The Open Group can be obtained online at http://www.opengroup.org/bookstore/. .\" The Institute of Electrical and Electronics Engineers and The Open Group, have given us permission to reprint portions of their documentation. In the following statement, the phrase "this text" refers to portions of the system documentation. Portions of this text .\" are reprinted and reproduced in electronic form in the Sun OS Reference Manual, from IEEE Std 1003.1, 2004 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2004 by the Institute of Electrical .\" and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between these versions and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html. .\" This notice shall appear on any product containing this material. .\" The contents of this file are subject to the terms of the Common Development and Distribution License (the "License"). You may not use this file except in compliance with the License. You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE or http://www.opensolaris.org/os/licensing. .\" See the License for the specific language governing permissions and limitations under the License. When distributing Covered Code, include this CDDL HEADER in each file and include the License file at usr/src/OPENSOLARIS.LICENSE. If applicable, add the following below this CDDL HEADER, with .\" the fields enclosed by brackets "[]" replaced with your own identifying information: Portions Copyright [yyyy] [name of copyright owner] .TH CANCELLATION 5 "Oct 4, 2005" .SH NAME cancellation \- overview of concepts related to POSIX thread cancellation .SH DESCRIPTION .sp .sp .TS box; c | c l | l . FUNCTION ACTION _ \fBpthread_cancel()\fR Cancels thread execution. \fBpthread_setcancelstate()\fR Sets the cancellation \fIstate\fR of a thread. \fBpthread_setcanceltype()\fR Sets the cancellation \fItype\fR of a thread. \fBpthread_testcancel()\fR T{ Creates a cancellation point in the calling thread. T} \fBpthread_cleanup_push()\fR Pushes a cleanup handler routine. \fBpthread_cleanup_pop()\fR Pops a cleanup handler routine. .TE .SS "Cancellation" .sp .LP Thread cancellation allows a thread to terminate the execution of any application thread in the process. Cancellation is useful when further operations of one or more threads are undesirable or unnecessary. .sp .LP An example of a situation that could benefit from using cancellation is an asynchronously-generated cancel condition such as a user requesting to close or exit some running operation. Another example is the completion of a task undertaken by a number of threads, such as solving a maze. While many threads search for the solution, one of the threads might solve the puzzle while the others continue to operate. Since they are serving no purpose at that point, they should all be canceled. .SS "Planning Steps" .sp .LP Planning and programming for most cancellations follow this pattern: .RS +4 .TP 1. Identify which threads you want to cancel, and insert \fBpthread_cancel\fR(3C) statements. .RE .RS +4 .TP 2. Identify system-defined cancellation points where a thread that might be canceled could have changed system or program state that should be restored. See the \fBCancellation Points\fR for a list. .RE .RS +4 .TP 3. When a thread changes the system or program state just before a cancellation point, and should restore that state before the thread is canceled, place a cleanup handler before the cancellation point with \fBpthread_cleanup_push\fR(3C). Wherever a thread restores the changed state, pop the cleanup handler from the cleanup stack with \fBpthread_cleanup_pop\fR(3C). .RE .RS +4 .TP 4. Know whether the threads you are canceling call into cancel-unsafe libraries, and disable cancellation with \fBpthread_setcancelstate\fR(3C) before the call into the library. See \fBCancellation State\fR and \fBCancel-Safe\fR. .RE .RS +4 .TP 5. To cancel a thread in a procedure that contains no cancellation points, insert your own cancellation points with \fBpthread_testcancel\fR(3C). This function creates cancellation points by testing for pending cancellations and performing those cancellations if they are found. Push and pop cleanup handlers around the cancellation point, if necessary (see Step 3, above). .RE .SS "Cancellation Points" .sp .LP The system defines certain points at which cancellation can occur (cancellation points), and you can create additional cancellation points in your application with \fBpthread_testcancel()\fR. .sp .LP The following cancellation points are defined by the system (system-defined cancellation points): \fBcreat\fR(2), \fBaio_suspend\fR(3C), \fBclose\fR(2), \fBcreat\fR(2), \fBgetmsg\fR(2), \fBgetpmsg\fR(2), \fBlockf\fR(3C), \fBmq_receive\fR(3C), \fBmq_send\fR(3C), \fBmsgrcv\fR(2), \fBmsgsnd\fR(2), \fBmsync\fR(3C), \fBnanosleep\fR(3C), \fBopen\fR(2), \fBpause\fR(2), \fBpoll\fR(2), \fBpread\fR(2), \fBpthread_cond_timedwait\fR(3C), \fBpthread_cond_wait\fR(3C), \fBpthread_join\fR(3C), \fBpthread_testcancel\fR(3C), \fBputmsg\fR(2), \fBputpmsg\fR(2), \fBpwrite\fR(2), \fBread\fR(2), \fBreadv\fR(2), \fBselect\fR(3C), \fBsem_wait\fR(3C), \fBsigpause\fR(3C), \fBsigwaitinfo\fR(3C), \fBsigsuspend\fR(2), \fBsigtimedwait\fR(3C), \fBsigwait\fR(2), \fBsleep\fR(3C), \fBsync\fR(2), \fBsystem\fR(3C), \fBtcdrain\fR(3C), \fBusleep\fR(3C), \fBwait\fR(3C), \fBwaitid\fR(2), \fBwait3\fR(3C), \fBwaitpid\fR(3C), \fBwrite\fR(2), \fBwritev\fR(2), and \fBfcntl\fR(2), when specifying \fBF_SETLKW\fR as the command. .sp .LP When cancellation is asynchronous, cancellation can occur at any time (before, during, or after the execution of the function defined as the cancellation point). When cancellation is deferred (the default case), cancellation occurs only within the scope of a function defined as a cancellation point (after the function is called and before the function returns). See \fBCancellation Type\fR for more information about deferred and asynchronous cancellation. .sp .LP Choosing where to place cancellation points and understanding how cancellation affects your program depend upon your understanding of both your application and of cancellation mechanics. .sp .LP Typically, any call that might require a long wait should be a cancellation point. Operations need to check for pending cancellation requests when the operation is about to block indefinitely. This includes threads waiting in \fBpthread_cond_wait()\fR and \fBpthread_cond_timedwait()\fR, threads waiting for the termination of another thread in \fBpthread_join()\fR, and threads blocked on \fBsigwait()\fR. .sp .LP A mutex is explicitly not a cancellation point and should be held for only the minimal essential time. .sp .LP Most of the dangers in performing cancellations deal with properly restoring invariants and freeing shared resources. For example, a carelessly canceled thread might leave a mutex in a locked state, leading to a deadlock. Or it might leave a region of memory allocated with no way to identify it and therefore no way to free it. .SS "Cleanup Handlers" .sp .LP When a thread is canceled, it should release resources and clean up the state that is shared with other threads. So, whenever a thread that might be canceled changes the state of the system or of the program, be sure to push a cleanup handler with \fBpthread_cleanup_push\fR(3C) before the cancellation point. .sp .LP When a thread is canceled, all the currently-stacked cleanup handlers are executed in last-in-first-out (LIFO) order. Each handler is run in the scope in which it was pushed. When the last cleanup handler returns, the thread-specific data destructor functions are called. Thread execution terminates when the last destructor function returns. .sp .LP When, in the normal course of the program, an uncanceled thread restores state that it had previously changed, be sure to pop the cleanup handler (that you had set up where the change took place) using \fBpthread_cleanup_pop\fR(3C). That way, if the thread is canceled later, only currently-changed state will be restored by the handlers that are left in the stack. .sp .LP The \fBpthread_cleanup_push()\fR and \fBpthread_cleanup_pop()\fR functions can be implemented as macros. The application must ensure that they appear as statements, and in pairs within the same lexical scope (that is, the \fBpthread_cleanup_push()\fR macro can be thought to expand to a token list whose first token is '{' with \fBpthread_cleanup_pop()\fR expanding to a token list whose last token is the corresponding '}'). .sp .LP The effect of the use of \fBreturn\fR, \fBbreak\fR, \fBcontinue\fR, and \fBgoto\fR to prematurely leave a code block described by a pair of \fBpthread_cleanup_push()\fR and \fBpthread_cleanup_pop()\fR function calls is undefined. .SS "Cancellation State" .sp .LP Most programmers will use only the default cancellation state of \fBPTHREAD_CANCEL_ENABLE\fR, but can choose to change the state by using \fBpthread_setcancelstate\fR(3C), which determines whether a thread is cancelable at all. With the default \fIstate\fR of \fBPTHREAD_CANCEL_ENABLE\fR, cancellation is enabled and the thread is cancelable at points determined by its cancellation \fItype\fR. See \fBCancellation Type\fR. .sp .LP If the \fIstate\fR is \fBPTHREAD_CANCEL_DISABLE\fR, cancellation is disabled, the thread is not cancelable at any point, and all cancellation requests to it are held pending. .sp .LP You might want to disable cancellation before a call to a cancel-unsafe library, restoring the old cancel state when the call returns from the library. See \fBCancel-Safe\fR for explanations of cancel safety. .SS "Cancellation Type" .sp .LP A thread's cancellation \fBtype\fR is set with \fBpthread_setcanceltype\fR(3C), and determines whether the thread can be canceled anywhere in its execution or only at cancellation points. .sp .LP With the default \fItype\fR of \fBPTHREAD_CANCEL_DEFERRED\fR, the thread is cancelable only at cancellation points, and then only when cancellation is enabled. .sp .LP If the \fItype\fR is \fBPTHREAD_CANCEL_ASYNCHRONOUS\fR, the thread is cancelable at any point in its execution (assuming, of course, that cancellation is enabled). Try to limit regions of asynchronous cancellation to sequences with no external dependencies that could result in dangling resources or unresolved state conditions. Using asynchronous cancellation is discouraged because of the danger involved in trying to guarantee correct cleanup handling at absolutely every point in the program. .sp .sp .TS box; c | c | c l | l | l . Cancellation Type/State Table Type State Enabled (Default) Disabled _ Deferred (Default) T{ Cancellation occurs when the target thread reaches a cancellation point and a cancel is pending. (Default) T} T{ All cancellation requests to the target thread are held pending. T} Asynchronous T{ Receipt of a \fBpthread_cancel()\fR call causes immediate cancellation. T} T{ All cancellation requests to the target thread are held pending; as soon as cancellation is re-enabled, pending cancellations are executedimmediately. T} .TE .SS "Cancel-Safe" .sp .LP With the arrival of POSIX cancellation, the Cancel-Safe level has been added to the list of MT-Safety levels. See \fBattributes\fR(5). An application or library is Cancel-Safe whenever it has arranged for cleanup handlers to restore system or program state wherever cancellation can occur. The application or library is specifically Deferred-Cancel-Safe when it is Cancel-Safe for threads whose cancellation type is \fBPTHREAD_CANCEL_DEFERRED\fR. See \fBCancellation State\fR. It is specifically Asynchronous-Cancel-Safe when it is Cancel-Safe for threads whose cancellation type is \fBPTHREAD_CANCEL_ASYNCHRONOUS\fR. .sp .LP It is easier to arrange for deferred cancel safety, as this requires system and program state protection only around cancellation points. In general, expect that most applications and libraries are not Asynchronous-Cancel-Safe. .SS "POSIX Threads Only" .sp .LP The cancellation functions described in this manual page are available for POSIX threads, only (the Solaris threads interfaces do not provide cancellation functions). .SH EXAMPLES .LP \fBExample 1 \fRCancellation example .sp .LP The following short C++ example shows the pushing/popping of cancellation handlers, the disabling/enabling of cancellation, the use of \fBpthread_testcancel()\fR, and so on. The \fBfree_res()\fR cancellation handler in this example is a dummy function that simply prints a message, but that would free resources in a real application. The function \fBf2()\fR is called from the main thread, and goes deep into its call stack by calling itself recursively. .sp .LP Before \fBf2()\fR starts running, the newly created thread has probably posted a cancellation on the main thread since the main thread calls \fBthr_yield()\fR right after creating thread2. Because cancellation was initially disabled in the main thread, through a call to \fBpthread_setcancelstate()\fR, the call to \fBf2()\fR from \fBmain()\fR continues and constructs X at each recursive call, even though the main thread has a pending cancellation. .sp .LP When \fBf2()\fR is called for the fifty-first time (when \fB"i == 50"\fR), \fBf2()\fR enables cancellation by calling \fBpthread_setcancelstate()\fR. It then establishes a cancellation point for itself by calling \fBpthread_testcancel()\fR. (Because a cancellation is pending, a call to a cancellation point such as \fBread\fR(2) or \fBwrite\fR(2) would also cancel the caller here.) .sp .LP After the \fBmain()\fR thread is canceled at the fifty-first iteration, all the cleanup handlers that were pushed are called in sequence; this is indicated by the calls to \fBfree_res()\fR and the calls to the destructor for \fIX\fR. At each level, the C++ runtime calls the destructor for \fIX\fR and then the cancellation handler, \fBfree_res()\fR. The print messages from \fBfree_res()\fR and \fIX\fR's destructor show the sequence of calls. .sp .LP At the end, the main thread is joined by thread2. Because the main thread was canceled, its return status from \fBpthread_join()\fR is \fBPTHREAD_CANCELED\fR. After the status is printed, thread2 returns, killing the process (since it is the last thread in the process). .sp .in +2 .nf #include #include extern "C" void thr_yield(void); extern "C" void printf(...); struct X { int x; X(int i){x = i; printf("X(%d) constructed.\en", i);} ~X(){ printf("X(%d) destroyed.\en", x);} }; void free_res(void *i) { printf("Freeing `%d`\en",i); } char* f2(int i) { try { X dummy(i); pthread_cleanup_push(free_res, (void *)i); if (i == 50) { pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL); pthread_testcancel(); } f2(i+1); pthread_cleanup_pop(0); } catch (int) { printf("Error: In handler.\en"); } return "f2"; } void * thread2(void *tid) { void *sts; printf("I am new thread :%d\en", pthread_self()); pthread_cancel((pthread_t)tid); pthread_join((pthread_t)tid, &sts); printf("main thread cancelled due to %d\en", sts); return (sts); } main() { pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, NULL); pthread_create(NULL, NULL, thread2, (void *)pthread_self()); thr_yield(); printf("Returned from %s\en",f2(0)); } .fi .in -2 .SH ATTRIBUTES .sp .LP See \fBattributes\fR(5) for descriptions of the following attributes: .sp .sp .TS box; c | c l | l . ATTRIBUTE TYPE ATTRIBUTE VALUE _ MT-Level MT-Safe .TE .SH SEE ALSO .sp .LP \fBread\fR(2), \fBsigwait\fR(2), \fBwrite\fR(2), \fBIntro\fR(3), \fBcondition\fR(5), \fBpthread_cleanup_pop\fR(3C), \fBpthread_cleanup_push\fR(3C), \fBpthread_exit\fR(3C), \fBpthread_join\fR(3C), \fBpthread_setcancelstate\fR(3C), \fBpthread_setcanceltype\fR(3C), \fBpthread_testcancel\fR(3C), \fBsetjmp\fR(3C), \fBattributes\fR(5), \fBstandards\fR(5)