Standards, Environments, and Macros cancellation(5)
NAME
cancellation - overview of concepts related to POSIX thread
cancellation
DESCRIPTION
_____________________________________________________________________
| FUNCTION | ACTION ||_________________________|__________________________________________|
| pthread_cancel() | Cancels thread execution. |
| pthread_setcancelstate()| Sets the cancellation state of a thread.|
| pthread_setcanceltype() | Sets the cancellation type of a thread. |
| pthread_testcancel() | Creates a cancellation point in the cal-|
| | ling thread. || pthread_cleanup_push() | Pushes a cleanup handler routine. |
| pthread_cleanup_pop() | Pops a cleanup handler routine. |
|_________________________|__________________________________________|
CancellationThread cancellation allows a thread to terminate the execu-
tion of any application thread in the process. Cancellation is useful when further operations of one or more threads are undesirable or unnecessary.An example of a situation that could benefit from using can-
cellation 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. Planning StepsPlanning and programming for most cancellations follow this
pattern: 1. Identify which threads you want to cancel, andinsert pthread_cancel(3C) statements.
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 Cancellation Points for a list. 3. When a thread changes the system or program statejust before a cancellation point, and should
restore that state before the thread is canceled,place a cleanup handler before the cancellation
point with pthread_cleanup_push(3C). Wherever a
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Standards, Environments, and Macros cancellation(5)
thread restores the changed state, pop the cleanup handler from the cleanup stack withpthread_cleanup_pop(3C).
4. Know whether the threads you are canceling callinto cancel-unsafe libraries, and disable cancella-
tion with pthread_setcancelstate(3C) before the
call into the library. See Cancellation State andCancel-Safe.
5. To cancel a thread in a procedure that contains nocancellation points, insert your own cancellation
points with pthread_testcancel(3C). This function
creates cancellation points by testing for pending
cancellations and performing those cancellations if
they are found. Push and pop cleanup handlersaround the cancellation point, if necessary (see
Step 3, above). Cancellation PointsThe system defines certain points at which cancellation can
occur (cancellation points), and you can create additional
cancellation points in your application with
pthread_testcancel().
The following cancellation points are defined by the system
(system-defined cancellation points): creat(2),
aio_suspend(3C), close(2), creat(2), getmsg(2), getpmsg(2),
lockf(3C), mq_receive(3C), mq_send(3C), msgrcv(2),
msgsnd(2), msync(3C), nanosleep(3C), open(2), pause(2),poll(2), pread(2), pthread_cond_timedwait(3C),
pthread_cond_wait(3C), pthread_join(3C),
pthread_testcancel(3C), putmsg(2), putpmsg(2), pwrite(2),
read(2), readv(2), select(3C), sem_wait(3C), sigpause(3C),
sigwaitinfo(3C), sigsuspend(2), sigtimedwait(3C), sigwait(2), sleep(3C), sync(2), system(3C), tcdrain(3C), usleep(3C), wait(3C), waitid(2), wait3(3C), waitpid(3C),write(2), writev(2), and fcntl(2), when specifying F_SETLKW
as the command.When cancellation is asynchronous, cancellation can occur at
any time (before, during, or after the execution of thefunction defined as the cancellation point). When cancella-
tion is deferred (the default case), cancellation occurs
only within the scope of a function defined as a cancella-
tion point (after the function is called and before thefunction returns). See Cancellation Type for more informa-
tion about deferred and asynchronous cancellation.
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Standards, Environments, and Macros cancellation(5)
Choosing where to place cancellation points and understand-
ing how cancellation affects your program depend upon your
understanding of both your application and of cancellation
mechanics. Typically, any call that might require a long wait should bea cancellation point. Operations need to check for pending
cancellation requests when the operation is about to block
indefinitely. This includes threads waiting inpthread_cond_wait() and pthread_cond_timedwait(), threads
waiting for the termination of another thread inpthread_join(), and threads blocked on sigwait().
A mutex is explicitly not a cancellation point and should be
held for only the minimal essential time.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. Cleanup Handlers 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 cleanuphandler with pthread_cleanup_push(3C) before the cancella-
tion point.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 ter-
minates when the last destructor function returns. 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 wherethe change took place) using pthread_cleanup_pop(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.SunOS 5.11 Last change: 4 Oct 2005 3
Standards, Environments, and Macros cancellation(5)
The pthread_cleanup_push() and pthread_cleanup_pop() func-
tions can be implemented as macros. The application must ensure that they appear as statements, and in pairs withinthe same lexical scope (that is, the pthread_cleanup_push()
macro can be thought to expand to a token list whose firsttoken is '{' with pthread_cleanup_pop() expanding to a token
list whose last token is the corresponding '}'). The effect of the use of return, break, continue, and goto to prematurely leave a code block described by a pair ofpthread_cleanup_push() and pthread_cleanup_pop() function
calls is undefined. Cancellation StateMost programmers will use only the default cancellation
state of PTHREAD_CANCEL_ENABLE, but can choose to change
the state by using pthread_setcancelstate(3C), which deter-
mines whether a thread is cancelable at all. With thedefault state of PTHREAD_CANCEL_ENABLE, cancellation is
enabled and the thread is cancelable at points determined byits cancellation type. See Cancellation Type.
If the state is PTHREAD_CANCEL_DISABLE, cancellation is dis-
abled, the thread is not cancelable at any point, and allcancellation requests to it are held pending.
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 Cancel-Safe for
explanations of cancel safety. Cancellation TypeA thread's cancellation type is set with
pthread_setcanceltype(3C), and determines whether the thread
can be canceled anywhere in its execution or only at cancel-
lation points.With the default type of PTHREAD_CANCEL_DEFERRED, the
thread is cancelable only at cancellation points, and then
only when cancellation is enabled.
If the type is PTHREAD_CANCEL_ASYNCHRONOUS, the thread is
cancelable at any point in its execution (assuming, ofcourse, that cancellation is enabled). Try to limit regions
of asynchronous cancellation to sequences with no external
dependencies that could result in dangling resources orunresolved state conditions. Using asynchronous cancellation
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Standards, Environments, and Macros cancellation(5)
is discouraged because of the danger involved in trying to guarantee correct cleanup handling at absolutely every point in the program.__________________________________________________________________________
| Cancellation Type/State Table| | | | Type | State | | | | Enabled (Default) | Disabled |
|______________________________|_____________________|____________________|
| Deferred (Default) | Cancellation | All cancellation |
| | occurs when the | requests to the | | | target thread | target thread are || | reaches a cancel- | held pending. |
| | lation point and a | | | | cancel is pending. | | | | (Default) | || Asynchronous | Receipt of a | All cancellation |
| | pthread_cancel() | requests to the |
| | call causes | target thread are || | immediate cancel- | held pending; as |
| | lation. | soon as cancella- |
| | | tion is re- |
| | | enabled, pending || | | cancellations are |
| | | executedimmedi- |
| | | ately. ||______________________________|_____________________|____________________|
Cancel-Safe
With the arrival of POSIX cancellation, the Cancel-Safe
level has been added to the list of MT-Safety levels. See
attributes(5). An application or library is Cancel-Safe
whenever it has arranged for cleanup handlers to restoresystem 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 PTHREAD_CANCEL_DEFERRED. See Cancellation State. It is
specifically Asynchronous-Cancel-Safe when it is Cancel-Safe
for threads whose cancellation type is
PTHREAD_CANCEL_ASYNCHRONOUS.
It is easier to arrange for deferred cancel safety, as this requires system and program state protection only aroundcancellation points. In general, expect that most applica-
tions and libraries are not Asynchronous-Cancel-Safe.
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Standards, Environments, and Macros cancellation(5)
The cancellation functions described in this manual page are
available for POSIX threads, only (the Solaris threadsinterfaces do not provide cancellation functions).
EXAMPLES
Example 1 Cancellation example The following short C++ example shows the pushing/popping ofcancellation handlers, the disabling/enabling of cancella-
tion, the use of pthread_testcancel(), and so on. The
free_res() 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 f2() is called from the main thread, and goes deep into its call stack by calling itself recursively. Before f2() starts running, the newly created thread hasprobably posted a cancellation on the main thread since the
main thread calls thr_yield() right after creating thread2.
Because cancellation was initially disabled in the main
thread, through a call to pthread_setcancelstate(), the call
to f2() from main() continues and constructs X at each recursive call, even though the main thread has a pendingcancellation.
When f2() is called for the fifty-first time (when "i ==
50"), f2() enables cancellation by calling
pthread_setcancelstate(). It then establishes a cancellation
point for itself by calling pthread_testcancel(). (Because
a cancellation is pending, a call to a cancellation point
such as read(2) or write(2) would also cancel the caller here.)After the main() 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 tofree_res() and the calls to the destructor for X. At each
level, the C++ runtime calls the destructor for X and thenthe cancellation handler, free_res(). The print messages
from free_res() and X's destructor show the sequence of
calls.SunOS 5.11 Last change: 4 Oct 2005 6
Standards, Environments, and Macros cancellation(5)
At the end, the main thread is joined by thread2. Because the main thread was canceled, its return status frompthread_join() is PTHREAD_CANCELED. After the status is
printed, thread2 returns, killing the process (since it is the last thread in the process).#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.\n", i);}
~X(){ printf("X(%d) destroyed.\n", x);}
}; voidfree_res(void *i)
{printf("Freeing `%d`\n",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.\n"); } return "f2"; } void * thread2(void *tid) { void *sts;printf("I am new thread :%d\n", pthread_self());
pthread_cancel((pthread_t)tid);
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Standards, Environments, and Macros cancellation(5)
pthread_join((pthread_t)tid, &sts);
printf("main thread cancelled due to %d\n", sts);
return (sts); } main() {pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, NULL);
pthread_create(NULL, NULL, thread2, (void *)pthread_self());
thr_yield();
printf("Returned from %s\n",f2(0));
}ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| MT-Level | MT-Safe |
|_____________________________|_____________________________|
SEE ALSO
read(2), sigwait(2), write(2), Intro(3), condition(5),pthread_cleanup_pop(3C), pthread_cleanup_push(3C),
pthread_exit(3C), pthread_join(3C),
pthread_setcancelstate(3C), pthread_setcanceltype(3C),
pthread_testcancel(3C), setjmp(3C), attributes(5), stan-
dards(5)SunOS 5.11 Last change: 4 Oct 2005 8