CANCELLATION(7) Device and Network Interfaces CANCELLATION(7)


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 |
| | calling thread. |
|pthread_cleanup_push() | Pushes a cleanup handler routine. |
|pthread_cleanup_pop() | Pops a cleanup handler routine. |
+-------------------------+------------------------------------------+

Cancellation


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.


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.

Planning Steps


Planning and programming for most cancellations follow this pattern:

1. Identify which threads you want to cancel, and insert
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 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 pthread_cleanup_push(3C). Wherever a
thread restores the changed state, pop the cleanup handler
from the cleanup stack with pthread_cleanup_pop(3C).

4. Know whether the threads you are canceling call into cancel-
unsafe libraries, and disable cancellation with
pthread_setcancelstate(3C) before the call into the library.
See Cancellation State and Cancel-Safe.

5. To cancel a thread in a procedure that contains no
cancellation 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
handlers around the cancellation point, if necessary (see Step
3, above).

Cancellation Points


The 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 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 Cancellation Type for more information about deferred and
asynchronous cancellation.


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.


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 pthread_cond_wait() and pthread_cond_timedwait(),
threads waiting for the termination of another thread in pthread_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 cleanup handler with pthread_cleanup_push(3C) before the
cancellation 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
terminates 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 where the 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.


The pthread_cleanup_push() and pthread_cleanup_pop() 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
pthread_cleanup_push() macro can be thought to expand to a token list
whose first token 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 of pthread_cleanup_push() and
pthread_cleanup_pop() function calls is undefined.

Cancellation State


Most 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 determines whether a thread is
cancelable at all. With the default state of PTHREAD_CANCEL_ENABLE,
cancellation is enabled and the thread is cancelable at points determined
by its cancellation type. See Cancellation Type.


If the state is PTHREAD_CANCEL_DISABLE, cancellation is disabled, the
thread is not cancelable at any point, and all cancellation 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 Type


A 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 cancellation 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, 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.


+------------------------------+---------------------+----------------------+
|Cancellation Type/State Table | | |
|Type | State | |
| | Enabled (Default) | Disabled |
+------------------------------+---------------------+----------------------+
|Deferred (Default) | Cancellation occurs | All cancellation |
| | when the target | requests to the |
| | thread reaches a | target thread are |
| | cancellation point | held pending. |
| | and a cancel is | |
| | pending. (Default) | |
|Asynchronous | Receipt of a | All cancellation |
| | pthread_cancel() | requests to the |
| | call causes | target thread are |
| | immediate | held pending; as |
| | cancellation. | soon as cancellation |
| | | is re-enabled, |
| | | pending |
| | | cancellations are |
| | | executed |
| | | immediately. |
+------------------------------+---------------------+----------------------+

Cancel-Safe
With the arrival of POSIX cancellation, the Cancel-Safe level has been
added to the list of MT-Safety levels. See attributes(7). 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
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 around cancellation points. In
general, expect that most applications and libraries are not
Asynchronous-Cancel-Safe.

POSIX Threads Only


The cancellation functions described in this manual page are available
for POSIX threads, only (the Solaris threads interfaces do not provide
cancellation functions).

EXAMPLES


Example 1: Cancellation example




The following short C++ example shows the pushing/popping of cancellation
handlers, the disabling/enabling of cancellation, 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 has probably 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 pending cancellation.


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 to free_res() and the calls to the destructor for
X. At each level, the C++ runtime calls the destructor for X and then the
cancellation handler, free_res(). The print messages from free_res() and
X's destructor show the sequence of calls.


At the end, the main thread is joined by thread2. Because the main thread
was canceled, its return status from pthread_join() is PTHREAD_CANCELED.
After the status is printed, thread2 returns, killing the process (since
it is the last thread in the process).


#include <pthread.h>
#include <sched.h>
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);}
};

void
free_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);

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(7) for descriptions of the following attributes:


+---------------+-----------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+-----------------+
|MT-Level | MT-Safe |
+---------------+-----------------+

SEE ALSO


read(2), sigwait(2), write(2), Intro(3), 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(7), condition(7),
standards(7)


October 4, 2005 CANCELLATION(7)