Cancelling a thread using pthread_cancel : good practice or bad

Question

I have a C++ program on Linux (CentOS 5.3) spawning multiple threads which are in an infinite loop to perform a job and sleep for certain minutes. Now I have to cancel the running threads in case a new configuration notification comes in and freshly start new set of threads, for which i have used pthread_cancel. What I observed was, the threads were not getting stopped even after receiving cancel indication,even some sleeping threads were coming up after the sleep was completed.

As the behavior was not desired, usage of pthread_cancel in the mentioned scenario raises question about being good or bad practice.

Please comment on the pthread_cancel usage in above mentioned scenario.

Answer 1

In general thread cancellation is not a really good idea. It is better, whenever possible, to have a shared flag, that is used by the threads to break out of the loop. That way, you will let the threads perform any cleanup they might need to do before actually exiting.

On the issue of the threads not actually cancelling, the POSIX specification determines a set of cancellation points ( man 7 pthreads ). Threads can be cancelled only at those points. If your infinite loop does not contain a cancellation point you can add one by calling pthread_testcancel. If pthread_cancel has been called, then it will be acted upon at this point.

Answer 2

If you are writing exception safe C++ code (see http://www.boost.org/community/exception_safety.html) than your code is naturally ready for thread cancellation. glibs throws C++ exception on thread cancel, so that your destructors can do the appropriate clean-up.

Answer 3

You can do the equivalent of the code below.

#include <pthread.h>
#include <cxxabi.h>
#include <unistd.h>
...
void *Control(void* pparam)
{
    try
    {
        // do your work here, maybe long loop
    }   
    catch (abi::__forced_unwind&)
    {  // handle pthread_cancel stack unwinding exception
        throw;
    }
    catch (exception &ex) 
    {
        throw ex;
    }
}

int main()
{
    pthread_t tid;
    int rtn;
    rtn = pthread_create( &tid, NULL, Control, NULL );

    usleep(500);
    // some other work here

    rtn = pthtead_cancel( tid );
}