Subj : Re: pthread_cancel
To   : comp.programming.threads
From : Ed Skinner
Date : Tue May 10 2005 11:09 am

On Mon, 09 May 2005 03:39:52 +0000, Greg Law wrote:

> Hi,
> 
> I am calling pthread_cancel on a thread that has canceling enabled, of
> deferred type.  The thread I am canceling never makes any calls to
> testcancel, or waits on any condition variables.  So it surprises me
> that the cancelation has any effect at all.  That is, in the following
> program the thread I've created is terminated (to my surprise).  I guess
> I've not understood correctly when threads may be canceled?
> 
> Cheers,
> 
> Greg
> 
> 
> #include <pthread.h>
> #include <assert.h>
> 
> void
> Start (void *a) {
>     int tmp;
>     int r = pthread_setcancelstate (PTHREAD_CANCEL_ENABLE, &tmp); assert
>     (!r);
>     r = pthread_setcanceltype (PTHREAD_CANCEL_DEFERRED, &tmp); assert
>     (!r);
> 
>     while (1) {
>        printf ("%d\n", tmp++);
>     }
>     }
>     }
> int
> main (void) {
>     int r = 0;
>     pthread_t p;
> 
>     r = pthread_create (&p, NULL, (void*)Start, 0); printf ("%d: %ld\n",
>     r, p);
> 
>     r = pthread_cancel (p);
>     printf ("back from cancel (%d)\n", r); while (1);
> }
> }
> }
     I think main() is firing off the pthread_cancel() BEFORE the thread
has an opportunity to run and set the deferred cancel state. (In a simple
situation such as this it is highly likely that main() will continue to be
the thing that executes -- doing a pthread_create() merely creates the new
thread but does not guarantee that the new thread will actually execute --
main() needs to do something to give the new thread an opportunity to
execute and a sleep(1) will do precisely that.)
     Try adding a sleep(1) in main() between the pthread_create() and the
pthread_cancel() to give the thread an opportunity to run and protect
itself.

.