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Diffstat (limited to 'FAQ')
| -rw-r--r-- | FAQ | 76 |
1 files changed, 76 insertions, 0 deletions
@@ -12,6 +12,8 @@ Q 2 Now that pthreads-win32 builds under Mingw32, why do I get Q 3 How do I use pthread.dll for Win32 (Visual C++ 5.0) +Q 4 Cancelation doesn't work for me, why? + ============================================================================= Q 1 Should I use Cygwin or Mingw32 as a development environment? @@ -177,3 +179,77 @@ is included for information. Cheers. Ross + +------------------------------------------------------------------------------ + +Q 4 Cancelation doesn't work for me, why? +--- + +A 4 +--- + +> I'm investigating a problem regarding thread cancelation. The thread I want +> to cancel has PTHREAD_CANCEL_ASYNCHRONOUS, however, this piece of code +> blocks on the join(): +> +> if ((retv = Pthread_cancel( recvThread )) == 0) +> { +> retv = Pthread_join( recvThread, 0 ); +> } +> +> Pthread_* are just macro's; they call pthread_*. +> +> The thread recvThread seems to block on a select() call. It doesn't get +> cancelled. +> +> Two questions: +> +> 1) is this normal behaviour? +> +> 2) if not, how does the cancel mechanism work? I'm not very familliar to +> win32 programming, so I don't really understand how the *Event() family of +> calls work. + +The answer to your first question is, normal POSIX behaviour would +be to asynchronously cancel the thread. + +However ... + +Pthreads-win32 doesn't support asynchronous cancelation, only +deferred, which is also very limited. The reason there is no async +cancelation is that it's too hard, if not impossible, to implement +on top of Win32. Incidently, Butenhof "Programming with POSIX +Threads" recommends not using async cancelation because the +possible side effects are unpredictable, especially if you're +trying to write portable code. + +Using deferred cancelation would normally be the way to go, however, +even though the POSIX threads standard lists a number of C library +functions that are defined as deferred cancelation points, there is +no hookup between those which are provided by Windows and the +pthreads-win32 library. + +Incidently, it's worth noting for code portability that the POSIX +threads standard list doesn't include "select" because (as I read in +Butenhof) it isn't recognised by POSIX. + +Effectively, the only cancelation points that pthreads-win32 can +recognise are those the library implements itself, ie. + + pthread_testcancel + pthread_cond_wait + pthread_cond_timedwait + pthread_join + sem_wait + +Pthreads-win32 also provides two functions that allow you to create +cancelation points within your application, but only for cases where +a thread is going to block on a Win32 handle. These are: + + pthreadCancelableWait(HANDLE waitHandle) /* Infinite wait */ + + pthreadCancelableTimedWait(HANDLE waitHandle, DWORD timeout) + +Regards. +Ross + |