On Sun, 30 Aug 2020 09:59:15 +0100 Barry Scott <ba...@barrys-emacs.org> wrote:
> > > > On 29 Aug 2020, at 18:01, Dennis Lee Bieber <wlfr...@ix.netcom.com> > > wrote: > > > > On Sat, 29 Aug 2020 18:24:10 +1000, John O'Hagan > > <resea...@johnohagan.com> declaimed the following: > > > >> There's no error without the sleep(1), nor if the Process is > >> started before the Thread, nor if two Processes are used instead, > >> nor if two Threads are used instead. IOW the error only occurs if > >> a Thread is started first, and a Process is started a little later. > >> > >> Any ideas what might be causing the error? > >> > > > > Under Linux, multiprocessing creates processes using > > fork(). That means that, for some fraction of time, you have TWO > > processes sharing the same thread and all that entails (if it > > doesn't overlay the forked process with a new executable, they are > > sharing the thread until the thread exits). > > In the parent you have 1 or more threads. > > After fork the new process has 1 thread, which is not shared with the > parent. Any extra threads are not in the new process. But the memory > in the new process will have data structures from the parents other > threads. > > So no you never have two processes sharing an threads. > > This leads to problems with locks. > > Barry > > > > > > https://stackoverflow.com/questions/54466572/how-to-properly-multithread-in-opencv-in-2019 > > (which points to) > > https://answers.opencv.org/question/32415/thread-safe/?answer=32452#post-id-32452 > > """ > > The library itself is thread safe in that you can have multiple > > calls into the library at the same time, however the data is not > > always thread safe. """ > > > > The sleep(1), when compounded with the overhead of starting > > the thread, and then starting the process, likely means the thread > > had exited before the process actually is started. Try replacing > > the sleep(2) in the work code with something like sleep(30) -- I > > hypothesize that you'll get the same error condition even with the > > sleep(1) in place. > > > > > > -- > > Wulfraed Dennis Lee Bieber AF6VN > > wlfr...@ix.netcom.com > > http://wlfraed.microdiversity.freeddns.org/ > > > > -- > > https://mail.python.org/mailman/listinfo/python-list > > > Thanks to all who replied. I'm still none the wiser about the error, but I learned something about multiprocessing! John -- https://mail.python.org/mailman/listinfo/python-list
