On Wed, 9 Apr 2025, Jeremy Drake via Cygwin wrote: > On Thu, 10 Apr 2025, Kevin Schnitzius via Cygwin wrote: > > > On Wednesday, April 9, 2025 at 06:54:34 PM EDT, Jeremy Drake via Cygwin > > <cygwin@cygwin.com> wrote: > > > > > The recent issue with pthread_atfork handlers reminded me of a scenario > > > that I know glibc handles, but it seems that Cygwin does not. Test case: > > > > <... code that loads a shared lib, registers some functions in shared lib > > with pthread_atfork(), unloads the shared lib, and crashes on fork...> > > > > Calling functions in an unloaded library should result in undefined > > behavior. > > > > However, further investigation reveals that the Linux pthread_atfork() > > registered functions are not being called and POSIX does not proved a > > mechanism for un-registering these functions. Note: pthread_atfork() is > > not bumping the ref count on the shared lib--those functions are definitely > > unavailable after the dlclose() > > > > In the Cygwin version, calling the functions in the unloaded library when > > the fork happens causes the crash. > > > > This seems to be a bug with fork(), if it is a bug at all. > > I did a quick search, and found a write-up of *my* bug report :D > > https://developers.redhat.com/articles/2022/12/14/how-we-addressed-unforeseen-use-case-pthreadatfork > > "Now, dlclose()'ing a module means that any fork handlers registered by it > should not be executed after the dlclose and should therefore implicitly > be deregistered."
It seems glibc takes care of this implicit deregistration in __cxa_finalize, after calling __cxa_atexit functions, it unregisters any at_quick_exit or pthread_atfork callbacks from the DSO being unloaded. https://sourceware.org/git/?p=glibc.git;a=blob;f=stdlib/cxa_finalize.c;h=2bb35602edc6bf842e5d2c93ad03454d7b57ee65;hb=HEAD It doesn't look like newlib deals with either at_quick_exit or pthread_atfork handlers in its __cxa_finalize implementation. -- Problem reports: https://cygwin.com/problems.html FAQ: https://cygwin.com/faq/ Documentation: https://cygwin.com/docs.html Unsubscribe info: https://cygwin.com/ml/#unsubscribe-simple
