Strict aliasing issue in threads

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
48/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Quiet
Tech stack
cpp
Domain
operating-systems

Research direction

Start with src/plugins/streamers/lstream/runtime/kj/kj/thread.cc around the reported calls and thread.h around the threadId declaration. Check how the thread identifier is represented and used across platforms, then build with -Werror=strict-aliasing to reproduce the reported diagnostics. Done means the strict-aliasing errors are resolved without assuming a particular representation for pthread_t.

Written by the indexing model from the issue text.

Description

Originally reported downstream in Gentoo at https://bugs.gentoo.org/973164. We build with -Werror=strict-aliasing to help catch future compiler incompatibilities.

x86_64-pc-linux-gnu-g++ -c -O2 -pipe -march=x86-64 -mtune=generic -g -Werror=odr -Werror=lto-type-mismatch -Werror=strict-aliasing -Wno-unused-parameter -fPIC -Wall -Wextra -mno-direct-extern-access -D_REENTRANT -DCAPNP_LITE -DQT_NO_DEBUG -I/var/tmp/portage/sci-electronics/klayout-0.30.7/work/ruby33/klayout-0.30.7/src/plugins/streamers/lstream/runtime/kj -I/var/tmp/portage/sci-electronics/klayout-0.30.7/work/ruby33/klayout-0.30.7/plugins/streamers/lstream/runtime/kj -I/var/tmp/portage/sci-electronics/klayout-0.30.7/work/ruby33/klayout-0.30.7/src/plugins/streamers/lstream/runtime/capnp -I/var/tmp/portage/sci-electronics/klayout-0.30.7/work/ruby33/klayout-0.30.7/src/plugins/streamers/lstream/runtime/kj -I/usr/lib64/qt6/mkspecs/linux-g++ -o thread.o /var/tmp/portage/sci-electronics/klayout-0.30.7/work/ruby33/klayout-0.30.7/src/plugins/streamers/lstream/runtime/kj/kj/thread.cc
/var/tmp/portage/sci-electronics/klayout-0.30.7/work/ruby33/klayout-0.30.7/src/plugins/streamers/lstream/runtime/kj/kj/thread.cc: In destructor ‘kj::Thread::~Thread()’:
/var/tmp/portage/sci-electronics/klayout-0.30.7/work/ruby33/klayout-0.30.7/src/plugins/streamers/lstream/runtime/kj/kj/thread.cc:82:39: error: dereferencing type-punned pointer will break strict-aliasing rules
[-Werror=strict-aliasing]
   82 |     int pthreadResult = pthread_join(*reinterpret_cast<pthread_t*>(&threadId), nullptr);
      |                                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/var/tmp/portage/sci-electronics/klayout-0.30.7/work/ruby33/klayout-0.30.7/src/plugins/streamers/lstream/runtime/kj/kj/thread.cc: In member function ‘void kj::Thread::sendSignal(int)’:
/var/tmp/portage/sci-electronics/klayout-0.30.7/work/ruby33/klayout-0.30.7/src/plugins/streamers/lstream/runtime/kj/kj/thread.cc:96:37: error: dereferencing type-punned pointer will break strict-aliasing rules
[-Werror=strict-aliasing]
   96 |   int pthreadResult = pthread_kill(*reinterpret_cast<pthread_t*>(&threadId), signo);
      |                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/var/tmp/portage/sci-electronics/klayout-0.30.7/work/ruby33/klayout-0.30.7/src/plugins/streamers/lstream/runtime/kj/kj/thread.cc: In member function ‘void kj::Thread::detach()’:
/var/tmp/portage/sci-electronics/klayout-0.30.7/work/ruby33/klayout-0.30.7/src/plugins/streamers/lstream/runtime/kj/kj/thread.cc:103:39: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
  103 |   int pthreadResult = pthread_detach(*reinterpret_cast<pthread_t*>(&threadId));
      |                                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1plus: some warnings being treated as errors
make[5]: *** [Makefile:1227: thread.o] Error 1
make[5]: *** Waiting for unfinished jobs....

The violation is at https://github.com/KLayout/klayout/blob/5ccf6260f1769e7431901101bf27d01b68df177e/src/plugins/streamers/lstream/runtime/kj/kj/thread.cc#L70

pthread_t is an opaque type, it shouldn't be assumed to be unsigned long long:
https://github.com/KLayout/klayout/blob/5ccf6260f1769e7431901101bf27d01b68df177e/src/plugins/streamers/lstream/runtime/kj/kj/thread.h#L69

I assume this is done for compatibility with Windows, otherwise pthread_t would've been used. But pthread_t is an opaque type: https://stackoverflow.com/questions/33285562/why-and-in-what-sense-is-pthread-t-an-opaque-type

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