Re: Fwd: Futex using boost, threads never wakeup

From: oliver.kowalke_at_[hidden]
Date: 2023-02-23 16:14:52

Code from namespace detail is not part of the public API. Do  not use this code (boost::fiber::detail::futex).

23.02.2023 14:51:39 tsoumplekas_giorgos via Boost <boost_at_[hidden]>:

> Subject: Futex using boost, threads never wakeup
> Hello all
> I posted the following question in stackoverflow
> https://stackoverflow.com/questions/75538799/futex-using-boost-threads-never-wakeup
> 0
> <https://stackoverflow.com/posts/75538799/timeline>
> 
> I am studying about futex synchronisation Following up the manual
> <https://man7.org/linux/man-pages/man2/futex.2.html> , I tried to
> syncroniaze threads using boost futex However the threads never wake up
> 
> #include <iostream>#include <atomic>#include
> <boost/fiber/detail/futex.hpp>#include <thread>#include
> <vector>#include <chrono>void wait(std::atomic<int>Â  & futexp){
> Â Â Â  int one = 1;
> Â Â Â  while(std::atomic_compare_exchange_strong(&futexp, &one, 0))
> Â Â Â  {}
> Â Â Â  std::cout << std::this_thread::get_id() << "," << futexp.load() <<
> std::endl;
> Â Â Â  boost::fibers::detail::futex_wait(&futexp,0);
> Â Â  // std::this_thread::sleep_for(std::chrono::milliseconds(5000));
> 
> }void fpost(std::atomic<int>Â  & futexp){
> Â Â Â  int zero = 0;
> Â Â Â  if (std::atomic_compare_exchange_strong(&futexp, &zero, 1)) {
> Â Â Â Â  boost::fibers::detail::futex_wake(&futexp);
> Â Â Â  }
> }
> 
> std::atomic<int>Â  futexp{1};int main() {
> Â Â Â  std::vector<std::thread>Â  vec;
> Â Â Â  for(int i =0 ; i< 5;++i)
> Â Â Â  {
> Â Â Â Â Â Â Â  vec.emplace_back([]()
> Â Â Â Â Â Â Â  {
> Â Â Â Â Â Â Â Â Â Â Â  wait(futexp);
> Â Â Â Â Â Â Â Â Â Â Â  fpost(futexp);
> Â Â Â Â Â Â Â  });
> Â Â Â  }
> 
> 
> Code demo <https://godbolt.org/z/94hMsrW4a>
> 
> I am using boost 1.66 and gcc 9.1 What goes wrong with this piece of code ?
> 
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