Re: Compiler gives wrong output

[U.Mutlu]

Richard Kettlewell wrote on 06/09/2015 10:16 AM:
> "U.Mutlu" <for-gmane@mutluit.com> writes:
> > Hi,
> > on this page:
> > http://en.cppreference.com/w/cpp/thread/recursive_mutex/try_lock
> > they give the following C++11 example:
> >
> > #include <iostream>
> > #include <mutex>
> >
> > int main()
> > {
> >      std::mutex test;
> >      if (test.try_lock()==true) {
> >          std::cout << "lock acquired" << std::endl;
> >          test.unlock();    //now unlock the mutex
> >      } else {
> >          std::cout << "lock not acquired" << std::endl;
> >      }
> >
> >      test.lock();    //to lock it again
> >      if (test.try_lock()) {  //true can be left out
> >          std::cout << "lock acquired" << std::endl;
> >      } else {
> >          std::cout << "lock not acquired" << std::endl;
> >      }
> >      test.unlock();
> > }
> >
> > and they say it would give this output:
> > Output:
> > lock acquired
> > lock not acquired
> >
> > I wonder why my compiler (g++ 4.9.2 from Debian repo) gives a different output:
> > lock acquired
> > lock acquired
> >
> > Which compiler is wrong, and why?
>
> AIUI, neither is wrong.  The code violates the preconditions of
> try_lock(), and therefore results in undefined behavior.

Mystery solved: when using such concurrency/thread objects,
then one has to link with pthread (-lpthread):
 g++ -Wall -O2 -std=gnu++11 file.cpp -lpthread
Now my compiler gives the same result.