Re: Bug#749290: g++-4.8: broken std::thread on Hurd

[Pino Toscano <pino@debian.org>]

On 2014-05-26 02:31, Gabriele Giacone wrote:
> $ wget
>
> https://bugs.launchpad.net/ubuntu/+source/gcc-defaults/+bug/1228201/+attachment/3831609/+files/thread.cpp
> $ g++ thread.cpp -pthread -std=c++11 -o thread
> $ ./thread
> terminate called after throwing an instance of 'std::system_error'
>   what():  Enable multithreading to use std::thread: Operation not 
> permitted
> Aborted

This happens because in gcc/src/libstdc++-v3/src/c++11/thread.cc,
thread::_M_start_thread throws EPERM if __gthread_active_p is false.

__gthread_active_p, implemented inline in bits/gthr-default.h, does
the following (simplified):

  __gthrw2(__gthrw_(__pthread_key_create),
           __pthread_key_create,
           pthread_key_create)
  # define GTHR_ACTIVE_PROXY      __gthrw_(__pthread_key_create)
  [...]
  static inline int
  __gthread_active_p (void)
  {
    static void *const __gthread_active_ptr
      = __extension__ (void *) &GTHR_ACTIVE_PROXY;
    return __gthread_active_ptr != 0;
  }

so checks for the presence of __pthread_key_create to determine
whether threads are available, with a lengthy comment saying why that
symbol is checked when using the GNU C library.
Indeed, creating a simple extern "C" __pthread_key_create which just
calls pthread_key_create in the test case makes it run fine.

It looks like to me there are two solutions:

a) fix the GCC detection of threads on Hurd, so it uses only
   pthread_key_create (or another internal symbol of Hurd's
   libpthread)

b) fix pthread_key_create in Hurd's libpthread, changing it to
   __pthread_key_create and declaring pthread_key_create as strong
   alias, just like it is done in NPTL

IMHO most probably (b) is the most realistic and easy to do.

--
Pino Toscano