Re: boost::thread and detaching

From: Aatu Koskensilta (yg-boost-users_at_[hidden])
Date: 2003-05-22 11:15:03

William E. Kempf wrote:
> Aatu Koskensilta said:
> 
>>Hello.
>>
>>I have a very simple server, in which there is a loop that looks like
>>
>>while( (c_sock = accept(...)) != -1)
>>{
>>  try
>>  {
>>   boost::thread thr( boost::bind(&ServerClass::client_connection, this,
>>                                  c_sock));
>>  }catch ( const std::exception & e)
>>  {
>>   std::cout << "Exception " << e << std::endl;
>>   close(c_sock);
>>  }
>>}
>>
>>Now, it used to look more like
>>
>>while( (c_sock = accept(...)) != -1)
>>{
>>     pthread_t thr;
>>     pthread_attr_t attrs;
>>     pthread_attr_init(&attrs);
>>     pthread_attr_setdetachstate(&attrs, PTHREAD_CREATE_DETACHED);
>>thr_data * data = new thr_data(this, c_sock);
>>     pthread_create(&thr, &attrs, thread_proxy_function, thr_data);
>>}
>>
>>(Actually, the pthread specific stuff and thread parameter wrapping and
>>all that that was done with wrappers written by me. Also, there was
>>proper error checking and all.)
>>
>>Now, the older version worked without any problems. However, the newer
>>version using boost::thread ceased to work after approximately 256
>>threads had been created *all of which had exited*, pthread_create
>>reporting an error Memory allocation failed which was reported as a
>>resource allocation error by boost::thread.
>>
>>It seems that this problem was caused by the fact that the thread
>>function usually got to execute and finish *before* the temporary thread
>> object was destroyed (boost::thread destructor calls pthread_detach).
>>Apparently LinuxThreads didn't consider it prudent to release the
>>resources allocated to the thread upon call of pthread_detach if the
>>thread had exited but no one had joined it yet.
>>
>>Is my analysis credible?
> 
> 
> Sounds credible... but I didn't know the POSIX standard behaved in this
> manner (i.e. detaching a thread that had already ended would leak
> resources instead of behaving like a join in this case).  I'll have to
> research both the standard and my own code here.
> 
> 
>>Am I doing something completely idiotic here?
> 
> 
> No.
> 
> 
>>I
>> managed to get my server code functioning by poking at the
>>boost::thread  code and making it create threads detached by default,
>>but that's hardly  a very elegant solution.
> 
> 
> Agreed.
> 
> 
>>I'm testing this code in a RedHat 9 Linux box.
> 
> 
> Do you have a short test case for this?

The following program produces the strange behaviour. On my machine it 
dies after 255 threads.

#include <boost/thread.hpp>
#include <iostream>

void thread_function()
{
  return;
}

int main()
{
  int num_threads = 0;
  while(1)
  {
   try
   {
    boost::thread thr(thread_function);
    num_threads++;
   }catch ( const std::exception & e)
   {
     std::cout << "Exception " << e.what() << " after " << num_threads <<
                  " threads" << std::endl;
     break;
   }
  }

}

-- 
Aatu Koskensilta (aatu.koskensilta_at_[hidden])
"Wovon man nicht sprechen kann, daruber muss man schweigen"
  - Ludwig Wittgenstein, Tractatus Logico-Philosophicus