Why is there a difference using std::thread::hardw

问题:

The description of the problem itself is pretty simple. I'm testing the differences of std::thread library in C++11 and boost::thread library.

The output of these:

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

int main() {
  std::cout << std::thread::hardware_concurrency() << std::endl;
  std::cout << boost::thread::hardware_concurrency() << std::endl;
  return 0;
}

gives me different results:

0
4

Why is that?

PS: The version of the gcc package is 4.6.2-1.fc16 (x86_64). I'm using

g++ test.cc -Wall -std=c++0x -lboost_thread-mt -lpthread

回答1:

After reviewing /usr/include/c++/4.6.2/thread

it can be seen that the implementation is actually:

// Returns a value that hints at the number of hardware thread contexts.
static unsigned int
hardware_concurrency()
{ return 0; }

So problem solved. It's just another feature that hasn't been implemented in gcc 4.6.2

回答2:

The method employed by your compiler installation of boost is supported for your target, whereas your installation of boost compiler does not support this feature for your target.

TFM says:

The number of hardware threads available on the current system (e.g. number of CPUs or cores or hyperthreading units), or 0 if this information is not available.

EDIT: scratch that, reverse it.

EDIT2: This feature is present on the trunk, but absent in 4.6.2:

~/tmp/gcc-4.6.2/libstdc++-v3/src> wc -l thread.cc
104 thread.cc
~/tmp/gcc-4.6.2/libstdc++-v3/src> grep concurrency thread.cc | wc -l
0
~/tmp/gcc-4.6.2/libstdc++-v3> grep -C 2 VERIFY testsuite/30_threads/thread/members/hardware_concurrency.cc

  // Current implementation punts on this.
  VERIFY( std::thread::hardware_concurrency() == 0 );

  return 0;