Why is there a difference using std::thread::hardware_concurrency() and boost::thread::hardware_concurrency()? - linux

Question

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

Answer 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

Answer 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;