Compiling C++ source file using Boost.Thread

Question

I am trying to learn how to use the C++ Boost.Thread library. I have installed the Boost libraries on my Ubuntu 11.10 system. I am following the book "The Boost C++ Libraries" by Schaling - specifically example 6.1 on page 66. I am trying to compile the following code example:

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

void wait(int seconds)
{ 
  boost::this_thread::sleep(boost::posix_time::seconds(seconds));
}

void thread()
{
  for(int i = 0; i < 5; ++i)
  {
    wait(1);
    std::cout << i << std::endl;
  }
}

int main() 
{
  boost::thread t(thread);
  t.join();
}

However, when I compile this with the following from the command line:

$ g++ example61.cpp -o example61 -I /usr/local/include

I get the following output:

/tmp/cc6bVu1F.o: In function `main':
example6.cpp:(.text+0x9d): undefined reference to `boost::thread::join()'
example6.cpp:(.text+0xae): undefined reference to `boost::thread::~thread()'
example6.cpp:(.text+0xc6): undefined reference to `boost::thread::~thread()'
/tmp/cc6bVu1F.o: In function `boost::detail::thread_data_base::thread_data_base()':
example6.cpp:(.text._ZN5boost6detail16thread_data_baseC2Ev[_ZN5boost6detail16thread_data_baseC5Ev]+0x24): undefined reference to `vtable for boost::detail::thread_data_base'
/tmp/cc6bVu1F.o: In function `void boost::this_thread::sleep<boost::posix_time::seconds>(boost::posix_time::seconds const&)':
example6.cpp:(.text._ZN5boost11this_thread5sleepINS_10posix_time7secondsEEEvRKT_[void boost::this_thread::sleep<boost::posix_time::seconds>(boost::posix_time::seconds const&)]+0x35): undefined reference to `boost::this_thread::sleep(boost::posix_time::ptime const&)'
/tmp/cc6bVu1F.o: In function `boost::thread::thread<void (*)()>(void (*)(), boost::disable_if<boost::is_convertible<void (*&)(), boost::detail::thread_move_t<void (*)()> >, boost::thread::dummy*>::type)':
example6.cpp:(.text._ZN5boost6threadC2IPFvvEEET_NS_10disable_ifINS_14is_convertibleIRS4_NS_6detail13thread_move_tIS4_EEEEPNS0_5dummyEE4typeE[_ZN5boost6threadC5IPFvvEEET_NS_10disable_ifINS_14is_convertibleIRS4_NS_6detail13thread_move_tIS4_EEEEPNS0_5dummyEE4typeE]+0x30): undefined reference to `boost::thread::start_thread()'
/tmp/cc6bVu1F.o: In function `boost::detail::thread_data<void (*)()>::~thread_data()':
example6.cpp:(.text._ZN5boost6detail11thread_dataIPFvvEED2Ev[_ZN5boost6detail11thread_dataIPFvvEED5Ev]+0x1f): undefined reference to `boost::detail::thread_data_base::~thread_data_base()'
/tmp/cc6bVu1F.o:(.rodata._ZTIN5boost6detail11thread_dataIPFvvEEE[typeinfo for boost::detail::thread_data<void (*)()>]+0x10): undefined reference to `typeinfo for boost::detail::thread_data_base'
collect2: ld returned 1 exit status

I don't know how to interpret this. Can anyone help? Thank you so much!

Answer 1

That is a linking error. It means your code is correct and you include the correct headers, but the compiler doesn't link against the boost threading library. To fix this, you need to compile like this:

g++ example61.cpp -o example61 -I /usr/local/include -lboost_thread

If you've installed the Boost threading library to a non-standard path, you must also add it to the search path:

g++ example61.cpp -o example61 -I /usr/local/include -lboost_thread -L/usr/local/lib

Answer 2

You need to link with the library. Some Boost libraries are implemented entirely in the header files and do not need a library. But others, like thread, are implemented partly in headers and partly in compiled library code.

I believe that you need to add -lboost_thread-mt to your compile command.