undefined reference to `pthread_create' Error when making C++11 application with ASIO and std::thread

Question

I set Eclipse (Actually Xilinx SDK but based on Eclipse), and g++4.9.2, to compile a project which uses standalone ASIO and I used -std=c++11 in the Properties -> C/C++ Build -> Settings -> Tool Settings -> Other flags so it can compile using all C++11 features.

I also set ASIO_HAS_STD_THREAD, ASIO_STANDALONE and so on in C/C++ General Symbols, and I expect that the ASIO header will use std::thread instead of pthread. However, I still see Error from make:

undefined reference to pthread_create, 
..asio-1.10.6\include\asio\detail\impl\posix_thread.ipp
and posix_tss_ptr.hpp

so the problem is, since I am using C++11, and specified ASIO_HAS_STD_THREAD but not ASIO_HAS_PTHREADS, the posix_thread.ipp should not be even included (through posix_thread.hpp), according to thread.hpp in ASIO:

#if !defined(ASIO_HAS_THREADS)
# include "asio/detail/null_thread.hpp"
#elif defined(ASIO_WINDOWS)
# if defined(UNDER_CE)
#  include "asio/detail/wince_thread.hpp"
# else
#  include "asio/detail/win_thread.hpp"
# endif
#elif defined(ASIO_HAS_PTHREADS)
# include "asio/detail/posix_thread.hpp"
#elif defined(ASIO_HAS_STD_THREAD)
# include "asio/detail/std_thread.hpp"
#else
# error Only Windows, POSIX and std::thread are supported!
#endif

Suspect 1 -pthread

Contrary to most people believe, C++11 doesn't need the -pthread and I have tried to compile a simple project without -pthread in Eclipse. However, you can correct me if I am wrong. When I put -pthread in the linker option, it does compile, however I felt I don't want the pthread if not necessary.

Suspect 2 - ASIO makefile

When I search the posix_tss_ptr.hpp, I found also in Makefile.am. I wonder whether this affect the error?

So what is the cause of the problem? If not the above two suspect? I hope the solution can be still using pure C++11 way, not to use pthread if my reasoning is correct.

Update

I found that ASIO_HAS_PTHREADS is defined not by me and that's why ASIO uses POSIX threads somewhere and the linker then needs the option -pthread. Then I traced down to the asio/detail/signal_blocker.hpp by using #error directive. There are only two places that it is defined and they are in ASIO config.hpp

#  if defined(ASIO_HAS_BOOST_CONFIG) && defined(BOOST_HAS_PTHREADS)
#   define ASIO_HAS_PTHREADS 1
#  elif defined(_POSIX_THREADS)
#   define ASIO_HAS_PTHREADS 1

ASIO still replies on POSIX THREADS or Windows for signal_blocker.hpp shown below. That's why ASIO still needs pthread.

#if !defined(ASIO_HAS_THREADS) || defined(ASIO_WINDOWS) \
  || defined(ASIO_WINDOWS_RUNTIME) \
  || defined(__CYGWIN__) || defined(__SYMBIAN32__)
typedef null_signal_blocker signal_blocker;
#elif defined(ASIO_HAS_PTHREADS)
typedef posix_signal_blocker signal_blocker;
#endif

And _PTHREADS is defined from the gnu cross compiler (arm-xilinx-linux-gnueabi) include files such as features.h, posix_opt.h, and etc. I am not going to track down which truly defined the macro, but ASIO is the source which uses _POSIX_THREADS and so the linker option -pthread should be there.

Again, non ASIO C++11 thread doesn't need -pthread for g++ 4.9.2 but standalone ASIO needs it. The following code is built correctly without -pthread in g++4.9.2 (Xilinx SDK which is based on Eclipse):

#include <thread>
void test() {
    for(int i=0;i<100;i++);
}
int main()
{
    std::thread thread1(test);
    thread1.join();
    return 0;
}

Answer 1

The fact that a program is written in C++11 has no bearing on with whether or not it needs to be linked with the pthread library. It needs to link that library if it requires Posix threads.

C++11 provides the std::thread class and each conforming compiler's standard library must implement the functionality of that class using some native threads API hosted by the target system. GCC implements it using pthreads, so you cannot any build a program that creates std::thread objects with GCC unless you link it with -pthread. This fact is unconnected with asio.

LATER

I actually built a program without pthread using std::std just using -std=c++11

I think you are mistaken and confused by the fact that some Windows ports of GCC link libpthread by default. E.g. if your example program is in thread.cpp I can successfully build it in Windows with TDM-GCC 4.9.2 so:

>g++ -std=c++11 -o test_thread thread.cpp

But if you build it in verbose mode:

>g++ -v -std=c++11 -o test_thread thread.cpp

you can see that a great many library options are passed to the linker behind the scenes, in particular -lpthread:

>g++ -v -std=c++11 -o test_thread thread.cpp 2>&1 | grep -Po 'pass-through=-lpthread' -
pass-through=-lpthread

And on Linux you will not link libpthread unless you ask:

$ g++ -std=c++11 -o test_thread thread.cpp
/tmp/ccpyEles.o: In function `std::thread::thread<void (&)()>(void (&)())':
thread.cpp:(.text._ZNSt6threadC2IRFvvEJEEEOT_DpOT0_[_ZNSt6threadC5IRFvvEJEEEOT_DpOT0_]+0x7d): undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status