what's the differences between pthread and -pthread options in cmake? - c++

Question

Environment
ubuntu 16.04, gcc 5.4.0, cmake 3.5.1
Question
target_link_libraries(promise pthread)
target_link_libraries(promise -pthread)
target_link_libraries(promise -lpthread)
What's the differences, which is better ?
Problem
promise.cpp
std::promise<int> pr;
auto fut = pr.get_future();
pr.set_value(10); // throw std::exception and terminate
CMakeLists.txt
add_executable(promise promise.cpp)
target_link_libraries(promise pthread)
Solution
Modify CMakeLists.txt slightly.
add_executable(promise promise.cpp)
target_link_libraries(promise -pthread)
I found the answer from here. But I don't know why ?
But, the best solution is portable.
set(THREADS_PREFER_PTHREAD_FLAG ON)
find_package(Threads)
target_link_libraries(promise Threads::Threads)

Answer 1

All calls are generally wrong. As answered by #vre, you should use find_package(Threads) instead.
But all these calls are effectively the same!
Calls
target_link_libraries(promise pthread)
and
target_link_libraries(promise -lpthread)
are transformed into the same linker's command line: for parameters not starting with -, CMake will add -l automatically (from target_link_libraries documentation):
A plain library name: The generated link line will ask the linker to search for the library (e.g. foo becomes -lfoo or foo.lib).
While calls
target_link_libraries(promise -lpthread)
and
target_link_libraries(promise -pthread)
are translated into different flags, for linking process these flags means the same.
Option -pthread, passed to gcc, would add additional compile definitions. But parameters for target_link_libraries are not used for compilation.
Why using find_package(Threads) is correct
If one uses
set(THREADS_PREFER_PTHREAD_FLAG ON) # Without this flag CMake may resort to just '-lpthread'
find_package(Threads)
a library target Threads::Threads is created, with additional compile and link options -pthread attached to it as an interface.
When use
target_link_libraries(promise Threads::Threads)
CMake automatically propagates interface compile and link options, thus promise target is both compiled and linked with -pthread option.

Answer 2

First, we can use cmake -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON to see the underlying commands of make.
target_link_libraries(promise pthread) and target_link_libraries(promise -lpthread) will result the same link option: -lpthread, like:
/usr/bin/c++ -std=c++11 -rdynamic CMakeFiles/promise.dir/promise.cpp.o -o promise -lpthread
However, target_link_libraries(promise -pthread) will give you -pthread option:
/usr/bin/c++ -std=c++11 -rdynamic CMakeFiles/promise.dir/promise.cpp.o -o promise -pthread
The difference between -pthread and -lpthread is well explained here. In general, you should use -pthread and target_link_libraries(promise -pthread).
btw, clang built binary seems ok with both options.

Answer 3

I would suggest using the modern CMake way by means of imported targets:
set(THREADS_PREFER_PTHREAD_FLAG ON)
find_package(Threads)
target_link_libraries(promise Threads::Threads)
This does not only add the library dependency, it also sets the compile option and works on almost all platforms. See the answers to the following post for further details:
Difference between -pthread and -pthreads for C/C++ on Ubuntu 14.04
And have a look into the fine documentation of FindThreads.cmake module:
https://cmake.org/cmake/help/v3.11/module/FindThreads.html