static linking with cmake fails [duplicate] - c++

Question

This question already has answers here:
cmake and libpthread
(4 answers)
Closed 3 years ago.
I am trying to link to a static c++ Library with cmake on ubuntu.
the library is installed here: /usr/local/lib/libfrnetlib.a and headers are located here: /usr/local/include/frnetlib
Thanks for any help!
my cmakelists.txt looks like this:
###### setup
cmake_minimum_required(VERSION 3.2)
set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
###### names + sources
project(ws_server)
set(SOURCE_FILES ws_server.cpp TcpListener.h WebFrame.h WebSocket.h)
###### include
include_directories(/usr/local/lib)
include_directories(/usr/local/include/frnetlib)
link_directories(/usr/local/lib/libfrnetlib.a)
###### library
find_library(frnetlib NAMES libfrnetlib.a HINTS /usr/local/lib)
if(NOT frnetlib)
message([status] "
frnetlib library not found
")
else(frnetlib)
message([status] "---------------frnetlib library found :-)")
endif()
###### executable + linking
add_executable(${PROJECT_NAME} ${SOURCE_FILES})
target_link_libraries(${PROJECT_NAME} ${frnetlib})
install(TARGETS ${PROJECT_NAME} DESTINATION bin)
and the output of cmake like this:
> Executing task: if [ -d build ]; then rm -Rf build; fi && mkdir build && cd build && cmake -v -DCMAKE_BUILD_TYPE=release .. && cmake --build . -v <
-- The C compiler identification is GNU 5.4.0
-- The CXX compiler identification is GNU 5.4.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
[status]--------------------------------------frnetlib library found :-)
-- Configuring done
-- Generating done
-- Build files have been written to: /home/pandapc/vscode/own_ws_server/build
/usr/local/bin/cmake -S/home/pandapc/vscode/own_ws_server -B/home/pandapc/vscode/own_ws_server/build --check-build-system CMakeFiles/Makefile.cmake 0
/usr/local/bin/cmake -E cmake_progress_start /home/pandapc/vscode/own_ws_server/build/CMakeFiles /home/pandapc/vscode/own_ws_server/build/CMakeFiles/progress.marks
/usr/bin/make -f CMakeFiles/Makefile2 all
make[1]: Entering directory '/home/pandapc/vscode/own_ws_server/build'
/usr/bin/make -f CMakeFiles/ws_server.dir/build.make CMakeFiles/ws_server.dir/depend
make[2]: Entering directory '/home/pandapc/vscode/own_ws_server/build'
cd /home/pandapc/vscode/own_ws_server/build && /usr/local/bin/cmake -E cmake_depends "Unix Makefiles" /home/pandapc/vscode/own_ws_server /home/pandapc/vscode/own_ws_server /home/pandapc/vscode/own_ws_server/build /home/pandapc/vscode/own_ws_server/build /home/pandapc/vscode/own_ws_server/build/CMakeFiles/ws_server.dir/DependInfo.cmake --color=
Dependee "/home/pandapc/vscode/own_ws_server/build/CMakeFiles/ws_server.dir/DependInfo.cmake" is newer than depender "/home/pandapc/vscode/own_ws_server/build/CMakeFiles/ws_server.dir/depend.internal".
Dependee "/home/pandapc/vscode/own_ws_server/build/CMakeFiles/CMakeDirectoryInformation.cmake" is newer than depender "/home/pandapc/vscode/own_ws_server/build/CMakeFiles/ws_server.dir/depend.internal".
Scanning dependencies of target ws_server
make[2]: Leaving directory '/home/pandapc/vscode/own_ws_server/build'
/usr/bin/make -f CMakeFiles/ws_server.dir/build.make CMakeFiles/ws_server.dir/build
make[2]: Entering directory '/home/pandapc/vscode/own_ws_server/build'
[ 50%] Building CXX object CMakeFiles/ws_server.dir/ws_server.cpp.o
/usr/bin/c++ -I/usr/local/lib -I/usr/local/include/frnetlib -std=c++11 -O3 -DNDEBUG -std=gnu++11 -o CMakeFiles/ws_server.dir/ws_server.cpp.o -c /home/pandapc/vscode/own_ws_server/ws_server.cpp
[100%] Linking CXX executable ws_server
/usr/local/bin/cmake -E cmake_link_script CMakeFiles/ws_server.dir/link.txt --verbose=1
/usr/bin/c++ -std=c++11 -O3 -DNDEBUG -rdynamic CMakeFiles/ws_server.dir/ws_server.cpp.o -o ws_server -L/usr/local/lib/libfrnetlib.a -Wl,-rpath,/usr/local/lib/libfrnetlib.a: /usr/local/lib/libfrnetlib.a
CMakeFiles/ws_server.dir/ws_server.cpp.o: In function `main':
ws_server.cpp:(.text.startup+0x18e): undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status
CMakeFiles/ws_server.dir/build.make:84: recipe for target 'ws_server' failed
make[2]: *** [ws_server] Error 1
make[2]: Leaving directory '/home/pandapc/vscode/own_ws_server/build'
CMakeFiles/Makefile2:72: recipe for target 'CMakeFiles/ws_server.dir/all' failed
make[1]: *** [CMakeFiles/ws_server.dir/all] Error 2
make[1]: Leaving directory '/home/pandapc/vscode/own_ws_server/build'
Makefile:129: recipe for target 'all' failed
make: *** [all] Error 2
The terminal process terminated with exit code: 2
Terminal will be reused by tasks, press any key to close it.

Answer 1

thanks some programmer dude,
the solution is either to set this flag:
#set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pthread")
or use this cmakelists.txt
###### setup
cmake_minimum_required(VERSION 3.2)
set(CMAKE_CXX_STANDARD 11)
#set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pthread")
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ")
###### names + sources
project(ws_server)
set(SOURCE_FILES ws_server.cpp TcpListener.h WebFrame.h WebSocket.h)
###### library
find_library(frnetlib NAMES libfrnetlib.a HINTS /usr/local/lib)
find_package( Threads )
if(NOT frnetlib)
message([status] "
frnetlib library not found!
")
else(frnetlib)
message([status] "--------------------------------------frnetlib library found :-)")
endif()
###### executable + linking
add_executable(${PROJECT_NAME} ${SOURCE_FILES})
target_link_libraries(${PROJECT_NAME} ${frnetlib} ${CMAKE_THREAD_LIBS_INIT})
install(TARGETS ${PROJECT_NAME} DESTINATION bin)