Using asio but link error in pthread

edit

Hi guys,

I am trying to compile a simple UDP server in ROS 2. I am using the async library ASIO as stand-alone. It's a header only library so I don't need to link to it. I tried to compile the code but I got the following link errors for pthread

--- stderr: oam_api                                                                          
/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/Scrt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
CMakeFiles/udp_server_main.dir/src/udp_server_main.cpp.o: In function `asio::detail::posix_event::posix_event()':
udp_server_main.cpp:(.text._ZN4asio6detail11posix_eventC2Ev[_ZN4asio6detail11posix_eventC5Ev]+0x4d): undefined reference to `pthread_condattr_setclock'
CMakeFiles/udp_server_main.dir/src/udp_server_main.cpp.o: In function `asio::detail::posix_thread::~posix_thread()':
udp_server_main.cpp:(.text._ZN4asio6detail12posix_threadD2Ev[_ZN4asio6detail12posix_threadD5Ev]+0x26): undefined reference to `pthread_detach'
CMakeFiles/udp_server_main.dir/src/udp_server_main.cpp.o: In function `asio::detail::posix_thread::join()':
udp_server_main.cpp:(.text._ZN4asio6detail12posix_thread4joinEv[_ZN4asio6detail12posix_thread4joinEv]+0x2b): undefined reference to `pthread_join'
collect2: error: ld returned 1 exit status
make[2]: *** [udp_server_main] Error 1
make[1]: *** [CMakeFiles/udp_server_main.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....
make: *** [all] Error 2

I am not sure what is the problem since I added the -pthread option to the compiler flags. Here are the CMakeLists.txt and package.xml file:

CMakeLists.txt

cmake_minimum_required(VERSION 3.5)
project(oam_api)

# Default to C99
if(NOT CMAKE_C_STANDARD)
    set(CMAKE_C_STANDARD 99)
endif()

# Default to C++14
if(NOT CMAKE_CXX_STANDARD)
    set(CMAKE_CXX_STANDARD 14)
endif()

if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
    add_compile_options(-Wall -Wextra -Wpedantic)
endif()

# for compiling asio
add_compile_options(-DASIO_STANDALONE -pthread)

# find dependencies
find_package(ament_cmake REQUIRED)
find_package(rclcpp REQUIRED)

include_directories(include)

set(CONFIGURATOR_LIB_SRC
    src/oam_api.cpp
)

add_library(${PROJECT_NAME} SHARED
    ${CONFIGURATOR_LIB_SRC}
)


add_executable(udp_server_main src/udp_server_main.cpp)
target_include_directories(udp_server_main PUBLIC
    $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
    $<INSTALL_INTERFACE:include>)

install(TARGETS udp_server_main
    EXPORT export_${PROJECT_NAME}
    DESTINATION lib/${PROJECT_NAME})

if(BUILD_TESTING)
    find_package(ament_lint_auto REQUIRED)
    find_package(ament_cmake_gtest REQUIRED)

    # exclude some default linters from ament_lint
    list(APPEND AMENT_LINT_AUTO_EXCLUDE
        ament_cmake_uncrustify
    )
    list(APPEND AMENT_LINT_AUTO_EXCLUDE
        ament_cmake_copyright
    )
    ament_lint_auto_find_test_dependencies()

    # Add a gtest executables
    ament_add_gtest(${PROJECT_NAME}-test
        test/test_oam_api.cpp
    )
    target_link_libraries(${PROJECT_NAME}-test ${PROJECT_NAME})

endif()    # BUILD_TESTING

ament_package()

The package.xml file

<?xml version="1.0"?>
<?xml-model href="http://download.ros.org/schema/package_format3.xsd" schematypens="http://www.w3.org/2001/XMLSchema"?>
<package format="3">
    <name>oam_api</name>
    <version>0.0.0</version>
    <description>TODO: Package description</description>
    <maintainer email="root@todo.todo">root</maintainer>
    <license>TODO: License declaration</license>

    <buildtool_depend>ament_cmake_auto</buildtool_depend>
    <depend>rclcpp</depend>

    <test_depend>ament_cmake</test_depend>
    <test_depend>ament_cmake_gtest</test_depend>
    <test_depend>rclcpp</test_depend>
    <test_depend>ament_lint_auto</test_depend>
    <test_depend>ament_lint_common</test_depend>

    <export>
        <build_type>ament_cmake</build_type>
    </export>
</package>

Edit

I found the reason for the

undefined reference to `main'

error. I put the main() function inside a namespace so there was no global main. newbie mistake