How to include a static library using cmake?

Question

I want to add the library ixwebsocket my project. Following the instructions of the library I get the libixwebsocket.a (the compiled static library, correct me if I am wrong). How I can use this .a file in my project?

For testing I have created the following program: main.cpp

#CMakeLists.txt
cmake_minimum_required(VERSION 3.0.0)
project(cpptest VERSION 0.1.0)

include(CTest)
enable_testing()

add_executable(cpptest main.cpp)

set(CPACK_PROJECT_NAME ${PROJECT_NAME})
set(CPACK_PROJECT_VERSION ${PROJECT_VERSION})
include(CPack)

target_link_libraries(cpptest "${CMAKE_SOURCE_DIR}/libixwebsocket.a")

//main.cpp
#include <iostream>
#include <ixwebsocket/IXNetSystem.h>
#include <ixwebsocket/IXWebSocket.h>

int main(int, char**) {
    std::cout << "Hello, world!\n";

    ix::WebSocket webSocket;
}

In the directory I have CMakeLists.txt, libixwebsocket.a and main.cpp.

I get a long list of undefined reference errors, the firsts ones:

[build] [ 50%] Linking CXX executable cpptest
[build] /usr/bin/ld: ../libixwebsocket.a(IXWebSocket.cpp.o): in function `ix::WebSocket::start()':
[build] IXWebSocket.cpp:(.text+0x2240): undefined reference to `pthread_create'
ld] /usr/bin/ld: ../libixwebsocket.a(IXWebSocket.cpp.o): in function `ix::WebSocket::checkConnection(bool)':
[build] IXWebSocket.cpp:(.text+0x5b8d): undefined reference to `pthread_cond_clockwait'
[build] /usr/bin/ld: ../libixwebsocket.a(IXWebSocketPerMessageDeflateCodec.cpp.o): in function `bool ix::WebSocketPerMessageDeflateCompressor::compressData<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&) [clone .part.0]':
[build] IXWebSocketPerMessageDeflateCodec.cpp:(.text+0x67): undefined reference to `deflate'
[build] /usr/bin/ld: ../libixwebsocket.a(IXWebSocketPerMessageDeflateCodec.cpp.o): in function `ix::WebSocketPerMessageDeflateCompressor::~WebSocketPerMessageDeflateCompressor()':
[build] IXWebSocketPerMessageDeflateCodec.cpp:(.text+0x35c): undefined reference to `deflateEnd'
[build] /usr/bin/ld: ../libixwebsocket.a(IXWebSocketPerMessageDeflateCodec.cpp.o): in function `ix::WebSocketPerMessageDeflateCompressor::init(unsigned char, bool)':
[build] IXWebSocketPerMessageDeflateCodec.cpp:(.text+0x3a6): undefined reference to `deflateInit2_'
[build] /usr/bin/ld: ../libixwebsocket.a(IXWebSocketPerMessageDeflateCodec.cpp.o): in function `ix::WebSocketPerMessageDeflateDecompressor::~WebSocketPerMessageDeflateDecompressor()':
[build] IXWebSocketPerMessageDeflateCodec.cpp:(.text+0x64c): undefined reference to `inflateEnd'
[build] /usr/bin/ld: ../libixwebsocket.a(IXWebSocketPerMessageDeflateCodec.cpp.o): in function `ix::WebSocketPerMessageDeflateDecompressor::init(unsigned char, bool)':
[build] IXWebSocketPerMessageDeflateCodec.cpp:(.text+0x685): undefined reference to `inflateInit2_'
[build] /usr/bin/ld: ../libixwebsocket.a(IXWebSocketPerMessageDeflateCodec.cpp.o): in function `ix::WebSocketPerMessageDeflateDecompressor::decompress(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&)':
[build] IXWebSocketPerMessageDeflateCodec.cpp:(.text+0x7d6): undefined reference to `inflate'
[build] /usr/bin/ld: ../libixwebsocket.a(IXWebSocketPerMessageDeflateCodec.cpp.o): in function `bool ix::WebSocketPerMessageDeflateCompressor::compressData<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::vector<unsigned char, std::allocator<unsigned char> > >(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::vector<unsigned char, std::allocator<unsigned char> >&) [clone .part.0]':
[build] IXWebSocketPerMessageDeflateCodec.cpp:(.text+0x90d): undefined reference to `deflate'
[build] /usr/bin/ld: ../libixwebsocket.a(IXSocketOpenSSL.cpp.o): in function `ix::SocketOpenSSL::openSSLInitialize()':
[build] IXSocketOpenSSL.cpp:(.text+0x1c): undefined reference to `OPENSSL_init_ssl'
[build] /usr/bin/ld: IXSocketOpenSSL.cpp:(.text+0x29): undefined reference to `OPENSSL_init_ssl'

How may these undefined refrence errors be resolved? Thanks in advance!

Answer 1

It would be always best to use find_package() calls. This way you ensure that the library is indeed installed on your system, or is exported in the build tree.

If you have the library installed on your system you can use this call, like this:

find_package(ixwebsocket REQUIRED)
target_link_libraries(cpptest ixwebsocket::ixwebsocket)

In your case, since the library is compiled independently (and it is not a target in your tree), you need to use find_library(IXWEBSOCKET_LIB ixwebsocket) and specify additional hints/names if you have the library in your own location, following the docs.

Answer 2

I ran into a similar issue today, the library was linking just fine but a static lib is just a collection of .o files and has no understanding of it's own dependencies...

This command worked for me:

g++ -o output/myapp -std=c++17 myapp.cpp -Llib -framework CoreFoundation -framework Security -lmylib

I'm no expert in CMake but this answer seems to indicate that the below would achieve the same effect

target_link_libraries(program "-framework CoreFoundation" "-framework Security")

So if we apply it to your case:

#CMakeLists.txt
cmake_minimum_required(VERSION 3.0.0)
project(cpptest VERSION 0.1.0)

include(CTest)
enable_testing()

add_executable(cpptest main.cpp)

set(CPACK_PROJECT_NAME ${PROJECT_NAME})
set(CPACK_PROJECT_VERSION ${PROJECT_VERSION})
include(CPack)

target_link_libraries(cpptest "${CMAKE_SOURCE_DIR}/libixwebsocket.a" "-framework CoreFoundation")

I've got a feeling pthread_create is likely to live in CoreFoundation but I'm just guessing... You may need to try a few frameworks to find the right one.

Hope this helps a little :)