Undefined reference with cmake using own libraries

Question

I have the folllowing directory structure:

-project -helper -build -include -h_a.h -h_b.h -src -h_a -h_a.cpp -CMakeLists.txt -h_b -h_b.cpp -CMakeLists.txt -CMakeLists.txt -proj_c -build -src -main.cpp -CMakeLists.txt

In the helper project two libraries are generated: libh_a.a and libh_b.a. libh_a.a is used to build libh_b.a. The files are the following:

helper/src/CMakeLists.txt:

cmake_minimum_required(VERSION 2.6) project(helper) set(CMAKE_CXX_FLAGS "-Wall -g -std=c++11") add_subdirectory(h_a) add_subdirectory(h_b)

helper/src/h_a/CMakeLists.txt:

project(h_a) add_library(h_a h_a.cpp)

helper/src/h_a/h_a.cpp

void func_a(){}

helper/src/h_b/CMakeLists.txt:

project(h_b) add_library(h_b h_b.cpp) target_link_libraries( h_b STATIC h_a )

helper/src/h_b/h_b.cpp:

#include "../../include/h_a.h" void func_b(){ func_a(); }

proj_c/src/CMakeLists.txt:

cmake_minimum_required(VERSION 3.2) project(proj_c) find_library(h_a PATHS ../../helper/build/h_a) link_directories(${CMAKE_CURRENT_SOURCE_DIR}/../../helper/build/h_a) find_library(h_b PATHS ../../helper/build/h_b) link_directories(${CMAKE_CURRENT_SOURCE_DIR}/../../helper/build/h_b) set(CMAKE_RUNTIME_OUTPUT_DIRECTORY "../bin/") add_executable(proj_c main.cpp) target_link_libraries( proj_c h_a h_b )

proj_c/src/main.cpp:

#include "../../helper/include/h_b.h" int main(){ func_b(); }

First I run cmake ../src from helper/build (no error messsages) than cmake ../src from proj_c/build and I got

proj_c/src/../../helper/build/h_b/libh_b.a(h_b.cpp.o): In function `func_b()': helper/src/h_b/h_b.cpp:4: undefined reference to `func_a()'

It seems that the problem is with h_b.cpp, but libh_b.a was built previously without errors.

Unrelated generic CMake suggestion: set( CMAKE_CXX_STANDARD 11 ) to set C++11 platform-independently. Set CMAKE_BUILD_TYPE to Debug (instead of adding -g to CMAKE_CXX_FLAGS) to get debug information compiled in platform-independently. Check for CMAKE_COMPILER_IS_GNUCC before setting -Wall. — DevSolar
– DevSolar, Commented Feb 1, 2016 at 9:02

DevSolar · Accepted Answer · 2016-02-01 10:25:00Z

The first problem is that your libraries aren't found. In proj_c/build/CMakeCache.txt:

//Path to a library. h_a:FILEPATH=h_a-NOTFOUND //Path to a library. h_b:FILEPATH=h_b-NOTFOUND

(Use the DOC option to find_library() to get the generic "Path to a library" replaced with something more helpful.)

This kind of "silent failure" can be overcome by adding a check after find_library():

if ( NOT h_a ) message( FATAL_ERROR "Library h_a not found in ${CMAKE_CURRENT_SOURCE_DIR}/../../helper/build/h_a" ) endif()

(For larger projects you will have a FindXyz.cmake module set up for finding dependencies, which you can call via find_package( Xyz REQUIRED ) to avoid silent failure. But that, and the helper functions available to you when writing such a module, are a subject all of its own.)

The failure itself is due to a problem with your use of find_library:

find_library(h_a PATHS ../../helper/build/h_a)

You tell CMake where to look, and in which variable to store the result, but not what to look for.

Use:

find_library(h_a NAMES h_a PATHS ../../helper/build/h_a)

This will resolve the -NOTFOUND. (Apply for h_b as well.)

You will still get the linker error though, because of...

target_link_libraries( proj_c h_a h_b )

...linking order. proj_c looks for func_b(), does not find it in h_a, does find it in h_b, which in turn requires func_a(), which isn't found because the linker is making a single pass over the library list only.

It's the same with specifyling -lh_a -lh_b on the command line.

Switch the libraries around according to the dependency chain (proj_c depends on h_b depends on h_a):

target_link_libraries( proj_c h_b h_a )

That will work.

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