Undefined link with cmake using native libraries

Question

I have the following 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

helperTwo libraries are created in the project : libh_a.aand libh_b.a. libh_a.aused to build libh_b.a. The files are as follows:

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 ../srcfrom helper/build(no error messages) than cmake ../srcfrom 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()'

The problem seems to be related to h_b.cpp, but libh_b.awas built earlier without errors.

Answer 1

The first problem is that your libraries were not 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

( DOC find_library(), " " .)

" " , find_library():

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

( FindXyz.cmake, , find_package( Xyz REQUIRED ), . , , .)

find_library:

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

CMake, , , , .

:

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

-NOTFOUND. ( h_b.)

, ...

target_link_libraries(
    proj_c
    h_a
    h_b
)

... . proj_c func_b(), h_a, h_b, , , func_a(), , .

-lh_a -lh_b .

(proj_c h_b h_a):

target_link_libraries(
    proj_c
    h_b
    h_a
)

.