install the library to a non-standard location fails on find lib and undefined headers

[SaKa1979]

I'm building Paho_cpp as part of a project (Linux). The paho_cpp repository (commit #2ff3d155dcd10564f1816675789284b4efd79eb7) is added as a submodule. The paho_c repository (commit #f7799da95e347bbc930b201b52a1173ebbad45a7) is added as a submodule of paho_cpp.
The /usr/local/lib does not contain any paho libraries.

To build the libraries and install them in a non-standard location I use the following:

C library:

cmake -Bbuild -H. -DPAHO_WITH_SSL=OFF \
                  -DPAHO_BUILD_SHARED=OFF \
                  -DPAHO_BUILD_STATIC=ON \
                  -DPAHO_BUILD_DOCUMENTATION=OFF \
                  -DPAHO_BUILD_SAMPLES=OFF \
                  -DPAHO_BUILD_DEB_PACKAGE=OFF \
                  -DPAHO_ENABLE_TESTING=OFF \
                  -DPAHO_ENABLE_CPACK=OFF \
                  -DPAHO_HIGH_PERFORMANCE=OFF \
                  -DPAHO_USE_SELECT=OFF \
                  -DCMAKE_INSTALL_PREFIX=./build/_install

cmake --build build/ --target install

CPP library:

cmake -Bbuild -H. -DPAHO_BUILD_STATIC=ON \
                  -DPAHO_BUILD_SHARED=OFF \
                  -DPAHO_WITH_SSL=OFF \
                  -DPAHO_BUILD_DEB_PACKAGE=OFF \
                  -DPAHO_BUILD_SAMPLES=ON \
                  -DPAHO_BUILD_TESTS=OFF \
                  -DPAHO_BUILD_DOCUMENTATION=OFF \
                  -DCMAKE_INSTALL_PREFIX=./build/_install \
                  -DCMAKE_PREFIX_PATH=paho_c/build/_install

cmake --build build/ --target install

Without changing anything to the paho_cpp files, find_package(PahoMqttC REQUIRED) fails. So I added the specific paths to the FindPahoMqttC.cmake like so:

find_library(PAHO_MQTT_C_LIBRARIES NAMES ${_PAHO_MQTT_C_LIB_NAME} PATHS ${PROJECT_SOURCE_DIR}/paho_c/build/_install/lib/)
find_path(PAHO_MQTT_C_INCLUDE_DIRS NAMES MQTTAsync.h PATHS ${PROJECT_SOURCE_DIR}/paho_c/build/_install/include/)

In my own project I've added a FindPahoMqttCpp.cmake file to find PahoMqttCpp. This works, the lib is found.
The project's CMakeList contains:

add_library(${LIB_NAME} STATIC ${SOURCES})
target_link_libraries(${LIB_NAME} PRIVATE paho-mqttpp3)

target_include_directories(${LIB_NAME} PUBLIC ${CMAKE_CURRENT_LIST_DIR}
                            ${PROJECT_SOURCE_DIR}/paho_cpp/build/_install/include/

Building results in /usr/bin/ld: cannot find -lpaho-mqttpp3: No such file or directory. So, it's apparently unable to find the library.
I also need to explicitly target the _install/include directory so the mqtt/async_client.h file is located.

First I don't understand why it will not build out-of-the box when using a non-standard installation directory.
Second, why the linker error when the libraries are found by the build system?
Third, Why is necessary to explicitly target the include directory?

I hope I'm in the right place to post these questions and I especially hope somebody can help me out here. I've been struggling with this for multiple days now. Thanks in advance.

[fpagliughi]

Yes, this is the right place! I've been trying to get up to speed on CMake recently, as the build system was implemented by someone else who wandered off a while back. But I'm starting to get the hang of it now, and will see if I can help figure this out.

[philipp-schmidt]

+1 for updates to cmake, @SaKa1979 I agree the exported targets are not of much help.
I will have a look and see what I can do later today.
@fpagliughi you may have to check the windows build in the end, no way of testing that for me.

[philipp-schmidt]

I have opted to use the c api directly instead, it works out of the box with cmake.

[philipp-schmidt]

@SaKa1979 I revisited the cmake build for paho.mqtt.cpp and it turns out it is was easier than I thought.
You only have to target_link_libraries against the exported target PahoMqttCpp::paho-mqttpp3. It will take care of importing all includes and libraries.

So in your case, remove all find_library, find_path and target_include_directories calls, anything that you added in your issue description. All you need is roughly this:

find_package(PahoMqttCpp REQUIRED)
add_executable(myapp ${MYSOURCES})
target_link_libraries(myapp PahoMqttCpp::paho-mqttpp3)

Also you have to set your CMAKE_PREFIX_PATH so that both import targets can be found, paho.mqtt.cpp as well as paho.mqtt.c:

cmake .. -DCMAKE_PREFIX_PATH="/pathto/paho.mqtt.c/build/install/;/pathto/paho.mqtt.cpp/build/install"

[philipp-schmidt]

I got the final clue from this Stackoverflow post, even though the naming of the targets provided in the answer are wrong. But the general idea was correct:
https://stackoverflow.com/questions/55799122/compile-pahomqttcpp-sample-as-standalone-with-cmake-on-linux

[SaKa1979]

Thank you very much!
I will try and come back to tell how it went.

[dimonoid]

Thank you very much! I will try and come back to tell how it went.

How did it go? What was your solution?