PAHO_WITH_MQTT_C OFF export error

[ryanjmulder]

When I set PAHO_WITH_MQTT_C to OFF, I get the cmake error:

export called with target "paho-mqttpp3-static" which requires target
[cmake]   "paho-mqtt3as-static" that is not in any export set.

It seems to be because these lines only happen in the PAHO_WITH_MQTT_C block

## install paho.mqtt.c library (appending to PahoMqttCpp export)
    install(TARGETS ${PAHO_MQTT_C_LIBS}
        EXPORT PahoMqttCpp
        ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
        LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
        RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
    )

[fpagliughi]

The way the build is set up for this C++ library is that the shared library links against the C shared library, and the static library links against the static C library.

Assuming that you configured the C++ lib for a static build, the two possibilities I can think of is that (1) The C lib was compiled without the static option, or (2) It's an old C lib that didn't properly name the static target.

But I will look if there is an issue with this library's build or if there is any sort of work-around.

[fpagliughi]

Yes, I believe my previous explanation is correct.

When you use PAHO_WITH_MQTT_C=ON , the CMake for the C++ library will not only build the C lib, but also make sure that it's a compatible version and that the build is properly configured to match those chosen for the C++ library. So, if you request the static build for C++, it will also build and link the static C lib.

If you then turn off PAHO_WITH_MQTT_C and attempt to build against a C lib that was not configured to match the C++ build, you may get a configuration error.

How to reproduce:

$ mkdir build ; cd build/
$ cmake .. -DPAHO_BUILD_EXAMPLES=ON -DPAHO_BUILD_TESTS=ON -DPAHO_BUILD_DEB_PACKAGE=ON -DPAHO_BUILD_STATIC=ON -DPAHO_WITH_MQTT_C=ON
...
$ cmake --build .

Thus should build fine. Now, turn off the automatic C build and use what's already installed:

$ cmake .. -DPAHO_WITH_MQTT_C=OFF  -DCMAKE_PREFIX_PATH=/usr/local
-- Found Paho C v1.3.16
-- Creating shared library
-- Found Catch2 v2.13.10
-- Configuring done (0.0s)
CMake Error at src/CMakeLists.txt:100 (target_link_libraries):
  Target "paho-mqttpp3-static" links to:

    eclipse-paho-mqtt-c::paho-mqtt3as-static

  but the target was not found.  Possible reasons include:

    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.



-- Generating done (0.1s)
CMake Generate step failed.  Build files cannot be regenerated correctly.

Here, the installed Paho C lib was not built with the static library enabled; so having it enabled for C++ causes an error.

If you turn off the static option in the C++ lib, then it should configure and build properly:

$ cmake .. -DPAHO_BUILD_STATIC=OFF -DPAHO_WITH_MQTT_C=OFF -DCMAKE_PREFIX_PATH=$HOME/usr/local
-- Found Paho C v1.3.16
-- Creating shared library
-- Found Catch2 v2.13.10
-- Configuring done (0.1s)
-- Generating done (0.1s)
-- Build files have been written to: /home/fmp/mqtt/paho-mqttpp/build

[fpagliughi]

I believe this is the problem. If there's any additional problem, feel free to re-open.

[ryanjmulder]

Thanks for taking a look at this.
I now see that I left out some critical information, I'm so sorry!
I'm using FetchContent to get the latest tag of the C lib and C++ lib, setting PAHO_WITH_MQTT_C to OFF, and then building both statically.

I've attached a minimal example:
main.cpp
CMakeLists.txt
CMakeLists-should-work.txt

In the working CMakeLists.txt, I'm adding these lines:

install(TARGETS paho-mqtt3a-static
        EXPORT PahoMqttCpp
        ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
        LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
        RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
    )

to prevent this error:

[cmake]   export called with target "paho-mqttpp3-static" which requires target
[cmake]   "paho-mqtt3a-static" that is not in any export set.

but, I think the CMakeLists-should-work.txt should work

[fpagliughi]

Let's reopen this to keep the discussion going.

Do you know which version of each library are found?

[ryanjmulder]

$ git -C build/_deps/paho.mqtt.cpp-src/ status
HEAD detached at v1.5.3
nothing to commit, working tree clean

$ git -C build/_deps/eclipse-paho-mqtt-c-src/ status
HEAD detached at v1.3.16
nothing to commit, working tree clean