编译C++ Paho MQTT代码报错：找不到Paho MQTT C库

It seems your Paho MQTT C library is installed properly, but the CMake build system for the C++ library can't locate it. This is a common hiccup on Ubuntu 16.04, usually tied to dynamic link cache updates or CMake's default search path limitations. Here are targeted fixes to resolve this:

1. Refresh the system dynamic link cache #

When you install libraries to /usr/local/lib (the default location for sudo make install), Ubuntu 16.04 doesn't automatically update its dynamic linker cache. This means the system (and CMake) might not recognize the newly installed Paho C library. Run this command first:

sudo ldconfig

Then try rebuilding the C++ library again, clearing old build artifacts to avoid conflicts:

cd ~/Paho_MQTT/paho_cpp/paho.mqtt.cpp/build
rm -rf *
cmake ..
make

2. Manually specify the Paho C library path to CMake #

If the first step doesn't work, CMake might not be checking /usr/local by default. You can explicitly point it to the C library's headers and binaries:

First, confirm the default installation paths (these are correct if you used the standard make install):

• Header files: /usr/local/include
• Library file: /usr/local/lib/libpaho-mqtt3a.so (shared library) or /usr/local/lib/libpaho-mqtt3a.a (static library)

Run CMake with these paths specified:

cd ~/Paho_MQTT/paho_cpp/paho.mqtt.cpp/build
rm -rf *
cmake -DPAHO_MQTT_C_INCLUDE_DIR=/usr/local/include -DPAHO_MQTT_C_LIBRARY=/usr/local/lib/libpaho-mqtt3a.so ..
make

If you built the static version of the Paho C library, replace .so with .a in the library path.

3. Verify shared library support in Paho C library (optional) #

If issues persist, check if you enabled shared library builds when compiling the Paho C library—the C++ library typically expects this by default:

1. Navigate back to your Paho C build directory:

   cd ~/Paho_MQTT/paho_c/paho.mqtt.c/build
   

2. Reopen the CMake configuration:

   ccmake ..
   

3. Use arrow keys to navigate to BUILD_SHARED_LIBS and toggle it to ON (press Enter to change the value).
4. Recompile and reinstall the C library:

   make
   sudo make install
   sudo ldconfig
   

5. Return to the C++ library build directory and try compiling again.

内容的提问来源于stack exchange，提问作者Salman