Create a shared library for dlib

Question

Following the instructions to compile dlib using cmake (here) generates a static dlib library:

cd examples
mkdir build
cd build
cmake ..
cmake --build . --config Release

How can I instruct cmake to generate a shared (.so) library instead?

Bellman

Answer 1

If you want to make a .so file then do this:

cd dclib/dlib
mkdir build
cd build
cmake -DBUILD_SHARED_LIBS=1 ..
make
sudo make install

On a unix system, that will install dlib system wide. This means installing the .so file and also the header files so you can compile programs with a command like g++ main.cpp -ldlib. Finally, on linux systems you will also need to run sudo ldconfig after installing any new shared libraries.

However, for most users, I would recommend using CMake as shown in the examples. That way would allow you to enable or disable debugging modes whenever you want and also makes distributing the project easier, both in source form and compiled form. For example, if you wanted to compile on windows then shared libraries are definitely not the way to go. Moreover, using CMake as shown in the examples will always work in a straightforward manner without any setup.

Auger

Thanks Davis! The last line does not work for me. It seems that there is no rule to make target install. I am running the make install in the build folder. – Bellman

This doesn't create any .so files anywhere on Linux (Ubuntu 16.04) for me. Just puts a .a file in /usr/local/lib/. locate libdlib just returns: /usr/local/lib/libdlib.a – Helvetian

Yeah. These instructions are dated. I just fixed them. – Auger

@DavisKing How to link an example file say face_recognition_ex.cpp? I created the -o file using g++ -c -I../ -std=c++11 dnn_face_recognition_ex.cpp Then to link, I tried g++ dnn_face_recognition_ex.o -o dnn_face_recognition_ex -ldlib but get this error ld: library not found for -ldlib – Decomposer

Answer 2

According to dlib/CMakeLists.txt, standalone (not from examples) building of dlib also creates shared library named dlib-shared:

mkdir shared_build # Build directory can be any
cd shared_build
cmake ..
cmake --build . --config Release
make install # Install library for make it acessible for others

For use this library in examples, you need to add definition of dlib library into your examples/CMakeLists.txt before include(../dlib/cmake).

examples/CMakeLists.txt:

...
PROJECT(examples)

add_library(dlib SHARED IMPORTED) # Imported(!) dlib target
set_target_properties(dlib PROPERTIES IMPORTED_LOCATION "<full path to the installed dlib-shared library file>")

# Now it is safe to include other dlib infrustucture - it won't build dlib again.
include(../dlib/cmake)
...

Awaken

This isn't how you compile a dlib shared library. – Auger

Except missed make install(just forgot about it, thinking it goes without saying), what is wrong with my answer? – Awaken

The add_library statement is not going to work in a sensible way. – Auger

The add_library statement is not going to work in a sensible way - Why? add_library(... IMPORTED) is one of the common ways for use 3d-party libraries. – Awaken

If you want to use cmake to link with an installed library use cmake's find_package command. Or at the very least don't include dlib/cmake. That's for static builds. – Auger

That's for static builds. - Why dlib/cmake is for static build only? I haven't find any evidence of that by looking into code. The reason of using add_library(dlib SHARED IMPORTED) is exactly for being combined with include(dlib/cmake), which does useful things aside from the linking. find_package() approach will not work in this case, as it declares dlib target as static, so example will be again linked statically. – Awaken