Build dependency in CMake

Question

I want to build and use Botan linked statically in my application, which means first

python configure.py --disable shared

followed by

make

And then I'd like to create a dependency target for libbotan.a as "libbotan" that I can use in the rest of my CMake files. Since Botan not using CMake I'm not sure how to include the dependency in my project.

My current attempt looks like this:

in deps/Botan.1.11.7/CmakeLists.txt

add_custom_command(OUTPUT botan-configure COMMAND ./configure.py --disable-shared)
add_custom_command(OUTPUT botan-build COMMAND make DEPENDS botan-configure)
add_custom_target(botan DEPENDS botan-configure botan-build)

but when I added botan as a dependency in core/CMakeLists.txt like this

add_executable(console console.cpp)
target_link_libraries(console messages core botan ${Boost_LIBRARIES})

I get

CMake Error at simpleclient/CMakeLists.txt:5 (target_link_libraries):
 Target "botan" of type UTILITY may not be linked into another target.  One
 may link only to STATIC or SHARED libraries, or to executables with the
 ENABLE_EXPORTS property set.

I tried using ExternalProject_Add like this

ExternalProject_Add(botan
    GIT_REPOSITORY https://github.com/randombit/botan.git
    CONFIGURE_COMMAND python configure.py --disable-shared
    BUILD_COMMAND make
    INSTALL_COMMAND make install
)

But that gives me the same error.

Answer 1

Take a look at the ExternalProject module:

The 'ExternalProject_Add' function creates a custom target to drive download, update/patch, configure, build, install and test steps of an external project

Note that this will only create a utility target. That is, you may run the target to build the library and you may add dependencies from your project's targets to the utility target. You can not however link to the target directly.

You still need to obtain the paths to the library and include directories of the external project manually. Since the project in question does not seem to use CMake by itself, that means writing your own calls to find_library, find_path, etc. and use the results from those calls to properly integrate the dependency.

Since the external project is built as part of your normal CMake run it should be quite easy to obtain the correct values by searching in the install path given to ExternalProject_Add.