Question

I’m using cmake for my project. No I want to split some parts into a library and use this for 2 different applications.

Now I don’t how how to do this subprojects in cmake. My first attempt was to use the add_subdirectory command:

cmake_minimum_required(VERSION 2.8)

add_subdirectory(MSI)

message("Building MsiQtWizard with: ${MSI_INCLUDE_DIR}")
add_subdirectory(MsiQtWizard)

So MSI would be my library. Inside the MSI folder is another cmakelists which is basically a standalone list for building the library. I thought I could make the MsiQtWizard project also a standalone cmakelists, so I could theoretically build MSI and use the library to build MsiQtWizard (and other projects).

The cmakelists in the root directory would just be a helper to build the library and the GUI in one single step.

The problem is, for building MsiQtWizard, I need the include path to msi and the static library binaries. I tried to do something like that at the end of MIS/CMakelists.txt:

### Set variables, other scripts may use ###
SET(MSI_INCLUDE_DIR include)
MESSAGE("Include directory is: ${MSI_INCLUDE_DIR}")

and in the MsiQtWizard/CMakelists:

##### external libraries #####

#MSI
find_path(MSI_INCLUDE_DIR REQUIRED msi/Image.hpp
            PATH_SUFFIXES MSI/include include)

My intend is, that MsiQtWizard will search for msi, if the varaible was not previously set (so that you could use this cmakelists as a standalone). When building MSI, I want to save the include path (and later binary locations) and pass it to MsiQtWizard – but the value is gone as soon as I’m back in my root cmakelists.

So that is, what I tried. My Question is now: How would I correctly split my project into a library and a (later multiple) application and can I do it in a way that I can also build them independently?

Or, more specific: How can I pass values from a node CMakelist to the root CMakeList (like I tried with MSI_INCLUDE_DIR)?

Answer 1

If your building a library – its best to completely separate it from the application build. Otherwise you are coupling your library with your application with cmake, which in my view defeats the purpose of building a library.

When building your library you will want something like

project (MSILibrary)
ADD_LIBRARY(MSILibrary  src/MSI1.cpp src/MSI2.cpp)
install (TARGETS MSILibrary DESTINATION lib)

where src contains your library code. You can then make then sudo make install your library to your standard library location (e.g. /usr/lib).

You can then use your library in any subsequent project. Put these in a new directory and create a new CMakeLists.txt for them.

You will want something like,

#include find modules
set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/Modules/")

project (MSI-project-1)

find_package(MSILibrary REQUIRED)
IF(MSILibrary_FOUND)
  include_directories(${MSILibrary_INCLUDE_DIRS} 
ENDIF(MSILibrary_FOUND )


target_link_libraries (MSI-project-1 ${MSILibrary_LIBRARIES})
install (TARGETS MSI-project-1 DESTINATION bin)

Now all you need to do is help cmake find you library.
You can include a module for this. In the file ./cmake/Modules/FindMSILibrary.cmake type something like:

# - Try to find MSILibrary library
# Once done, this will define
#
#  MSILibrary_FOUND - system has MSILibrary
#  MSILibrary_INCLUDE_DIRS - the MSILibrary include directories
#  MSILibrary_LIBRARIES - link these to use MSILibrary

## Google this script (I think its fairly standard, but was not bundled with my CMAKE)   - it helps find the macros.
include(LibFindMacros)

# Dependencies
libfind_package(MSILibrary)

# Use pkg-config to get hints about paths
libfind_pkg_check_modules(MSILibrary_PKGCONF MSILibrary)

# Include dir
find_path(MSILibrary_INCLUDE_DIR
  NAMES MSI.hpp
  PATHS ${MSI_Library_PKGCONF_INCLUDE_DIRS} 
)

# Finally the library itself
find_library(MSILibrary_LIBRARY
  NAMES MSILibrary 
  PATHS ${MSILibrary_PKGCONF_LIBRARY_DIRS} 
)

# Set the include dir variables and the libraries and let libfind_process do the rest.
# NOTE: Singular variables for this library, plural for libraries this this lib depends on.
set(MSILibrary_PROCESS_INCLUDES MSILibrary_INCLUDE_DIR MSILibrary_INCLUDE_DIRS)
set(MSILibrary_PROCESS_LIBS MSILibrary_LIBRARY MSILibrary_LIBRARIES)
libfind_process(MSILibrary)

That should be it.

EDIT:

If you really want to package your applications with your library (perhaps some example applications), then you can do something like so:

In your root CMakeLists.txt

cmake_minimum_required (VERSION 2.6)
project (MSIProject)

# The version number.
set (MSIProject_VERSION_MAJOR 0)
set (MSIProject_VERSION_MINOR 1)
set (MSIProject_PATCH_LEVEL 3 )

# project options
OPTION( BUILD_SHARED_LIBS "Set to OFF to build static libraries" ON )
OPTION( BUILD_EXAMPLES "Set to OFF to skip building the examples" ON )

# Put the libaries and binaries that get built into directories at the
# top of the build tree rather than in hard-to-find leaf
# directories.
set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)


##########################################################################
#  Build the library 
##########################################################################

add_subdirectory(MSI-src)

##################
#  Build your example Apps if requested
############


IF( BUILD_EXAMPLES )
  add_subdirectory(example/MSI-project-1)
  add_subdirectory(example/MSI-project-2)
ENDIF( BUILD_EXAMPLES )

Your library MSI-src/CMakeFiles.txt will be as before, and your example/MSI-project-1/CMakeLists.txt will be something like

## Make the InferData example project
project (MSI-project-1)

#include MSI library
include_directories ("${MSILibrary_SOURCE_DIR}/include")
#include the includes of this project
include_directories ("${MSI-project-1_SOURCE_DIR}/../include")

#build
add_executable(MSI-project-1  src/P1.cpp)
target_link_libraries (MSI-project-1 MSILibrary) #link

install (TARGETS MSI-project-1 DESTINATION bin)