qiBuild and CMake

Introduction

This section is targeted to users already using CMake for their build system.

qiBuild is built on top of CMake.

Ultimately, every qi_ functions call standard CMake functions, so for instance qi_create_bin accepts every argument the standard add_executable CMake function does.

After installing a qiBuild project, you can use the -config.cmake files generated by qiBuild even from a pure CMake project.

A very high-level point of view

One way to think about the qibuild is to compare this two few samples


            
             # Find some deps for the whole project
find_package(foo COMPONENT spam eggs)

# Add include path for every target in this file
include_directories(${FOO_INCLUDE_DIRS})

# Create a bar library
add_library(bar bar.h bar.c)

# Link the bar target with foo libraries
# (both libfoo_spam.so and libfoo_eggs.so)
target_link_libraries(bar ${FOO_LIBRARIES})

# Create a baz library
add_library(baz bar.h bar.c)

# Link the baz target with foo libraries
# (both libfoo_spam.so and libfoo_eggs.so)
target_link_libraries(bar ${FOO_LIBRARIES})

# Lots and lots of install rules

            
           

            
             qi_create_lib(bar bar.h bar.c)

# Find the SPAM component of foo,
# add the foo/spam include directory, and only link with
# libfoo_spam.so
qi_use_lib(bar FOO_SPAM)

# Find the EGGS component of foo,
# add the foo/eggs include directory, and only link with
# libfoo_eggs.so
qi_use_lib(baz FOO_EGGS)

# Install rules for bar, baz already created,
# bar-config and baz-config generated and ready to be installed to

            
           

You can see than in fact everything in qibuild is close to a target: we tie together the include directories and the libraries, making it easier to avoid weird link errors (i.e the include directory was correct but you forget to link with the library)

Also note how easy it is to make sure that someone using bar will only depend on FOO_SPAM , and not the whole FOO package.

CMake variables

To make sure you do not break qiBuild behavior you should manipulate the CMAKE_FIND_ROOT_PATH and CMAKE_MODULE_PATH variables carefully.

For instance:


            
             # This will break finding packages in the toolchain:

set(CMAKE_FIND_ROOT_PATH "/path/to/something")

# This won't:

# (create an empty list if CMAKE_FIND_ROOT_PATH does not exist)
if(NOT CMAKE_FIND_ROOT_PATH)
  set(CMAKE_FIND_ROOT_PATH)
endif()
list(APPEND CMAKE_FIND_ROOT_PATH "/path/to/something")

            
           

            
             # This will break finding the qibuild framework
#  include (qibuild/general) will no longer work

set (CMAKE_MODULE_PATH "/path/to/something")

# This won't
# (create an empty list if CMAKE_FIND_ROOT_PATH does not exist)
if(NOT CMAKE_MODULE_PATH)
  set(CMAKE_MODULE_PATH)
endif()
list(APPEND CMAKE_MODULE_PATH "/path/to/something")

            
           

CMake functions

Creating executables

Using :cmake:function: qi_create_bin will make sure that:

  • The executable is generated in build/sdk/bin
  • An install rule is created to <prefix>/bin
  • On linux, rpath is set to $ORIGIN/../lib

You can change this behavior using various NO_ arguments to qi_create_bin (for instance NO_INSTALL , NO_RPATH ...), or simply call set_target_properties yourself

Creating libraries

Using qi_create_lib will make sure that:

  • If the library is static, it is generated in build/sdk/lib
  • If the library is shared, it is generated in build/sdk/bin on Windows, and in build/sdk/lib on linux
  • The install rules are created accordingly
  • On linux, -fPIC is used so that you can use the static library inside a shared library
  • On mac, the install name dir is set to @executable_path/../lib

You can change this behavior using various NO_ arguments to qi_create_bin (for instance NO_FPIC , NO_INSTALL ...), or simply call set_target_properties yourself

The library will be:

  • built as a shared library on UNIX
  • built as a static library on Windows

You can can set BUILD_SHARED_LIBS=OFF to compile everything in static by default.

Installing

Using qi_install functions will make sure that:

  • You will get an error if the files you want to install do not exist at configuration time, not at install time.

Exporting targets

The export() and install(EXPORT ...) command do exist in standard CMake but they are a bit clumsy to use.

(See qi_stage_lib versus export for details)

In qibuild , you have a much nicer API


             
              qi_stage_lib(world)

qi_use_lib(hello world)

             
            

Using qi_use_lib in conjunction with qi_stage_lib work in any of the following cases:

  • world and hello are both targets in the same project
  • world and hello are two targets in two different projects in the same worktree (providing a small configuration file)
  • world is a package in a toolchain
  • world is a library that has been found by a custom qibuild module in cmake/qibuild/modules/world-config.cmake
  • world is a library installed on the system that has been found by an upstream CMake module in /usr/share/cmake/modules/FindWorld.cmake

Plus, qi_use_lib will export sane defaults for you:

  • include directories will be set to the last call to include_directories
  • WORLD_DEPENDS will be set using the call to qi_use_lib(... world)

And still, you will be able to stage different include directories or dependencies if you want.

Even better, you can still use standard CMake code:


             
              find_package(world)

include_directories(${WORLD_INCLUDE_DIRS})

add_library(hello)

target_libraries(hello ${WORLD_LIBRARIES})

             
            

You do not need to read the world-config.cmake because you know the exported variables will always have the same name: <target>_INCLUDE_DIRS and <target>_LIBRARIES

qitest versus CTest

See Note for CTest users

qi_stage_lib versus export

You may wonder why qi_stage_lib does not use export .

There are several reasons but the main reason is that we did not like the idea of the “global CMake package registry”.

One workflow we needed to support since the beginning was to be able to use the same worktree to compile for two different targets (say linux64 and cross-compiling)

Also, export does not work that well when you want to work with several versions of the same target (say master and a release-1.12 branch).

You can kind of solve that using version numbers (in a FooConfigVersion.cmake ) for instance, but that’s a bit clumsy too.