3.5. Project declaration

Next must-have command is project. Command project(foo) will set languages to C and C++ (default), declare some foo_* variables and run basic build tool checks.

CMake documentation

3.5.1. Tools discovering

By default on calling project command CMake will try to detect compilers for default languages: C and C++. Let’s add some variables and check where they are defined:

cmake_minimum_required(VERSION 2.8)

message("Before 'project':")
message("  C: '${CMAKE_C_COMPILER}'")
message("  C++: '${CMAKE_CXX_COMPILER}'")

project(Foo)

message("After 'project':")
message("  C: '${CMAKE_C_COMPILER}'")
message("  C++: '${CMAKE_CXX_COMPILER}'")

Examples on GitHub

Run test on Linux:

[project-examples]> rm -rf _builds
[project-examples]> cmake -Hset-compiler -B_builds
Before 'project':
  C: ''
  C++: ''
-- The C compiler identification is GNU 4.8.4
-- The CXX compiler identification is GNU 4.8.4
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
After 'project':
  C: '/usr/bin/cc'
  C++: '/usr/bin/c++'
-- Configuring done
-- Generating done
-- Build files have been written to: /.../project-examples/_builds

CMake will run tests for other tools as well, so try to avoid checking of anything before project, place all checks after project declared.

Also project is a place where toolchain file will be read.

# CMakeLists.txt

cmake_minimum_required(VERSION 2.8)

message("Before 'project'")

project(Foo)

message("After 'project'")
# toolchain.cmake

message("Processing toolchain")
[project-examples]> rm -rf _builds
[project-examples]> cmake -Htoolchain -B_builds -DCMAKE_TOOLCHAIN_FILE=toolchain.cmake
Before 'project'
Processing toolchain
Processing toolchain
-- The C compiler identification is GNU 4.8.4
-- The CXX compiler identification is GNU 4.8.4
-- Check for working C compiler: /usr/bin/cc
Processing toolchain
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
Processing toolchain
-- Detecting C compiler ABI info - done
-- Detecting C compile features
Processing toolchain
Processing toolchain
Processing toolchain
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
Processing toolchain
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
Processing toolchain
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
Processing toolchain
Processing toolchain
Processing toolchain
-- Detecting CXX compile features - done
After 'project'
-- Configuring done
-- Generating done
-- Build files have been written to: /.../project-examples/_builds

Note

You may notice that toolchain read several times

3.5.2. Languages

If you don’t have or don’t need support for one of the default languages you can set language explicitly after name of the project. This is how to setup C-only project:

cmake_minimum_required(VERSION 2.8)

message("Before 'project':")
message("  C: '${CMAKE_C_COMPILER}'")
message("  C++: '${CMAKE_CXX_COMPILER}'")

project(Foo C)

message("After 'project':")
message("  C: '${CMAKE_C_COMPILER}'")
message("  C++: '${CMAKE_CXX_COMPILER}'")

There is no checks for C++ compiler and variable with path to C++ compiler is empty now:

[project-examples]> rm -rf _builds
[project-examples]> cmake -Hc-compiler -B_builds
Before 'project':
  C: ''
  C++: ''
-- The C compiler identification is GNU 4.8.4
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
After 'project':
  C: '/usr/bin/cc'
  C++: ''
-- Configuring done
-- Generating done
-- Build files have been written to: /.../project-examples/_builds

Of course you will not be able to build C++ targets anymore. Since CMake thinks that *.cpp extension is for C++ sources (by default) there will be error reported if C++ is not listed (discovering of C++ tools will not be triggered):

cmake_minimum_required(VERSION 2.8)
project(Foo C)

add_library(foo foo.cpp)
[project-examples]> rm -rf _builds
[project-examples]> cmake -Hcpp-not-found -B_builds
-- The C compiler identification is GNU 4.8.4
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Configuring done
CMake Error: Cannot determine link language for target "foo".
CMake Error: CMake can not determine linker language for target: foo
-- Generating done
-- Build files have been written to: /.../project-examples/_builds

We can save some time by using special language NONE when we don’t need any tools at all:

cmake_minimum_required(VERSION 2.8)
project(foo NONE)

No checks for C or C++ compiler as you can see:

[project-examples]> rm -rf _builds
[project-examples]> cmake -Hno-language -B_builds
-- Configuring done
-- Generating done
-- Build files have been written to: /.../project-examples/_builds

Note

Such form will be used widely in examples in cases when we don’t need to build targets.

Note

For CMake 3.0+ sub-option LANGUAGES added, since it will be:

cmake_minimum_required(VERSION 3.0)
project(foo LANGUAGES NONE)

3.5.3. Variables

Command project declare *_{SOURCE,BINARY}_DIR variables. Since version 3.0 you can add VERSION which additionally declare *_VERSION_{MAJOR,MINOR,PATCH,TWEAK} variables:

cmake_minimum_required(VERSION 3.0)

message("Before project:")
message("  Source: ${PROJECT_SOURCE_DIR}")
message("  Binary: ${PROJECT_BINARY_DIR}")
message("  Version: ${PROJECT_VERSION}")
message("  Version (alt): ${PROJECT_VERSION_MAJOR}.${PROJECT_VERSION_MINOR}.${PROJECT_VERSION_PATCH}")

project(Foo VERSION 1.2.7)

message("After project:")
message("  Source: ${PROJECT_SOURCE_DIR}")
message("  Binary: ${PROJECT_BINARY_DIR}")
message("  Version: ${PROJECT_VERSION}")
message("  Version (alt): ${PROJECT_VERSION_MAJOR}.${PROJECT_VERSION_MINOR}.${PROJECT_VERSION_PATCH}")
[project-examples]> rm -rf _builds
[project-examples]> cmake -Hvariables -B_builds
Before project:
  Source:
  Binary:
  Version:
  Version (alt): ..
-- The C compiler identification is GNU 4.8.4
-- The CXX compiler identification is GNU 4.8.4
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
After project:
  Source: /.../project-examples/variables
  Binary: /.../project-examples/_builds
  Version: 1.2.7
  Version (alt): 1.2.7
-- Configuring done
-- Generating done
-- Build files have been written to: /.../project-examples/_builds

You can use alternative foo_{SOURCE,BINARY}_DIRS/ foo_VERSION_{MINOR,MAJOR,PATCH} synonyms. This is useful when you have hierarchy of projects:

# Top level CMakeLists.txt

cmake_minimum_required(VERSION 2.8)
project(foo)

message("From top level:")
message("  Source (general): ${PROJECT_SOURCE_DIR}")
message("  Source (foo): ${foo_SOURCE_DIR}")

add_subdirectory(boo)
# CMakeLists.txt from 'boo' directory

cmake_minimum_required(VERSION 2.8)
project(boo)

message("From subdirectory 'boo':")
message("  Source (general): ${PROJECT_SOURCE_DIR}")
message("  Source (foo): ${foo_SOURCE_DIR}")
message("  Source (boo): ${boo_SOURCE_DIR}")
[project-examples]> rm -rf _builds
[project-examples]> cmake -Hhierarchy -B_builds
-- The C compiler identification is GNU 4.8.4
-- The CXX compiler identification is GNU 4.8.4
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
From top level:
  Source (general): /.../project-examples/hierarchy
  Source (foo): /.../project-examples/hierarchy
From subdirectory 'boo':
  Source (general): /.../project-examples/hierarchy/boo
  Source (foo): /.../project-examples/hierarchy
  Source (boo): /.../project-examples/hierarchy/boo
-- Configuring done
-- Generating done
-- Build files have been written to: /.../project-examples/_builds

As you can see we are still able to use foo_* variables even if new command project(boo) called.

3.5.4. When not declared

CMake will implicitly declare project in case there is no such command in top-level CMakeLists.txt. This will be equal to calling project before any other commands. It means that project will be called before cmake_minimum_required so can lead to problems described in previous section:

# Top level CMakeLists.txt

message("Before 'cmake_minimum_required'")
cmake_minimum_required(VERSION 2.8)

add_subdirectory(boo)
# CMakeLists.txt in directory 'boo'

cmake_minimum_required(VERSION 2.8)
project(boo)
[project-examples]> rm -rf _builds
[project-examples]> cmake -Hnot-declared -B_builds
-- The C compiler identification is GNU 4.8.4
-- The CXX compiler identification is GNU 4.8.4
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
Before 'cmake_minimum_required'
-- Configuring done
-- Generating done
-- Build files have been written to: /.../project-examples/_builds

3.5.5. Summary

  • You must have project command in your top-level CMakeLists.txt
  • Use project to declare non divisible monolithic hierarchy of targets
  • Try to minimize the number of instructions before project and verify that variables are declared in such block of CMake code