docs/cmake.rst

Build systems
#############

Building with setuptools
========================

For projects on PyPI, building with setuptools is the way to go. Sylvain Corlay
has kindly provided an example project which shows how to set up everything,
including automatic generation of documentation using Sphinx. Please refer to
the [pbtest]_ repository.

.. [pbtest] https://github.com/pybind/pbtest

.. _cmake:

Building with CMake
===================

For C++ codebases that already have an existing CMake-based build system, the
following snippet should be a good starting point to create bindings across
platforms. It assumes that the code is located in a file named
:file:`example.cpp`, and that the pybind11 repository is located in a
subdirectory named :file:`pybind11`.

.. code-block:: cmake

    cmake_minimum_required(VERSION 2.8)

    project(example)

    # Add a CMake parameter for choosing a desired Python version
    set(EXAMPLE_PYTHON_VERSION "" CACHE STRING "Python version to use for compiling the example library")
    
    include(CheckCXXCompilerFlag)

    # Set a default build configuration if none is specified. 'MinSizeRel' produces the smallest binaries
    if(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
      message(STATUS "Setting build type to 'MinSizeRel' as none was specified.")
      set(CMAKE_BUILD_TYPE MinSizeRel CACHE STRING "Choose the type of build." FORCE)
      set_property(CACHE CMAKE_BUILD_TYPE PROPERTY STRINGS "Debug" "Release"
        "MinSizeRel" "RelWithDebInfo")
    endif()
    string(TOUPPER "${CMAKE_BUILD_TYPE}" U_CMAKE_BUILD_TYPE)

    # Try to autodetect Python (can be overridden manually if needed)
    set(Python_ADDITIONAL_VERSIONS 3.4 3.5 3.6 3.7)
    if (NOT ${EXAMPLE_PYTHON_VERSION} STREQUAL "")
      find_package(PythonLibs ${EXAMPLE_PYTHON_VERSION} EXACT)
      if (NOT PythonLibs_FOUND)
        find_package(PythonLibs ${EXAMPLE_PYTHON_VERSION} REQUIRED)
      endif()
    else()
      find_package(PythonLibs REQUIRED)
    endif()

    # The above sometimes returns version numbers like "3.4.3+"; the "+" must be removed for the next lines to work
    string(REPLACE "+" "" PYTHONLIBS_VERSION_STRING "+${PYTHONLIBS_VERSION_STRING}")

    # Uncomment the following line if you will also require a matching Python interpreter
    # find_package(PythonInterp ${PYTHONLIBS_VERSION_STRING} EXACT REQUIRED)

    if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" OR CMAKE_CXX_COMPILER_ID MATCHES "GNU")
      CHECK_CXX_COMPILER_FLAG("-std=c++14" HAS_CPP14_FLAG)
      CHECK_CXX_COMPILER_FLAG("-std=c++11" HAS_CPP11_FLAG)

      if (HAS_CPP14_FLAG)
        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++14")
      elseif (HAS_CPP11_FLAG)
        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
      else()
        message(FATAL_ERROR "Unsupported compiler -- at least C++11 support is needed!")
      endif()

      # Enable link time optimization and set the default symbol
      # visibility to hidden (very important to obtain small binaries)
      if (NOT ${U_CMAKE_BUILD_TYPE} MATCHES DEBUG)
        # Default symbol visibility
        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fvisibility=hidden")

        # Check for Link Time Optimization support
        CHECK_CXX_COMPILER_FLAG("-flto" HAS_LTO_FLAG)
        if (HAS_LTO_FLAG)
          set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -flto")
        endif()
      endif()
    endif()

    # Include path for Python header files
    include_directories(${PYTHON_INCLUDE_DIR})

    # Include path for pybind11 header files -- this may need to be changed depending on your setup
    include_directories(${PROJECT_SOURCE_DIR}/pybind11/include)

    # Create the binding library
    add_library(example SHARED
      example.cpp
      # ... extra files go here ...
    )

    # Don't add a 'lib' prefix to the shared library
    set_target_properties(example PROPERTIES PREFIX "")

    if (WIN32)
      if (MSVC)
        # /bigobj is needed for bigger binding projects due to the limit to 64k
        # addressable sections. /MP enables multithreaded builds (relevant when
        # there are many files).
        set_target_properties(example PROPERTIES COMPILE_FLAGS "/MP /bigobj ")

        if (NOT ${U_CMAKE_BUILD_TYPE} MATCHES DEBUG)
          # Enforce size-based optimization and link time code generation on MSVC
          # (~30% smaller binaries in experiments).
          set_target_properties(example APPEND_STRING PROPERTY COMPILE_FLAGS "/Os /GL ")
          set_target_properties(example APPEND_STRING PROPERTY LINK_FLAGS "/LTCG ")
        endif()
      endif()

      # .PYD file extension on Windows
      set_target_properties(example PROPERTIES SUFFIX ".pyd")

      # Link against the Python shared library
      target_link_libraries(example ${PYTHON_LIBRARY})
    elseif (UNIX)
      # It's quite common to have multiple copies of the same Python version
      # installed on one's system. E.g.: one copy from the OS and another copy
      # that's statically linked into an application like Blender or Maya.
      # If we link our plugin library against the OS Python here and import it
      # into Blender or Maya later on, this will cause segfaults when multiple
      # conflicting Python instances are active at the same time (even when they
      # are of the same version).

      # Windows is not affected by this issue since it handles DLL imports
      # differently. The solution for Linux and Mac OS is simple: we just don't
      # link against the Python library. The resulting shared library will have
      # missing symbols, but that's perfectly fine -- they will be resolved at
      # import time.

      # .SO file extension on Linux/Mac OS
      set_target_properties(example PROPERTIES SUFFIX ".so")

      # Strip unnecessary sections of the binary on Linux/Mac OS
      if(APPLE)
        set_target_properties(example PROPERTIES MACOSX_RPATH ".")
        set_target_properties(example PROPERTIES LINK_FLAGS "-undefined dynamic_lookup ")
        if (NOT ${U_CMAKE_BUILD_TYPE} MATCHES DEBUG)
          add_custom_command(TARGET example POST_BUILD COMMAND strip -u -r ${PROJECT_BINARY_DIR}/example.so)
        endif()
      else()
        if (NOT ${U_CMAKE_BUILD_TYPE} MATCHES DEBUG)
          add_custom_command(TARGET example POST_BUILD COMMAND strip ${PROJECT_BINARY_DIR}/example.so)
        endif()
      endif()
    endif()