Introduction to cmake and ninja
===============================

Cmake plus ninja is approximately equal to GNU autotools plus GNU
make, respectively. Both cmake and GNU autotools support self and
cross-compilation, checking for required components and versions.

- For a decent-sized project - such as vpp - build performance is drastically better with (cmake, ninja).

- The cmake input language looks like an actual language, rather than a shell scripting scheme on steroids.

- Ninja doesn't pretend to support manually-generated input files. Think of it as a fast, dumb robot which eats mildly legible byte-code.

See the `cmake website <http://cmake.org>`_, and the `ninja website
<https://ninja-build.org>`_ for additional information.

vpp cmake configuration files
-----------------------------

The top of the vpp project cmake hierarchy lives in .../src/CMakeLists.txt.
This file defines the vpp project, and (recursively) includes two kinds
of files: rule/function definitions, and target lists.

- Rule/function definitions live in .../src/cmake/{\*.cmake}. Although the contents of these files is simple enough to read, it shouldn't be necessary to modify them very often

- Build target lists come from CMakeLists.txt files found in subdirectories, which are named in the SUBDIRS list in .../src/CMakeLists.txt

::
   
    ##############################################################################
    # subdirs - order matters
    ##############################################################################
    if("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux")
      find_package(OpenSSL REQUIRED)
      set(SUBDIRS
        vppinfra svm vlib vlibmemory vlibapi vnet vpp vat vcl plugins
        vpp-api tools/vppapigen tools/g2 tools/elftool tools/perftool)
    elseif("${CMAKE_SYSTEM_NAME}" STREQUAL "Darwin")
      set(SUBDIRS vppinfra)
    else()
      message(FATAL_ERROR "Unsupported system: ${CMAKE_SYSTEM_NAME}")
    endif()

    foreach(DIR ${SUBDIRS})
      add_subdirectory(${DIR})
    endforeach()

- The vpp cmake configuration hierarchy discovers the list of plugins to be built by searching for subdirectories in .../src/plugins which contain CMakeLists.txt files
  

::

    ##############################################################################
    # find and add all plugin subdirs
    ##############################################################################
    FILE(GLOB files RELATIVE
      ${CMAKE_CURRENT_SOURCE_DIR}
      ${CMAKE_CURRENT_SOURCE_DIR}/*/CMakeLists.txt
    )
    foreach (f ${files})
      get_filename_component(dir ${f} DIRECTORY)
      add_subdirectory(${dir})
    endforeach()
  
How to write a plugin CMakeLists.txt file
-----------------------------------------

It's really quite simple. Follow the pattern:

:: 

    add_vpp_plugin(mactime
      SOURCES
      mactime.c
      node.c

      API_FILES
      mactime.api

      INSTALL_HEADERS
      mactime_all_api_h.h
      mactime_msg_enum.h

      API_TEST_SOURCES
      mactime_test.c
    )   

Adding a target elsewhere in the source tree
--------------------------------------------

Within reason, adding a subdirectory to the SUBDIRS list in
.../src/CMakeLists.txt is perfectly OK. The indicated directory will
need a CMakeLists.txt file.

Here's how we build the g2 event data visualization tool:

:: 

    option(VPP_BUILD_G2 "Build g2 tool." OFF)
    if(VPP_BUILD_G2)
      find_package(GTK2 COMPONENTS gtk)
      if(GTK2_FOUND)
        include_directories(${GTK2_INCLUDE_DIRS})
        add_vpp_executable(g2
          SOURCES
          clib.c
          cpel.c
          events.c
          main.c
          menu1.c
          pointsel.c
          props.c
          g2version.c
          view1.c

          LINK_LIBRARIES vppinfra Threads::Threads m ${GTK2_LIBRARIES}
          NO_INSTALL
        )
      endif()
    endif()

The g2 component is optional, and is not built by default. There are
a couple of ways to tell cmake to include it in build.ninja [or in Makefile.]

When invoking cmake manually [rarely done and not very easy], specify
-DVPP_BUILD_G2=ON:

::

   $ cmake ... -DVPP_BUILD_G2=ON

Take a good look at .../build-data/packages/vpp.mk to see where and
how the top-level Makefile and .../build-root/Makefile set all of the
cmake arguments. One strategy to enable an optional component is fairly
obvious. Add -DVPP_BUILD_G2=ON to vpp_cmake_args.

That would work, of course, but it's not a particularly elegant solution. 

Tinkering with build options: ccmake
------------------------------------

The easy way to set VPP_BUILD_G2 - or frankly **any** cmake
parameter - is to install the "cmake-curses-gui" package and use
it.

- Do a straightforward vpp build using the top level Makefile, "make build" or "make build-release"
- Ajourn to .../build-root/build-vpp-native/vpp or .../build-root/build-vpp_debug-native/vpp
- Invoke "ccmake ." to reconfigure the project as desired

Here's approximately what you'll see:

::

     CCACHE_FOUND                     /usr/bin/ccache                              
     CMAKE_BUILD_TYPE                                                              
     CMAKE_INSTALL_PREFIX             /scratch/vpp-gate/build-root/install-vpp-nati
     DPDK_INCLUDE_DIR                 /scratch/vpp-gate/build-root/install-vpp-nati
     DPDK_LIB                         /scratch/vpp-gate/build-root/install-vpp-nati
     MBEDTLS_INCLUDE_DIR              /usr/include                                 
     MBEDTLS_LIB1                     /usr/lib/x86_64-linux-gnu/libmbedtls.so      
     MBEDTLS_LIB2                     /usr/lib/x86_64-linux-gnu/libmbedx509.so     
     MBEDTLS_LIB3                     /usr/lib/x86_64-linux-gnu/libmbedcrypto.so   
     MUSDK_INCLUDE_DIR                MUSDK_INCLUDE_DIR-NOTFOUND                   
     MUSDK_LIB                        MUSDK_LIB-NOTFOUND                           
     PRE_DATA_SIZE                    128                                          
     VPP_API_TEST_BUILTIN             ON                                           
     VPP_BUILD_G2                     OFF                                          
     VPP_BUILD_PERFTOOL               OFF                                          
     VPP_BUILD_VCL_TESTS              ON                                           
     VPP_BUILD_VPPINFRA_TESTS         OFF                                          

    CCACHE_FOUND: Path to a program.                                                
    Press [enter] to edit option Press [d] to delete an entry   CMake Version 3.10.2
    Press [c] to configure
    Press [h] for help           Press [q] to quit without generating
    Press [t] to toggle advanced mode (Currently Off)

Use the cursor to point at the VPP_BUILD_G2 line. Press the return key
to change OFF to ON. Press "c" to regenerate build.ninja, etc.

At that point "make build" or "make build-release" will build g2. And so on.

Note that toggling advanced mode ["t"] gives access to substantially
all of the cmake option, discovered directories and paths.