.. _module-pw_flatbuffers:

==============
pw_flatbuffers
==============
.. pigweed-module::
   :name: pw_flatbuffers

The flatbuffers module supports the use of cmake to build C++ headers from a
set of flatbuffers schema files. The user of these build files may either use
the provided flatbuffers distribution or provide their own.

-----------------------
Flatbuffers compilation
-----------------------
pw_flatbuffers currently only supports ``cmake`` as the build system.

CMake
=====
CMake provides a ``pw_flatbuffer_library`` function to build flatbuffer schema
files into C++ headers. The user of this function is expected to configure the
flatbuffers compiler as well as provide any needed dependencies such as
flatbuffers headers. Support for outputting other languages supported by
``flatc`` is currently not implemented.


**Arguments**

* ``NAME``: Name of dependency to create. The dependency target name will be
  ``NAME.cpp``
* ``SOURCES``: Source schema files to compile to headers.
* ``DEPS``: Dependencies needed to compile the passed in schema files
* ``PREFIX``: Prefix path to be appended to the generated header path.
* ``STRIP_PREFIX``: Prefix to be removed from the generated header path.
  This is applied before ``PREFIX``.
* ``FLATC_FLAGS``: flags to be passed to ``flatc`` when building

Note here that by default, the generated header path will be the relative path
between the ``CMakeLists.txt`` containing the build rule and the schema it
compiles. The name of the include file will be the name of the schema file,
suffixed with ``_generated.h``. For example, if a ``CMakeLists.txt`` at the
toplevel includes a schema ``foo/schema.fbs``, then the compiled header would
be included as ``#include "foo/schema_generated.h"``.

**Globals**

Additionally, several global cache variables influence the behavior of
``pw_flatbuffer_library``.

* ``pw_flatbuffers_FLATC``: Path to the flatbuffers compiler executable. This
  defaults to simply using ``flatc`` and relying on the shell ``PATH`` to
  locate the compiler.
* ``pw_flatbuffers_FLATC_FLAGS``: Global flags to pass to all invocations of
  the ``flatc`` compiler. Defaults to empty.
* ``pw_flatbuffers_LIBRARY``: Flatbuffer header library dependency to
  automatically add to all flatbuffers libraries built. Defaults to unset
  unless using the pigweed provided flatbuffers by setting
  ``dir_pw_third_party_flatbuffers``, in which case this will default to the
  pigweed provided third party flatbuffers. If left unset, the user takes
  responsibility for properly providing the flatbuffer dependencies to code.

**Example**

.. code-block:: cmake

   # Point to our installed directory
   set(dir_pw_third_party_flatbuffers ${path_to_flatbuffers_install})

   # Set global flags, such as the C++ version
   set(pw_flatbuffers_FLATC_FLAGS --cpp-std c++11 CACHE LIST "Global args to flatc")

   include($ENV{PW_ROOT}/pw_build/pigweed.cmake)
   include($ENV{PW_ROOT}/pw_flatbuffers/flatbuffers.cmake)

   # Build the fbs files to headers
   pw_flatbuffer_library(simple_test_schema
     SOURCES
       pw_flatbuffers_test_schema/simple_test.fbs
   )

   # Build a test exe
   pw_add_test(pw_flatbuffers.simple_test
     SOURCES
       simple_test.cc
     PRIVATE_DEPS
       simple_test_schema.cpp
     GROUPS
       modules
   )


GN
==
GN is not currently supported.

Bazel
=====
Bazel is not currently supported.