.. _build_overview:

Build Overview
##############

The Zephyr build process can be divided into two main phases: a configuration
phase (driven by CMake) and a build phase (driven by Make or Ninja).

.. _build_configuration_phase:

Configuration Phase
*******************

The configuration phase begins when the user invokes *CMake*,
specifying a source application directory and a board target.

.. figure:: build-config-phase.svg
    :align: center
    :alt: Zephyr's build configuration phase
    :figclass: align-center
    :width: 80%

CMake begins by processing the :file:`CMakeLists.txt` file in the application
directory, which refers to the :file:`CMakeLists.txt` file in the Zephyr
top-level directory, which in turn refers to :file:`CMakeLists.txt` files
throughout the build tree (directly and indirectly). Its primary output is a
set of Makefiles or Ninja files to drive the build process, but the CMake
scripts also do some processing of their own:

Devicetree
   :file:`*.dts` (*devicetree source*) and :file:`*.dtsi` (*devicetree source
   include*) files are collected from the target's architecture, SoC, board,
   and application directories.

   :file:`*.dtsi` files are included by :file:`*.dts` files via the C
   preprocessor (often abbreviated *cpp*, which should not be confused with
   C++). The C preprocessor is also used to merge in any devicetree
   :file:`*.overlay` files, and to expand macros in :file:`*.dts`,
   :file:`*.dtsi`, and :file:`*.overlay` files.

   The preprocessed devicetree sources (stored in :file:`*.dts.pre.tmp`) are
   parsed by :zephyr_file:`gen_defines.py <scripts/dts/gen_defines.py>` to
   generate a :file:`devicetree_unfixed.h` header with preprocessor macros.

   As a debugging aid, :file:`gen_defines.py` writes the final devicetree to
   :file:`zephyr.dts`. This file is just for reference. It is not used
   anywhere.

   The ``dtc`` devicetree compiler also gets run on the preprocessed devicetree
   sources to catch any extra warnings and errors generated by it. The output
   from ``dtc`` is unused otherwise.

   The above is just a brief overview. For more information on devicetree, see
   :ref:`dt-guide`.

Devicetree fixups
   Files named :file:`dts_fixup.h` from the target’s architecture, SoC, board,
   and application directories are concatenated into a single
   :file:`devicetree_fixups.h` file. :file:`dts_fixup.h` files are used to
   rename generated macros to names expected by the source code.

   Source code accesses preprocessor macros generated from devicetree by
   including the :zephyr_file:`devicetree.h <include/devicetree.h>` header,
   which includes :file:`devicetree_unfixed.h` and :file:`devicetree_fixups.h`.

Kconfig
   :file:`Kconfig` files define available configuration options for for the
   target architecture, SoC, board, and application, as well as dependencies
   between options.

   Kconfig configurations are stored in *configuration files*. The initial
   configuration is generated by merging configuration fragments from the board
   and application (e.g. :file:`prj.conf`).

   The output from Kconfig is an :file:`autoconf.h` header with preprocessor
   assignments, and a :file:`.config` file that acts both as a saved
   configuration and as configuration output (used by CMake).

   Information from devicetree is available to Kconfig, through the functions
   defined in :zephyr_file:`kconfigfunctions.py
   <scripts/kconfig/kconfigfunctions.py>`.

   See :ref:`the Kconfig section of the manual <kconfig>` for more information.

Build Phase
***********

The build phase begins when the user invokes ``make`` or ``ninja``. Its
ultimate output is a complete Zephyr application in a format suitable for
loading/flashing on the desired target board (:file:`zephyr.elf`,
:file:`zephyr.hex`, etc.) The build phase can be broken down, conceptually,
into four stages: the pre-build, first-pass binary, final binary, and
post-processing.

Pre-build occurs before any source files are compiled, because during
this phase header files used by the source files are generated.

Pre-build
=========

Offset generation
   Access to high-level data structures and members is sometimes
   required when the definitions of those structures is not
   immediately accessible (e.g., assembly language). The generation of
   *offsets.h* (by *gen_offset_header.py*) facilitates this.

System call boilerplate
   The *gen_syscall.py* and *parse_syscalls.py*  scripts work
   together to bind potential system call functions with their
   implementations.

.. figure:: build-build-phase-1.svg
    :align: center
    :alt: Zephyr's build stage I
    :figclass: align-center
    :width: 80%

First-pass binary
=================

Compilation proper begins with the first-pass binary. Source files (C
and assembly) are collected from various subsystems (which ones is
decided during the configuration phase), and compiled into archives
(with reference to header files in the tree, as well as those
generated during the configuration phase and the pre-build stage).

If memory protection is enabled, then:

Partition grouping
   The gen_app_partitions.py script scans all the
   generated archives and outputs linker scripts to ensure that
   application partitions are properly grouped and aligned for the
   target’s memory protection hardware.

Then *cpp* is used to combine linker script fragments from the target’s
architecture/SoC, the kernel tree, optionally the partition output if
memory protection is enabled, and any other fragments selected during
the configuration process, into a *linker.cmd* file. The compiled
archives are then linked with *ld* as specified in the
*linker.cmd*.

In some configurations, this is the final binary, and the next stage
is skipped.

.. figure:: build-build-phase-2.svg
    :align: center
    :alt: Zephyr's build stage II
    :figclass: align-center
    :width: 80%

Final binary
============

In some configurations, the binary from the previous stage is
incomplete, with empty and/or placeholder sections that must be filled
in by, essentially, reflection. When :ref:`usermode_api` is enabled:

Kernel object hashing
   The *gen_kobject_list.py* scans the *ELF DWARF*
   debug data to find the address of the all kernel objects. This
   list is passed to *gperf*, which generates a perfect hash function and
   table of those addresses, then that output is optimized by
   *process_gperf.py*, using known properties of our special case.

Then, the link from the previous stage is repeated, this time with the
missing pieces populated.

.. figure:: build-build-phase-3.svg
    :align: center
    :alt: Zephyr's build stage III
    :figclass: align-center
    :width: 80%


Post processing
===============

Finally, if necessary, the completed kernel is converted from *ELF* to
the format expected by the loader and/or flash tool required by the
target. This is accomplished in a straightforward manner with *objdump*.

.. figure:: build-build-phase-4.svg
    :align: center
    :alt: Zephyr's build final stage
    :figclass: align-center
    :width: 80%


.. _build_system_scripts:

Supporting Scripts and Tools
****************************

The following is a detailed description of the scripts used during the build process.

.. _gen_syscalls.py:

:zephyr_file:`scripts/gen_syscalls.py`
========================================

.. include:: ../../../scripts/gen_syscalls.py
   :start-after: """
   :end-before: """

.. _gen_kobject_list.py:

:zephyr_file:`scripts/gen_kobject_list.py`
==========================================

.. include:: ../../../scripts/gen_kobject_list.py
   :start-after: """
   :end-before: """

.. _gen_offset_header.py:

:zephyr_file:`scripts/gen_offset_header.py`
===========================================

.. include:: ../../../scripts/gen_offset_header.py
   :start-after: """
   :end-before: """

.. _parse_syscalls.py:

:zephyr_file:`scripts/parse_syscalls.py`
========================================


.. include:: ../../../scripts/parse_syscalls.py
   :start-after: """
   :end-before: """

.. _gen_idt.py:

:zephyr_file:`arch/x86/gen_idt.py`
==================================

.. include:: ../../../arch/x86/gen_idt.py
   :start-after: """
   :end-before: """

.. _gen_gdt.py:

:zephyr_file:`arch/x86/gen_gdt.py`
===================================

.. include:: ../../../arch/x86/gen_gdt.py
   :start-after: """
   :end-before: """

.. _gen_relocate_app.py:

:zephyr_file:`scripts/gen_relocate_app.py`
===========================================

.. include:: ../../../scripts/gen_relocate_app.py
   :start-after: """
   :end-before: """

.. _process_gperf.py:

:zephyr_file:`scripts/process_gperf.py`
========================================

.. include:: ../../../scripts/process_gperf.py
   :start-after: """
   :end-before: """
