.. _cmake-details:

Build System (CMake)
********************


CMake is used to build your application together with the Zephyr kernel. A
CMake build is done in two stages. The first stage is called
**configuration**. During configuration, the CMakeLists.txt build scripts are
executed. After configuration is finished, CMake has an internal model of the
Zephyr build, and can generate build scripts that are native to the host
platform.

CMake supports generating scripts for several build systems, but only Ninja and
Make are tested and supported by Zephyr. After configuration, you begin the
**build** stage by executing the generated build scripts. These build scripts
can recompile the application without involving CMake following
most code changes. However, after certain changes, the configuration step must
be executed again before building. The build scripts can detect some of these
situations and reconfigure automatically, but there are cases when this must be
done manually.

Zephyr uses CMake's concept of a 'target' to organize the build. A
target can be an executable, a library, or a generated file. For
application developers, the library target is the most important to
understand. All source code that goes into a Zephyr build does so by
being included in a library target, even application code.

Library targets have source code, that is added through CMakeLists.txt
build scripts like this:

.. code-block:: cmake

   target_sources(app PRIVATE src/main.c)

In the above :file:`CMakeLists.txt`, an existing library target named ``app``
is configured to include the source file :file:`src/main.c`. The ``PRIVATE``
keyword indicates that we are modifying the internals of how the library is
being built. Using the keyword ``PUBLIC`` would modify how other
libraries that link with app are built. In this case, using ``PUBLIC``
would cause libraries that link with ``app`` to also include the
source file :file:`src/main.c`, behavior that we surely do not want. The
``PUBLIC`` keyword could however be useful when modifying the include
paths of a target library.

When introducing build system code using CMake or adding new CMake files,
please follow the style guidelines outlined :ref:`here <cmake-style>`.


Build and Configuration Phases
==============================

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* to generate a
build system, 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, which is explained here.

Note that paths beginning with :file:`build/` below refer to the build
directory you create when running CMake.

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 preprocessor output is
   placed in :file:`build/zephyr/zephyr.dts.pre`.

   The preprocessed devicetree sources are parsed by
   :zephyr_file:`gen_defines.py <scripts/dts/gen_defines.py>` to generate a
   :file:`build/zephyr/include/generated/zephyr/devicetree_generated.h` header with
   preprocessor macros.

   Source code should access preprocessor macros generated from devicetree by
   including the :zephyr_file:`devicetree.h <include/zephyr/devicetree.h>` header,
   which includes :file:`devicetree_generated.h`.

   :file:`gen_defines.py` also writes the final devicetree to
   :file:`build/zephyr/zephyr.dts` in the build directory. This file's contents
   may be useful for debugging.

   If the devicetree compiler ``dtc`` is installed, it is run on
   :file:`build/zephyr/zephyr.dts` to catch any extra warnings and errors
   generated by this tool. The output from ``dtc`` is unused otherwise, and
   this step is skipped if ``dtc`` is not installed.

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

Kconfig
   :file:`Kconfig` files define available configuration options 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). The definitions in
   :file:`autoconf.h` are automatically exposed at compile time, so there is no
   need to include this header.

   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
+++++++++

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

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%

Intermediate binaries
+++++++++++++++++++++

Compilation proper begins with the first intermediate 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(s)).

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

The exact number of intermediate binaries is decided during the configuration
phase.

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*.

Unfixed size binary
   The unfixed size intermediate binary is produced when :ref:`usermode_api`
   is enabled or :ref:`devicetree` is in use.
   It produces a binary where sizes are not fixed and thus it may be used
   by post-process steps that will impact the size of the final binary.

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

Fixed size binary
   The fixed size intermediate binary is produced when :ref:`usermode_api`
   is enabled or when generated IRQ tables are used,
   :kconfig:option:`CONFIG_GEN_ISR_TABLES`
   It produces a binary where sizes are fixed and thus the size must not change
   between the intermediate binary and the final binary.

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

Intermediate binaries post-processing
+++++++++++++++++++++++++++++++++++++

The binaries from the previous stage are incomplete, with empty and/or
placeholder sections that must be filled in by, essentially, reflection.

To complete the build procedure the following scripts are executed on the
intermediate binaries to produce the missing pieces needed for the final
binary.

When :ref:`usermode_api` is enabled:

Partition alignment
   The *gen_app_partitions.py* script scans the unfixed size binary and
   generates an app shared memory aligned linker script snippet where the
   partitions are sorted in descending order.

.. figure:: build-postprocess-1.svg
    :align: center
    :alt: Zephyr's intermediate binary post-process I
    :figclass: align-center
    :width: 80%

When :ref:`devicetree` is used:

Device dependencies
   The *gen_device_deps.py* script scans the unfixed size binary to determine
   relationships between devices that were recorded from devicetree data,
   and replaces the encoded relationships with values that are optimized to
   locate the devices actually present in the application.

.. figure:: build-postprocess-2.svg
    :align: center
    :alt: Zephyr's intermediate binary post-process II
    :figclass: align-center
    :width: 80%

When :kconfig:option:`CONFIG_GEN_ISR_TABLES` is enabled:
   The *gen_isr_tables.py* script scans the fixed size binary and creates
   an isr_tables.c source file with a hardware vector table and/or software
   IRQ table.

.. figure:: build-postprocess-3.svg
    :align: center
    :alt: Zephyr's intermediate binary post-process III
    :figclass: align-center
    :width: 80%

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.

.. figure:: build-postprocess-4.svg
    :align: center
    :alt: Zephyr's intermediate binary post-process IV
    :figclass: align-center
    :width: 80%

When no intermediate binary post-processing is required then the first
intermediate binary will be directly used as the final binary.

Final binary
++++++++++++

The binary from the previous stage is incomplete, with empty and/or
placeholder sections that must be filled in by, essentially, reflection.

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

.. figure:: build-build-phase-5.svg
    :align: center
    :alt: Zephyr's build final stage
    :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-6.svg
    :align: center
    :alt: Zephyr's build final stage post-process
    :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/build/gen_syscalls.py`
--------------------------------------------

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

.. _gen_device_deps.py:

:zephyr_file:`scripts/build/gen_device_deps.py`
-----------------------------------------------

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

.. _gen_kobject_list.py:

:zephyr_file:`scripts/build/gen_kobject_list.py`
------------------------------------------------

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

.. _gen_offset_header.py:

:zephyr_file:`scripts/build/gen_offset_header.py`
-------------------------------------------------

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

.. _parse_syscalls.py:

:zephyr_file:`scripts/build/parse_syscalls.py`
----------------------------------------------


.. include:: ../../../scripts/build/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/build/gen_relocate_app.py`
------------------------------------------------

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

.. _process_gperf.py:

:zephyr_file:`scripts/build/process_gperf.py`
---------------------------------------------

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

:zephyr_file:`scripts/build/gen_app_partitions.py`
--------------------------------------------------

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

.. _check_init_priorities.py:

:zephyr_file:`scripts/build/check_init_priorities.py`
-----------------------------------------------------

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