path: root/Help/command/add_custom_command.rst

add_custom_command
------------------

Add a custom build rule to the generated build system.

There are two main signatures for ``add_custom_command``.

Generating Files
^^^^^^^^^^^^^^^^

The first signature is for adding a custom command to produce an output:

.. code-block:: cmake

  add_custom_command(OUTPUT output1 [output2 ...]
                     COMMAND command1 [ARGS] [args1...]
                     [COMMAND command2 [ARGS] [args2...] ...]
                     [MAIN_DEPENDENCY depend]
                     [DEPENDS [depends...]]
                     [BYPRODUCTS [files...]]
                     [IMPLICIT_DEPENDS <lang1> depend1
                                      [<lang2> depend2] ...]
                     [WORKING_DIRECTORY dir]
                     [COMMENT comment]
                     [DEPFILE depfile]
                     [JOB_POOL job_pool]
                     [VERBATIM] [APPEND] [USES_TERMINAL]
                     [COMMAND_EXPAND_LISTS])

This defines a command to generate specified ``OUTPUT`` file(s).
A target created in the same directory (``CMakeLists.txt`` file)
that specifies any output of the custom command as a source file
is given a rule to generate the file using the command at build time.
Do not list the output in more than one independent target that
may build in parallel or the two instances of the rule may conflict
(instead use the :command:`add_custom_target` command to drive the
command and make the other targets depend on that one).
In makefile terms this creates a new target in the following form::

  OUTPUT: MAIN_DEPENDENCY DEPENDS
          COMMAND

The options are:

``APPEND``
  Append the ``COMMAND`` and ``DEPENDS`` option values to the custom
  command for the first output specified.  There must have already
  been a previous call to this command with the same output.
  The ``COMMENT``, ``MAIN_DEPENDENCY``, and ``WORKING_DIRECTORY``
  options are currently ignored when APPEND is given, but may be
  used in the future.

``BYPRODUCTS``
  .. versionadded:: 3.2

  Specify the files the command is expected to produce but whose
  modification time may or may not be newer than the dependencies.
  If a byproduct name is a relative path it will be interpreted
  relative to the build tree directory corresponding to the
  current source directory.
  Each byproduct file will be marked with the :prop_sf:`GENERATED`
  source file property automatically.

  Explicit specification of byproducts is supported by the
  :generator:`Ninja` generator to tell the ``ninja`` build tool
  how to regenerate byproducts when they are missing.  It is
  also useful when other build rules (e.g. custom commands)
  depend on the byproducts.  Ninja requires a build rule for any
  generated file on which another rule depends even if there are
  order-only dependencies to ensure the byproducts will be
  available before their dependents build.

  The :ref:`Makefile Generators` will remove ``BYPRODUCTS`` and other
  :prop_sf:`GENERATED` files during ``make clean``.

``COMMAND``
  Specify the command-line(s) to execute at build time.
  If more than one ``COMMAND`` is specified they will be executed in order,
  but *not* necessarily composed into a stateful shell or batch script.
  (To run a full script, use the :command:`configure_file` command or the
  :command:`file(GENERATE)` command to create it, and then specify
  a ``COMMAND`` to launch it.)
  The optional ``ARGS`` argument is for backward compatibility and
  will be ignored.

  If ``COMMAND`` specifies an executable target name (created by the
  :command:`add_executable` command), it will automatically be replaced
  by the location of the executable created at build time if either of
  the following is true:

  * The target is not being cross-compiled (i.e. the
    :variable:`CMAKE_CROSSCOMPILING` variable is not set to true).
  * .. versionadded:: 3.6
      The target is being cross-compiled and an emulator is provided (i.e.
      its :prop_tgt:`CROSSCOMPILING_EMULATOR` target property is set).
      In this case, the contents of :prop_tgt:`CROSSCOMPILING_EMULATOR` will be
      prepended to the command before the location of the target executable.

  If neither of the above conditions are met, it is assumed that the
  command name is a program to be found on the ``PATH`` at build time.

  Arguments to ``COMMAND`` may use
  :manual:`generator expressions <cmake-generator-expressions(7)>`.
  Use the ``TARGET_FILE`` generator expression to refer to the location of
  a target later in the command line (i.e. as a command argument rather
  than as the command to execute).

  Whenever one of the following target based generator expressions are used as
  a command to execute or is mentioned in a command argument, a target-level
  dependency will be added automatically so that the mentioned target will be
  built before any target using this custom command
  (see policy :policy:`CMP0112`).

    * ``TARGET_FILE``
    * ``TARGET_LINKER_FILE``
    * ``TARGET_SONAME_FILE``
    * ``TARGET_PDB_FILE``

  This target-level dependency does NOT add a file-level dependency that would
  cause the custom command to re-run whenever the executable is recompiled.
  List target names with the ``DEPENDS`` option to add such file-level
  dependencies.


``COMMENT``
  Display the given message before the commands are executed at
  build time.

``DEPENDS``
  Specify files on which the command depends.  Each argument is converted
  to a dependency as follows:

  1. If the argument is the name of a target (created by the
     :command:`add_custom_target`, :command:`add_executable`, or
     :command:`add_library` command) a target-level dependency is
     created to make sure the target is built before any target
     using this custom command.  Additionally, if the target is an
     executable or library, a file-level dependency is created to
     cause the custom command to re-run whenever the target is
     recompiled.

  2. If the argument is an absolute path, a file-level dependency
     is created on that path.

  3. If the argument is the name of a source file that has been
     added to a target or on which a source file property has been set,
     a file-level dependency is created on that source file.

  4. If the argument is a relative path and it exists in the current
     source directory, a file-level dependency is created on that
     file in the current source directory.

  5. Otherwise, a file-level dependency is created on that path relative
     to the current binary directory.

  If any dependency is an ``OUTPUT`` of another custom command in the same
  directory (``CMakeLists.txt`` file), CMake automatically brings the other
  custom command into the target in which this command is built.

  .. versionadded:: 3.16
    A target-level dependency is added if any dependency is listed as
    ``BYPRODUCTS`` of a target or any of its build events in the same
    directory to ensure the byproducts will be available.

  If ``DEPENDS`` is not specified, the command will run whenever
  the ``OUTPUT`` is missing; if the command does not actually
  create the ``OUTPUT``, the rule will always run.

  .. versionadded:: 3.1
    Arguments to ``DEPENDS`` may use
    :manual:`generator expressions <cmake-generator-expressions(7)>`.

``COMMAND_EXPAND_LISTS``
  .. versionadded:: 3.8

  Lists in ``COMMAND`` arguments will be expanded, including those
  created with
  :manual:`generator expressions <cmake-generator-expressions(7)>`,
  allowing ``COMMAND`` arguments such as
  ``${CC} "-I$<JOIN:$<TARGET_PROPERTY:foo,INCLUDE_DIRECTORIES>,;-I>" foo.cc``
  to be properly expanded.

``IMPLICIT_DEPENDS``
  Request scanning of implicit dependencies of an input file.
  The language given specifies the programming language whose
  corresponding dependency scanner should be used.
  Currently only ``C`` and ``CXX`` language scanners are supported.
  The language has to be specified for every file in the
  ``IMPLICIT_DEPENDS`` list.  Dependencies discovered from the
  scanning are added to those of the custom command at build time.
  Note that the ``IMPLICIT_DEPENDS`` option is currently supported
  only for Makefile generators and will be ignored by other generators.

``JOB_POOL``
  .. versionadded:: 3.15

  Specify a :prop_gbl:`pool <JOB_POOLS>` for the :generator:`Ninja`
  generator. Incompatible with ``USES_TERMINAL``, which implies
  the ``console`` pool.
  Using a pool that is not defined by :prop_gbl:`JOB_POOLS` causes
  an error by ninja at build time.

``MAIN_DEPENDENCY``
  Specify the primary input source file to the command.  This is
  treated just like any value given to the ``DEPENDS`` option
  but also suggests to Visual Studio generators where to hang
  the custom command. Each source file may have at most one command
  specifying it as its main dependency. A compile command (i.e. for a
  library or an executable) counts as an implicit main dependency which
  gets silently overwritten by a custom command specification.

``OUTPUT``
  Specify the output files the command is expected to produce.
  If an output name is a relative path it will be interpreted
  relative to the build tree directory corresponding to the
  current source directory.
  Each output file will be marked with the :prop_sf:`GENERATED`
  source file property automatically.
  If the output of the custom command is not actually created
  as a file on disk it should be marked with the :prop_sf:`SYMBOLIC`
  source file property.

``USES_TERMINAL``
  .. versionadded:: 3.2

  The command will be given direct access to the terminal if possible.
  With the :generator:`Ninja` generator, this places the command in
  the ``console`` :prop_gbl:`pool <JOB_POOLS>`.

``VERBATIM``
  All arguments to the commands will be escaped properly for the
  build tool so that the invoked command receives each argument
  unchanged.  Note that one level of escapes is still used by the
  CMake language processor before add_custom_command even sees the
  arguments.  Use of ``VERBATIM`` is recommended as it enables
  correct behavior.  When ``VERBATIM`` is not given the behavior
  is platform specific because there is no protection of
  tool-specific special characters.

``WORKING_DIRECTORY``
  Execute the command with the given current working directory.
  If it is a relative path it will be interpreted relative to the
  build tree directory corresponding to the current source directory.

  .. versionadded:: 3.13
    Arguments to ``WORKING_DIRECTORY`` may use
    :manual:`generator expressions <cmake-generator-expressions(7)>`.

``DEPFILE``
  .. versionadded:: 3.7

  Specify a ``.d`` depfile for the :generator:`Ninja` generator.
  A ``.d`` file holds dependencies usually emitted by the custom
  command itself.
  Using ``DEPFILE`` with other generators than Ninja is an error.

  If the ``DEPFILE`` argument is relative, it should be relative to
  :variable:`CMAKE_CURRENT_BINARY_DIR`, and any relative paths inside the
  ``DEPFILE`` should also be relative to :variable:`CMAKE_CURRENT_BINARY_DIR`
  (see policy :policy:`CMP0116`.)

Examples: Generating Files
^^^^^^^^^^^^^^^^^^^^^^^^^^

Custom commands may be used to generate source files.
For example, the code:

.. code-block:: cmake

  add_custom_command(
    OUTPUT out.c
    COMMAND someTool -i ${CMAKE_CURRENT_SOURCE_DIR}/in.txt
                     -o out.c
    DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/in.txt
    VERBATIM)
  add_library(myLib out.c)

adds a custom command to run ``someTool`` to generate ``out.c`` and then
compile the generated source as part of a library.  The generation rule
will re-run whenever ``in.txt`` changes.

Build Events
^^^^^^^^^^^^

The second signature adds a custom command to a target such as a
library or executable.  This is useful for performing an operation
before or after building the target.  The command becomes part of the
target and will only execute when the target itself is built.  If the
target is already built, the command will not execute.

.. code-block:: cmake

  add_custom_command(TARGET <target>
                     PRE_BUILD | PRE_LINK | POST_BUILD
                     COMMAND command1 [ARGS] [args1...]
                     [COMMAND command2 [ARGS] [args2...] ...]
                     [BYPRODUCTS [files...]]
                     [WORKING_DIRECTORY dir]
                     [COMMENT comment]
                     [VERBATIM] [USES_TERMINAL]
                     [COMMAND_EXPAND_LISTS])

This defines a new command that will be associated with building the
specified ``<target>``.  The ``<target>`` must be defined in the current
directory; targets defined in other directories may not be specified.

When the command will happen is determined by which
of the following is specified:

``PRE_BUILD``
  On :ref:`Visual Studio Generators`, run before any other rules are
  executed within the target.
  On other generators, run just before ``PRE_LINK`` commands.
``PRE_LINK``
  Run after sources have been compiled but before linking the binary
  or running the librarian or archiver tool of a static library.
  This is not defined for targets created by the
  :command:`add_custom_target` command.
``POST_BUILD``
  Run after all other rules within the target have been executed.

.. note::
  Because generator expressions can be used in custom commands,
  it is possible to define ``COMMAND`` lines or whole custom commands
  which evaluate to empty strings for certain configurations.
  For **Visual Studio 2010 (and newer)** generators these command
  lines or custom commands will be omitted for the specific
  configuration and no "empty-string-command" will be added.

  This allows to add individual build events for every configuration.

Examples: Build Events
^^^^^^^^^^^^^^^^^^^^^^

A ``POST_BUILD`` event may be used to post-process a binary after linking.
For example, the code:

.. code-block:: cmake

  add_executable(myExe myExe.c)
  add_custom_command(
    TARGET myExe POST_BUILD
    COMMAND someHasher -i "$<TARGET_FILE:myExe>"
                       -o "$<TARGET_FILE:myExe>.hash"
    VERBATIM)

will run ``someHasher`` to produce a ``.hash`` file next to the executable
after linking.