function
--------

Start recording a function for later invocation as a command.

.. code-block:: cmake

  function(<name> [<arg1> ...])
    <commands>
  endfunction()

Defines a function named ``<name>`` that takes arguments named
``<arg1>``, ...  The ``<commands>`` in the function definition
are recorded; they are not executed until the function is invoked.

Per legacy, the :command:`endfunction` command admits an optional
``<name>`` argument. If used, it must be a verbatim repeat of the
argument of the opening ``function`` command.

A function opens a new scope: see :command:`set(var PARENT_SCOPE)` for
details.

See the :command:`cmake_policy()` command documentation for the behavior
of policies inside functions.

See the :command:`macro()` command documentation for differences
between CMake functions and macros.

Invocation
^^^^^^^^^^

The function invocation is case-insensitive. A function defined as

.. code-block:: cmake

  function(foo)
    <commands>
  endfunction()

can be invoked through any of

.. code-block:: cmake

  foo()
  Foo()
  FOO()
  cmake_language(CALL foo)

and so on. However, it is strongly recommended to stay with the
case chosen in the function definition. Typically functions use
all-lowercase names.

.. versionadded:: 3.18
  The :command:`cmake_language(CALL ...)` command can also be used to
  invoke the function.

Arguments
^^^^^^^^^

When the function is invoked, the recorded ``<commands>`` are first
modified by replacing formal parameters (``${arg1}``, ...) with the
arguments passed, and then invoked as normal commands.

In addition to referencing the formal parameters you can reference the
``ARGC`` variable which will be set to the number of arguments passed
into the function as well as ``ARGV0``, ``ARGV1``, ``ARGV2``, ...  which
will have the actual values of the arguments passed in.  This facilitates
creating functions with optional arguments.

Furthermore, ``ARGV`` holds the list of all arguments given to the
function and ``ARGN`` holds the list of arguments past the last expected
argument.  Referencing to ``ARGV#`` arguments beyond ``ARGC`` have
undefined behavior.  Checking that ``ARGC`` is greater than ``#`` is
the only way to ensure that ``ARGV#`` was passed to the function as an
extra argument.
