3a197934fc
As both C and C++ standards require applications running under an OS to return 'int', adapt that for Zephyr to align with those standard. This also eliminates errors when building with clang when not using -ffreestanding, and reduces the need for compiler flags to silence warnings for both clang and gcc. Signed-off-by: Keith Packard <keithp@keithp.com>
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126 lines
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.. _language_cpp:
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C++ Language Support
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####################
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C++ is a general-purpose object-oriented programming language that is based on
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the C language.
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Enabling C++ Support
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********************
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Zephyr supports applications written in both C and C++. However, to use C++ in
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an application you must configure Zephyr to include C++ support by selecting
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the :kconfig:option:`CONFIG_CPP` in the application configuration file.
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To enable C++ support, the compiler toolchain must also include a C++ compiler
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and the included compiler must be supported by the Zephyr build system. The
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:ref:`toolchain_zephyr_sdk`, which includes the GNU C++ Compiler (part of GCC),
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is supported by Zephyr, and the features and their availability documented
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here assume the use of the Zephyr SDK.
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The default C++ standard level (i.e. the language enforced by the
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compiler flags passed) for Zephyr apps is C++11. Other standards are
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available via kconfig choice, for example
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:kconfig:option:`CONFIG_STD_CPP98`. The oldest standard supported and
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tested in Zephyr is C++98.
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When compiling a source file, the build system selects the C++ compiler based
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on the suffix (extension) of the files. Files identified with either a **cpp**
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or a **cxx** suffix are compiled using the C++ compiler. For example,
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:file:`myCplusplusApp.cpp` is compiled using C++.
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The C++ standard requires the ``main()`` function to have the return type of
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``int``. Your ``main()`` must be defined as ``int main(void)``. Zephyr ignores
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the return value from main, so applications should not return status
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information and should, instead, return zero.
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.. note::
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Do not use C++ for kernel, driver, or system initialization code.
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Language Features
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*****************
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Zephyr currently provides only a subset of C++ functionality. The following
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features are *not* supported:
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* Static global object destruction
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* OS-specific C++ standard library classes (e.g. ``std::thread``,
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``std::mutex``)
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While not an exhaustive list, support for the following functionality is
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included:
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* Inheritance
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* Virtual functions
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* Virtual tables
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* Static global object constructors
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* Dynamic object management with the **new** and **delete** operators
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* Exceptions
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* :abbr:`RTTI (runtime type information)`
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* Standard Template Library (STL)
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Static global object constructors are initialized after the drivers are
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initialized but before the application :c:func:`main()` function. Therefore,
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use of C++ is restricted to application code.
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In order to make use of the C++ exceptions, the
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:kconfig:option:`CONFIG_CPP_EXCEPTIONS` must be selected in the application
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configuration file.
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Zephyr Minimal C++ Library
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**************************
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Zephyr minimal C++ library (:file:`lib/cpp/minimal`) provides a minimal subset
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of the C++ standard library and application binary interface (ABI) functions to
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enable basic C++ language support. This includes:
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* ``new`` and ``delete`` operators
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* virtual function stub and vtables
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* static global initializers for global constructors
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The scope of the minimal C++ library is strictly limited to providing the basic
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C++ language support, and it does not implement any `Standard Template Library
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(STL)`_ classes and functions. For this reason, it is only suitable for use in
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the applications that implement their own (non-standard) class library and do
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rely on the Standard Template Library (STL) components.
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Any application that makes use of the Standard Template Library (STL)
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components, such as ``std::string`` and ``std::vector``, must enable the C++
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standard library support.
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C++ Standard Library
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********************
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The `C++ Standard Library`_ is a collection of classes and functions that are
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part of the ISO C++ standard (``std`` namespace).
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Zephyr does not include any C++ standard library implementation in source code
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form. Instead, it allows configuring the build system to link against the
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pre-built C++ standard library included in the C++ compiler toolchain.
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To enable C++ standard library, select an applicable toolchain-specific C++
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standard library type from the :kconfig:option:`CONFIG_LIBCPP_IMPLEMENTATION`
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in the application configuration file.
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For instance, when building with the :ref:`toolchain_zephyr_sdk`, the build
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system can be configured to link against the GNU C++ Library (``libstdc++.a``),
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which is a fully featured C++ standard library that provides all features
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required by the ISO C++ standard including the Standard Template Library (STL),
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by selecting :kconfig:option:`CONFIG_GLIBCXX_LIBCPP` in the application
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configuration file.
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The following C++ standard libraries are supported by Zephyr:
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* GNU C++ Library (:kconfig:option:`CONFIG_GLIBCXX_LIBCPP`)
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* ARC MetaWare C++ Library (:kconfig:option:`CONFIG_ARCMWDT_LIBCPP`)
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A Zephyr subsystem that requires the features from the full C++ standard
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library can select, from its config,
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:kconfig:option:`CONFIG_REQUIRES_FULL_LIBCPP`, which automatically selects a
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compatible C++ standard library unless the Kconfig symbol for a specific C++
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standard library is selected.
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.. _`C++ Standard Library`: https://en.wikipedia.org/wiki/C%2B%2B_Standard_Library
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.. _`Standard Template Library (STL)`: https://en.wikipedia.org/wiki/Standard_Template_Library
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