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165 lines
7.3 KiB
ReStructuredText
165 lines
7.3 KiB
ReStructuredText
.. _code_style:
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===================
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The libc code style
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===================
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Naming style
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============
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For the large part, the libc project follows the general `coding standards of
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the LLVM project <https://llvm.org/docs/CodingStandards.html>`_. The libc
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project differs from that standard with respect to the naming style. The
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differences are as follows:
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#. **Non-const variables** - This includes function arguments, struct and
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class data members, non-const globals and local variables. They all use the
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``snake_case`` style.
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#. **const and constexpr variables** - They use the capitalized
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``SNAKE_CASE`` irrespective of whether they are local or global.
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#. **Function and methods** - They use the ``snake_case`` style like the
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non-const variables.
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#. **Internal type names** - These are types which are internal to the libc
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implementation. They use the ``CaptilizedCamelCase`` style.
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#. **Public names** - These are the names as prescribed by the standards and
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will follow the style as prescribed by the standards.
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Macro style
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===========
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We define two kinds of macros: **code defined** and **build defined** macros.
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#. **Build defined** macros are generated by `CMake` or `Bazel` and are passed
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down to the compiler with the ``-D`` command line flag. They start with the
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``LIBC_COPT_`` prefix. They are used to tune the behavior of the libc.
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They either denote an action or define a constant.
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#. **Code defined** macros are defined within the ``src/__support/macros``
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folder. They all start with the ``LIBC_`` prefix. They are of two kinds
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* **Properties** - Build related properties like used compiler, target
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architecture or enabled CPU features defined by introspecting compiler
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defined preprocessor definitions. e.g., ``LIBC_TARGET_ARCH_IS_ARM``,
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``LIBC_TARGET_CPU_HAS_AVX2``, ``LIBC_COMPILER_IS_CLANG``, ...
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* **Attributes** - Compiler agnostic attributes or functions to handle
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specific operations. e.g., ``LIBC_INLINE``, ``LIBC_NO_LOOP_UNROLL``,
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``LIBC_LIKELY``, ``LIBC_INLINE_ASM``.
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Inline functions and variables defined in header files
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======================================================
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When defining functions and variables inline in header files, we follow certain
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rules:
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#. The functions should not be given file-static linkage. There can be class
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static methods defined inline however.
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#. Instead of using the ``inline`` keyword, functions should be tagged with the
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``LIBC_INLINE`` macro and variables should be tagged with the
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``LIBC_INLINE_VAR`` macro defined in ``src/__support/macros/attributes.h``.
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For example:
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.. code-block:: c++
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LIBC_INLINE_VAR constexpr bool foo = true;
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LIBC_INLINE ReturnType function_defined_inline(ArgType arg) {
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...
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}
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#. The ``LIBC_INLINE`` tag should also be added to functions which have
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definitions that are implicitly inline. Examples of such functions are
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class methods (static and non-static) defined inline and ``constexpr``
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functions.
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Setting ``errno`` from runtime code
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===================================
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Many libc functions set ``errno`` to indicate an error condition. If LLVM's libc
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is being used as the only libc, then the ``errno`` from LLVM's libc is affected.
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If LLVM's libc is being used in the :ref:`overlay_mode`, then the ``errno`` from
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the system libc is affected. When a libc function, which can potentially affect
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the ``errno``, is called from a unit test, we do not want the global ``errno``
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(as in, the ``errno`` of the process thread running the unit test) to be
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affected. If the global ``errno`` is affected, then the operation of the unit
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test infrastructure itself can be affected. To avoid perturbing the unit test
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infrastructure around the setting of ``errno``, the following rules are to be
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followed:
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#. A special macro named ``libc_errno`` defined in ``src/errno/libc_errno.h``
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should be used when setting ``errno`` from libc runtime code. For example,
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code to set ``errno`` to ``EINVAL`` should be:
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.. code-block:: c++
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libc_errno = EINVAL;
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#. ``errno`` should be set just before returning from the implementation of the
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public function. It should not be set from within helper functions. Helper
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functions should use idiomatic C++ constructs like
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`cpp::optional <https://github.com/llvm/llvm-project/blob/main/libc/src/__support/CPP/optional.h>`_
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and
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`ErrorOr <https://github.com/llvm/llvm-project/blob/main/libc/src/__support/error_or.h>`_
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to return error values.
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#. The header file ``src/errno/libc_errno.h`` is shipped as part of the target
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corresponding to the ``errno`` entrypoint ``libc.src.errno.errno``. We do
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not in general allow dependencies between entrypoints. However, the ``errno``
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entrypoint is the only exceptional entrypoint on which other entrypoints
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should explicitly depend on if they set ``errno`` to indicate error
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conditions.
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Assertions in libc runtime code
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===============================
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The libc developers should, and are encouraged to, use assertions freely in
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the libc runtime code. However, the assertion should be listed via the macro
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``LIBC_ASSERT`` defined in ``src/__support/libc_assert.h``. This macro can be
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used from anywhere in the libc runtime code. Internally, all it does is to
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print the assertion expression and exit. It does not implement the semantics
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of the standard ``assert`` macro. Hence, it can be used from any where in the
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libc runtime code without causing any recursive calls or chicken-and-egg
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situations.
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Allocations in the libc runtime code
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====================================
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Some libc functions allocate memory. For example, the ``strdup`` function
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allocates new memory into which the input string is duplicated. Allocations
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are typically done by calling a function from the ``malloc`` family of
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functions. Such functions can fail and return an error value to indicate
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allocation failure. To conform to standards, the libc should handle
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allocation failures gracefully and surface the error conditions to the user
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code as appropriate. Since LLVM's libc is implemented in C++, we want
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allocations and deallocations to employ C++ operators ``new`` and ``delete``
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as they implicitly invoke constructors and destructors respectively. However,
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if we use the default ``new`` and ``delete`` operators, the libc will end up
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depending on the C++ runtime. To avoid such a dependence, and to handle
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allocation failures gracefully, we use special ``new`` and ``delete`` operators
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defined in
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`src/__support/CPP/new.h <https://github.com/llvm/llvm-project/blob/main/libc/src/__support/CPP/new.h>`_.
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Allocations and deallocations using these operators employ a pattern like
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this:
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.. code-block:: c++
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#include "src/__support/CPP/new.h"
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...
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__llvm_libc::AllocChecker ac;
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auto *obj = new (ac) Type(...);
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if (!ac) {
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// handle allocator failure.
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}
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...
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delete obj;
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The only exception to using the above pattern is if allocating using the
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``realloc`` function is of value. In such cases, prefer to use only the
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``malloc`` family of functions for allocations and deallocations. Allocation
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failures will still need to be handled gracefully. Further, keep in mind that
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these functions do not call the constructors and destructors of the
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allocated/deallocated objects. So, use these functions carefully and only
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when it is absolutely clear that constructor and destructor invocation is
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not required.
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