.. _code_style: =================== The libc code style =================== Naming style ============ For the large part, the libc project follows the general `coding standards of the LLVM project `_. The libc project differs from that standard with respect to the naming style. The differences are as follows: #. **Non-const variables** - This includes function arguments, struct and class data members, non-const globals and local variables. They all use the ``snake_case`` style. #. **const and constexpr variables** - They use the capitalized ``SNAKE_CASE`` irrespective of whether they are local or global. #. **Function and methods** - They use the ``snake_case`` style like the non-const variables. #. **Internal type names** - These are types which are internal to the libc implementation. They use the ``CaptilizedCamelCase`` style. #. **Public names** - These are the names as prescribed by the standards and will follow the style as prescribed by the standards. Macro style =========== We define two kinds of macros: #. **Build defined** macros are generated by `CMake` or `Bazel` and are passed down to the compiler with the ``-D`` command line flag. They start with the ``LIBC_COPT_`` prefix. They are used to tune the behavior of the libc. They either denote an action or define a constant. #. **Code defined** macros are defined within the ``src/__support/macros`` folder. They all start with the ``LIBC_`` prefix. * ``src/__support/macros/properties/`` - Build related properties like target architecture or enabled CPU features defined by introspecting compiler defined preprocessor definitions. * ``architectures.h`` - Target architecture properties. e.g., ``LIBC_TARGET_ARCH_IS_ARM``. * ``compiler.h`` - Host compiler properties. e.g., ``LIBC_COMPILER_IS_CLANG``. * ``cpu_features.h`` - Target cpu feature availability. e.g., ``LIBC_TARGET_CPU_HAS_AVX2``. * ``types.h`` - Type properties and availability. e.g., ``LIBC_TYPES_HAS_FLOAT128``. * ``os.h`` - Target os properties. e.g., ``LIBC_TARGET_OS_IS_LINUX``. * ``src/__support/macros/config.h`` - Important compiler and platform features. Such macros can be used to produce portable code by parameterizing compilation based on the presence or lack of a given feature. e.g., ``LIBC_HAS_FEATURE`` * ``src/__support/macros/attributes.h`` - Attributes for functions, types, and variables. e.g., ``LIBC_UNUSED`` * ``src/__support/macros/optimization.h`` - Portable macros for performance optimization. e.g., ``LIBC_LIKELY``, ``LIBC_LOOP_NOUNROLL`` Inline functions and variables defined in header files ====================================================== When defining functions and variables inline in header files, we follow certain rules: #. The functions should not be given file-static linkage. There can be class static methods defined inline however. #. Instead of using the ``inline`` keyword, functions should be tagged with the ``LIBC_INLINE`` macro and variables should be tagged with the ``LIBC_INLINE_VAR`` macro defined in ``src/__support/macros/attributes.h``. For example: .. code-block:: c++ LIBC_INLINE_VAR constexpr bool foo = true; LIBC_INLINE ReturnType function_defined_inline(ArgType arg) { ... } #. The ``LIBC_INLINE`` tag should also be added to functions which have definitions that are implicitly inline. Examples of such functions are class methods (static and non-static) defined inline and ``constexpr`` functions. Setting ``errno`` from runtime code =================================== Many libc functions set ``errno`` to indicate an error condition. If LLVM's libc is being used as the only libc, then the ``errno`` from LLVM's libc is affected. If LLVM's libc is being used in the :ref:`overlay_mode`, then the ``errno`` from the system libc is affected. When a libc function, which can potentially affect the ``errno``, is called from a unit test, we do not want the global ``errno`` (as in, the ``errno`` of the process thread running the unit test) to be affected. If the global ``errno`` is affected, then the operation of the unit test infrastructure itself can be affected. To avoid perturbing the unit test infrastructure around the setting of ``errno``, the following rules are to be followed: #. A special macro named ``libc_errno`` defined in ``src/errno/libc_errno.h`` should be used when setting ``errno`` from libc runtime code. For example, code to set ``errno`` to ``EINVAL`` should be: .. code-block:: c++ libc_errno = EINVAL; #. ``errno`` should be set just before returning from the implementation of the public function. It should not be set from within helper functions. Helper functions should use idiomatic C++ constructs like `cpp::optional `_ and `ErrorOr `_ to return error values. #. The header file ``src/errno/libc_errno.h`` is shipped as part of the target corresponding to the ``errno`` entrypoint ``libc.src.errno.errno``. We do not in general allow dependencies between entrypoints. However, the ``errno`` entrypoint is the only exceptional entrypoint on which other entrypoints should explicitly depend on if they set ``errno`` to indicate error conditions. Assertions in libc runtime code =============================== The libc developers should, and are encouraged to, use assertions freely in the libc runtime code. However, the assertion should be listed via the macro ``LIBC_ASSERT`` defined in ``src/__support/libc_assert.h``. This macro can be used from anywhere in the libc runtime code. Internally, all it does is to print the assertion expression and exit. It does not implement the semantics of the standard ``assert`` macro. Hence, it can be used from any where in the libc runtime code without causing any recursive calls or chicken-and-egg situations. Allocations in the libc runtime code ==================================== Some libc functions allocate memory. For example, the ``strdup`` function allocates new memory into which the input string is duplicated. Allocations are typically done by calling a function from the ``malloc`` family of functions. Such functions can fail and return an error value to indicate allocation failure. To conform to standards, the libc should handle allocation failures gracefully and surface the error conditions to the user code as appropriate. Since LLVM's libc is implemented in C++, we want allocations and deallocations to employ C++ operators ``new`` and ``delete`` as they implicitly invoke constructors and destructors respectively. However, if we use the default ``new`` and ``delete`` operators, the libc will end up depending on the C++ runtime. To avoid such a dependence, and to handle allocation failures gracefully, we use special ``new`` and ``delete`` operators defined in `src/__support/CPP/new.h `_. Allocations and deallocations using these operators employ a pattern like this: .. code-block:: c++ #include "src/__support/CPP/new.h" ... LIBC_NAMESPACE::AllocChecker ac; auto *obj = new (ac) Type(...); if (!ac) { // handle allocator failure. } ... delete obj; The only exception to using the above pattern is if allocating using the ``realloc`` function is of value. In such cases, prefer to use only the ``malloc`` family of functions for allocations and deallocations. Allocation failures will still need to be handled gracefully. Further, keep in mind that these functions do not call the constructors and destructors of the allocated/deallocated objects. So, use these functions carefully and only when it is absolutely clear that constructor and destructor invocation is not required. Warnings in sources =================== We expect contributions to be free of warnings from the `minimum supported compiler versions`__ (and newer). .. __: https://libc.llvm.org/compiler_support.html#minimum-supported-versions Header Inclusion Policy ======================= Because llvm-libc supports `Overlay Mode `__ and `Fullbuild Mode `__ care must be taken when ``#include``'ing certain headers. The ``include/`` directory contains public facing headers that users must consume for fullbuild mode. As such, types defined here will have ABI implications as these definitions may differ from the underlying system for overlay mode and are NEVER appropriate to include in ``libc/src/`` without preprocessor guards for ``LLVM_LIBC_FULL_BUILD``. Consider the case where an implementation in ``libc/src/`` may wish to refer to a ``sigset_t``, what header should be included? ````, ````, ````? None of the above. Instead, code under ``src/`` should ``#include "hdr/types/sigset_t.h"`` which contains preprocessor guards on ``LLVM_LIBC_FULL_BUILD`` to either include the public type (fullbuild mode) or the underlying system header (overlay mode). Implementations in ``libc/src/`` should NOT be ``#include``'ing using ``<>`` or ``"include/*``, except for these "proxy" headers that first check for ``LLVM_LIBC_FULL_BUILD``. These "proxy" headers are similarly used when referring to preprocessor defines. Code under ``libc/src/`` should ``#include`` a proxy header from ``hdr/``, which contains a guard on ``LLVM_LIBC_FULL_BUILD`` to either include our header from ``libc/include/`` (fullbuild) or the corresponding underlying system header (overlay).