74 lines
2.9 KiB
ReStructuredText
74 lines
2.9 KiB
ReStructuredText
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Convention for implementing entrypoints
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=======================================
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LLVM-libc entrypoints are defined in the entrypoints document. In this document,
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we explain how the entrypoints are implemented. The source layout document
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explains that, within the high level ``src`` directory, there exists one
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directory for every public header file provided by LLVM-libc. The
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implementations of entrypoints live in the directory for the header they belong
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to. Some entrypoints are platform specific, and so their implementations are in
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a subdirectory with the name of the platform (e.g. ``stdio/linux/remove.cpp``).
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Implementation of entrypoints can span multiple ``.cpp`` and ``.h`` files, but
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there will be at least one header file with name of the form
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``<entrypoint name>.h`` for every entrypoint. This header file is called the
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implementation header file. For the ``isalpha`` function, the path to the
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implementation header file is ``src/ctype/isalpha.h``.
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Implementation Header File Structure
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------------------------------------
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We will use the ``isalpha`` function from the public ``ctype.h`` header file as an
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example. The ``isalpha`` function will be declared in an internal header file
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``src/ctype/isalpha.h`` as follows::
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// --- isalpha.h --- //
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#ifndef LLVM_LIBC_SRC_CTYPE_ISALPHA_H
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#define LLVM_LIBC_SRC_CTYPE_ISALPHA_H
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namespace LIBC_NAMESPACE {
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int isalpha(int c);
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} // namespace LIBC_NAMESPACE
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#endif LLVM_LIBC_SRC_CTYPE_ISALPHA_H
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Notice that the ``isalpha`` function declaration is nested inside the namespace
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``LIBC_NAMESPACE``. All implementation constructs in LLVM-libc are declared
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within the namespace ``LIBC_NAMESPACE``.
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``.cpp`` File Structure
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-----------------------
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The main ``.cpp`` file is named ``<entrypoint name>.cpp`` and is usually in the
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same folder as the header. It contains the signature of the entrypoint function,
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which must be defined with the ``LLVM_LIBC_FUNCTION`` macro. For example, the
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``isalpha`` function from ``ctype.h`` is defined as follows, in the file
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``src/ctype/isalpha.cpp``::
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// --- isalpha.cpp --- //
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namespace LIBC_NAMESPACE {
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LLVM_LIBC_FUNCTION(int, isalpha, (int c)) {
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// ... implementation goes here.
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}
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} // namespace LIBC_NAMESPACE
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Notice the use of the macro ``LLVM_LIBC_FUNCTION``. This macro helps us define
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a C alias symbol for the C++ implementation. For example, for a library build,
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the macro is defined as follows::
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#define LLVM_LIBC_FUNCTION(type, name, arglist)
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LLVM_LIBC_FUNCTION_IMPL(type, name, arglist)
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#define LLVM_LIBC_FUNCTION_IMPL(type, name, arglist)
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LLVM_LIBC_FUNCTION_ATTR decltype(LIBC_NAMESPACE::name)
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__##name##_impl__ __asm__(#name);
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decltype(LIBC_NAMESPACE::name) name [[gnu::alias(#name)]];
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type __##name##_impl__ arglist
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The LLVM_LIBC_FUNCTION_ATTR macro is normally defined to nothing, but can be
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defined by vendors who want to set their own attributes.
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