Direct-BT
v3.3.0-1-gc2d430c
Direct-BT - Direct Bluetooth Programming.
jaulib
include
jau
packed_attribute.hpp
Go to the documentation of this file.
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/*
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* Copyright (c) 2020 Gothel Software e.K.
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* Copyright (c) 2020 ZAFENA AB
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*
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* Permission is hereby granted, free of charge, to any person obtaining
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* a copy of this software and associated documentation files (the
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* "Software"), to deal in the Software without restriction, including
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* without limitation the rights to use, copy, modify, merge, publish,
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* distribute, sublicense, and/or sell copies of the Software, and to
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* permit persons to whom the Software is furnished to do so, subject to
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* the following conditions:
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*
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* The above copyright notice and this permission notice shall be
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* included in all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
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* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
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* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
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* NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
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* LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
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* OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
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* WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
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*
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*/
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#ifndef PACKED_ATTRIBUTE_HPP_
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#define PACKED_ATTRIBUTE_HPP_
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/** \addtogroup CppLang
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*
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* @{
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*/
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/** packed__: lead in macro, requires __packed lead out as well. Consider using __pack(...). */
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#ifndef packed__
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#ifdef _MSC_VER
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#define packed__ __pragma( pack(push, 1) )
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#else
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#define packed__
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#endif
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#endif
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/** __packed: lead out macro, requires packed__ lead in as well. Consider using __pack(...). */
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#ifndef __packed
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#ifdef _MSC_VER
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#define __packed __pragma( pack(pop))
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#else
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#define __packed __attribute__ ((packed))
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#endif
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#endif
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/** __pack(...): Produces MSVC, clang and gcc compatible lead-in and -out macros. */
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#ifndef __pack
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#ifdef _MSC_VER
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#define __pack(...) __pragma( pack(push, 1) ) __VA_ARGS__ __pragma( pack(pop))
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#else
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#define __pack(...) __VA_ARGS__ __attribute__ ((packed))
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#endif
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#endif
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/**@}*/
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namespace
jau
{
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/** \addtogroup CppLang
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*
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* @{
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*/
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/**
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* Support aligned memory transfer from and to potentially unaligned memory.
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*
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* This template causes little to no runtime costs.
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*
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* A casted data pointer to `packed_t<T>*` is similar to 'T * p = (T *) buffer', having `uint8_t * buffer`.
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* However, `packed_t<T>*` doesn't have any intrinsic alignment corrections due to
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* its used `__attribute__((__packed__))`.
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*
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* packed_t is used in \ref ByteUtils.
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*
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* @anchor packed_t_alignment_cast
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* Background for using packed_t:
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*
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* Due to the potentially unaligned memory address of `buffer`,
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* we can't just directly use pointer arithmetic like:
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* <pre>
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* // return uint16_t from memory
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* return *( (uint16_t *) ( buffer ) );
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*
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* // store uint16_t to memory
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* *( (uint16_t *) ( buffer ) ) = v;
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* </pre>
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*
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* The universal alternative using `memcpy()` is costly:
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* <pre>
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* // return uint16_t from memory
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* memcpy(&v, buffer, sizeof(v));
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* return v;
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*
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* // store uint16_t to memory
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* memcpy(buffer, &v, sizeof(v));
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* </pre>
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*
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* Solution is to use the *free of costs* high performance *compiler magic* 'struct __attribute__((__packed__))'.<br />
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* The offset memory is pointer_cast() into the desired packed_t type and its packed_t::store accessed thereafter:
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* <pre>
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* // return uint16_t from memory
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* return pointer_cast<const packed_t<uint16_t>*>( buffer )->store;
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*
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* // store uint16_t to memory
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* pointer_cast<packed_t<uint16_t>*>( buffer )->store = v;
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* </pre>
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*/
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template
<
typename
T> __pack (
struct
packed_t {
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T store;
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} ) ;
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/**@}*/
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}
// namespace jau
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/**@}*/
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#endif
/* PACKED_ATTRIBUTE_HPP_ */
jau
__pack(...): Produces MSVC, clang and gcc compatible lead-in and -out macros.
Definition:
backtrace.hpp:32
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