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