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The C and C++ Include Header Files

cat -n /usr/include/c++/15/bits/align.h

     1	// align implementation -*- C++ -*-
     2	
     3	// Copyright (C) 2014-2025 Free Software Foundation, Inc.
     4	//
     5	// This file is part of the GNU ISO C++ Library.  This library is free
     6	// software; you can redistribute it and/or modify it under the
     7	// terms of the GNU General Public License as published by the
     8	// Free Software Foundation; either version 3, or (at your option)
     9	// any later version.
    10	
    11	// This library is distributed in the hope that it will be useful,
    12	// but WITHOUT ANY WARRANTY; without even the implied warranty of
    13	// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    14	// GNU General Public License for more details.
    15	
    16	// Under Section 7 of GPL version 3, you are granted additional
    17	// permissions described in the GCC Runtime Library Exception, version
    18	// 3.1, as published by the Free Software Foundation.
    19	
    20	// You should have received a copy of the GNU General Public License and
    21	// a copy of the GCC Runtime Library Exception along with this program;
    22	// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
    23	// <http://www.gnu.org/licenses/>.
    24	
    25	/** @file bits/align.h
    26	 *  This is an internal header file, included by other library headers.
    27	 *  Do not attempt to use it directly. @headername{memory}
    28	 */
    29	
    30	#ifndef _GLIBCXX_ALIGN_H
    31	#define _GLIBCXX_ALIGN_H 1
    32	
    33	#include <bit>          // std::has_single_bit
    34	#include <debug/assertions.h> // _GLIBCXX_DEBUG_ASSERT
    35	#include <bits/version.h>
    36	
    37	namespace std _GLIBCXX_VISIBILITY(default)
    38	{
    39	_GLIBCXX_BEGIN_NAMESPACE_VERSION
    40	
    41	/**
    42	 *  @brief Fit aligned storage in buffer.
    43	 *
    44	 *  This function tries to fit @a __size bytes of storage with alignment
    45	 *  @a __align into the buffer @a __ptr of size @a __space bytes.  If such
    46	 *  a buffer fits then @a __ptr is changed to point to the first byte of the
    47	 *  aligned storage and @a __space is reduced by the bytes used for alignment.
    48	 *
    49	 *  C++11 20.6.5 [ptr.align]
    50	 *
    51	 *  @param __align   A fundamental or extended alignment value.
    52	 *  @param __size    Size of the aligned storage required.
    53	 *  @param __ptr     Pointer to a buffer of @a __space bytes.
    54	 *  @param __space   Size of the buffer pointed to by @a __ptr.
    55	 *  @return the updated pointer if the aligned storage fits, otherwise nullptr.
    56	 *
    57	 *  @ingroup memory
    58	 */
    59	inline void*
    60	align(size_t __align, size_t __size, void*& __ptr, size_t& __space) noexcept
    61	{
    62	  if (__space < __size)
    63	    return nullptr;
    64	  const auto __intptr = reinterpret_cast<__UINTPTR_TYPE__>(__ptr);
    65	  const auto __aligned = (__intptr - 1u + __align) & -__align;
    66	  const auto __diff = __aligned - __intptr;
    67	  if (__diff > (__space - __size))
    68	    return nullptr;
    69	  else
    70	    {
    71	      __space -= __diff;
    72	      return __ptr = reinterpret_cast<void*>(__aligned);
    73	    }
    74	}
    75	
    76	#ifdef __glibcxx_assume_aligned // C++ >= 20
    77	  /** @brief Inform the compiler that a pointer is aligned.
    78	   *
    79	   *  @tparam _Align An alignment value (i.e. a power of two)
    80	   *  @tparam _Tp    An object type
    81	   *  @param  __ptr  A pointer that is aligned to _Align
    82	   *
    83	   *  C++20 20.10.6 [ptr.align]
    84	   *
    85	   *  @ingroup memory
    86	   */
    87	  template<size_t _Align, class _Tp>
    88	    [[nodiscard,__gnu__::__always_inline__]]
    89	    constexpr _Tp*
    90	    assume_aligned(_Tp* __ptr) noexcept
    91	    {
    92	      static_assert(std::has_single_bit(_Align));
    93	      if (std::is_constant_evaluated())
    94		return __ptr;
    95	      else
    96		{
    97		  // This function is expected to be used in hot code, where
    98		  // __glibcxx_assert would add unwanted overhead.
    99		  _GLIBCXX_DEBUG_ASSERT((__UINTPTR_TYPE__)__ptr % _Align == 0);
   100		  return static_cast<_Tp*>(__builtin_assume_aligned(__ptr, _Align));
   101		}
   102	    }
   103	#endif // __glibcxx_assume_aligned
   104	
   105	_GLIBCXX_END_NAMESPACE_VERSION
   106	} // namespace
   107	
   108	#endif /* _GLIBCXX_ALIGN_H */