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The C and C++ Include Header Files

cat -n /usr/include/c++/15/ctime

     1	// -*- C++ -*- forwarding header.
     2	
     3	// Copyright (C) 1997-2025 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 include/ctime
    26	 *  This is a Standard C++ Library file.  You should @c \#include this file
    27	 *  in your programs, rather than any of the @a *.h implementation files.
    28	 *
    29	 *  This is the C++ version of the Standard C Library header @c time.h,
    30	 *  and its contents are (mostly) the same as that header, but are all
    31	 *  contained in the namespace @c std (except for names which are defined
    32	 *  as macros in C).
    33	 */
    34	
    35	//
    36	// ISO C++ 14882: 20.5  Date and time
    37	//
    38	
    39	#ifndef _GLIBCXX_CTIME
    40	#define _GLIBCXX_CTIME 1
    41	
    42	#ifdef _GLIBCXX_SYSHDR
    43	#pragma GCC system_header
    44	#endif
    45	
    46	#include <bits/c++config.h>
    47	#include <time.h>
    48	
    49	// Get rid of those macros defined in <time.h> in lieu of real functions.
    50	#undef clock
    51	#undef difftime
    52	#undef mktime
    53	#undef time
    54	#undef asctime
    55	#undef ctime
    56	#undef gmtime
    57	#undef localtime
    58	#undef strftime
    59	
    60	namespace std
    61	{
    62	  using ::clock_t;
    63	  using ::time_t;
    64	  using ::tm;
    65	
    66	  using ::clock;
    67	  using ::difftime;
    68	  using ::mktime;
    69	  using ::time;
    70	  using ::asctime;
    71	  using ::ctime;
    72	  using ::gmtime;
    73	  using ::localtime;
    74	  using ::strftime;
    75	} // namespace
    76	
    77	#if __cplusplus >= 201703L && defined(_GLIBCXX_HAVE_TIMESPEC_GET)
    78	#undef timespec_get
    79	namespace std
    80	{
    81	  using ::timespec;
    82	  using ::timespec_get;
    83	} // namespace std
    84	#endif
    85	
    86	#endif