1  /* Optimization of multithreaded code.
       2  
       3     Copyright (C) 2020-2023 Free Software Foundation, Inc.
       4  
       5     This file is free software: you can redistribute it and/or modify
       6     it under the terms of the GNU Lesser General Public License as
       7     published by the Free Software Foundation; either version 2.1 of the
       8     License, or (at your option) any later version.
       9  
      10     This file is distributed in the hope that it will be useful,
      11     but WITHOUT ANY WARRANTY; without even the implied warranty of
      12     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
      13     GNU Lesser General Public License for more details.
      14  
      15     You should have received a copy of the GNU Lesser General Public License
      16     along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
      17  
      18  /* Written by Bruno Haible <bruno@clisp.org>, 2020.  */
      19  
      20  #ifndef _THREAD_OPTIM_H
      21  #define _THREAD_OPTIM_H
      22  
      23  /* This file defines a way to optimize multithreaded code for the single-thread
      24     case, based on the variable '__libc_single_threaded', defined in
      25     glibc >= 2.32.  */
      26  
      27  /* Typical use: In a block or function, use
      28  
      29       bool mt = gl_multithreaded ();
      30       ...
      31       if (mt)
      32         if (pthread_mutex_lock (&lock)) abort ();
      33       ...
      34       if (mt)
      35         if (pthread_mutex_unlock (&lock)) abort ();
      36  
      37     The gl_multithreaded () invocation determines whether the program currently
      38     is multithreaded.
      39  
      40     if (mt) STATEMENT executes STATEMENT in the multithreaded case, and skips
      41     it in the single-threaded case.
      42  
      43     The code between the gl_multithreaded () invocation and any use of the
      44     variable 'mt' must not create threads or invoke functions that may
      45     indirectly create threads (e.g. 'dlopen' may, indirectly through C++
      46     initializers of global variables in the shared library being opened,
      47     create threads).
      48  
      49     The lock here is meant to synchronize threads in the same process.  The
      50     same optimization cannot be applied to locks that synchronize different
      51     processes (e.g. through shared memory mappings).  */
      52  
      53  /* This file uses HAVE_SYS_SINGLE_THREADED_H.  */
      54  #if !_GL_CONFIG_H_INCLUDED
      55   #error "Please include config.h first."
      56  #endif
      57  
      58  #if HAVE_SYS_SINGLE_THREADED_H /* glibc >= 2.32 */
      59  # include <sys/single_threaded.h>
      60  # define gl_multithreaded()  (!__libc_single_threaded)
      61  #else
      62  # define gl_multithreaded()  1
      63  #endif
      64  
      65  #endif /* _THREAD_OPTIM_H */