lib/thread-optim.h

   1 /* Optimization of multithreaded code.
   2
   3    Copyright (C) 2020 Free Software Foundation, Inc.
   4
   5    This program is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 3 of the License, or
   8    (at your option) any later version.
   9
  10    This program 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 General Public License for more details.
  14
  15    You should have received a copy of the GNU 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 #if HAVE_SYS_SINGLE_THREADED_H /* glibc >= 2.32 */
  54 # include <sys/single_threaded.h>
  55 # define gl_multithreaded()  !__libc_single_threaded
  56 #else
  57 # define gl_multithreaded()  1
  58 #endif
  59
  60 #endif /* _THREAD_OPTIM_H */