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/* Optimization of multithreaded code.
Copyright (C) 2020-2021 Free Software Foundation, Inc.
This file is free software: you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as
published by the Free Software Foundation; either version 2.1 of the
License, or (at your option) any later version.
This file is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License
along with this program. If not, see <https://www.gnu.org/licenses/>. */
/* Written by Bruno Haible <bruno@clisp.org>, 2020. */
#ifndef _THREAD_OPTIM_H
#define _THREAD_OPTIM_H
/* This file defines a way to optimize multithreaded code for the single-thread
case, based on the variable '__libc_single_threaded', defined in
glibc >= 2.32. */
/* Typical use: In a block or function, use
bool mt = gl_multithreaded ();
...
if (mt)
if (pthread_mutex_lock (&lock)) abort ();
...
if (mt)
if (pthread_mutex_unlock (&lock)) abort ();
The gl_multithreaded () invocation determines whether the program currently
is multithreaded.
if (mt) STATEMENT executes STATEMENT in the multithreaded case, and skips
it in the single-threaded case.
The code between the gl_multithreaded () invocation and any use of the
variable 'mt' must not create threads or invoke functions that may
indirectly create threads (e.g. 'dlopen' may, indirectly through C++
initializers of global variables in the shared library being opened,
create threads).
The lock here is meant to synchronize threads in the same process. The
same optimization cannot be applied to locks that synchronize different
processes (e.g. through shared memory mappings). */
#if HAVE_SYS_SINGLE_THREADED_H /* glibc >= 2.32 */
# include <sys/single_threaded.h>
# define gl_multithreaded() !__libc_single_threaded
#else
# define gl_multithreaded() 1
#endif
#endif /* _THREAD_OPTIM_H */
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