glibc/sysdeps/unix/sysv/linux/single-thread.h

/* Single thread optimization, Linux version.
   Copyright (C) 2019-2021 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library 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.

   The GNU C Library 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 the GNU C Library; if not, see
   <https://www.gnu.org/licenses/>.  */

#ifndef _SINGLE_THREAD_H
#define _SINGLE_THREAD_H

/* The default way to check if the process is single thread is by using the
   pthread_t 'multiple_threads' field.  However, for some architectures it is
   faster to either use an extra field on TCB or global variables (the TCB
   field is also used on x86 for some single-thread atomic optimizations).

   The ABI might define SINGLE_THREAD_BY_GLOBAL to enable the single thread
   check to use global variables instead of the pthread_t field.  */

#ifndef __ASSEMBLER__
extern int __libc_multiple_threads;
libc_hidden_proto (__libc_multiple_threads)
#endif

#if !defined SINGLE_THREAD_BY_GLOBAL || IS_IN (rtld)
# define SINGLE_THREAD_P \
  (THREAD_GETMEM (THREAD_SELF, header.multiple_threads) == 0)
#else
# define SINGLE_THREAD_P (__libc_multiple_threads == 0)
#endif

#define RTLD_SINGLE_THREAD_P SINGLE_THREAD_P

#endif /* _SINGLE_THREAD_H  */