nptl/pthread_rwlock_init.c

   1 /* Copyright (C) 2002-2016 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3    Contributed by Ulrich Drepper <drepper@redhat.com>, 2002.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library 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 GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <http://www.gnu.org/licenses/>.  */
  18
  19 #include "pthreadP.h"
  20 #include <string.h>
  21 #include <kernel-features.h>
  22
  23
  24 static const struct pthread_rwlockattr default_rwlockattr =
  25   {
  26     .lockkind = PTHREAD_RWLOCK_DEFAULT_NP,
  27     .pshared = PTHREAD_PROCESS_PRIVATE
  28   };
  29
  30
  31 int
  32 __pthread_rwlock_init (pthread_rwlock_t *rwlock,
  33                        const pthread_rwlockattr_t *attr)
  34 {
  35   const struct pthread_rwlockattr *iattr;
  36
  37   iattr = ((const struct pthread_rwlockattr *) attr) ?: &default_rwlockattr;
  38
  39   memset (rwlock, '\0', sizeof (*rwlock));
  40
  41   rwlock->__data.__flags
  42     = iattr->lockkind == PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP;
  43
  44   /* The __SHARED field is computed to minimize the work that needs to
  45      be done while handling the futex.  There are two inputs: the
  46      availability of private futexes and whether the rwlock is shared
  47      or private.  Unfortunately the value of a private rwlock is
  48      fixed: it must be zero.  The PRIVATE_FUTEX flag has the value
  49      0x80 in case private futexes are available and zero otherwise.
  50      This leads to the following table:
  51
  52                  |     pshared     |     result
  53                  | shared  private | shared  private |
  54      ------------+-----------------+-----------------+
  55      !avail 0    |     0       0   |     0       0   |
  56       avail 0x80 |  0x80       0   |     0    0x80   |
  57
  58      If the pshared value is in locking functions XORed with avail
  59      we get the expected result.  */
  60   rwlock->__data.__shared = (iattr->pshared == PTHREAD_PROCESS_PRIVATE
  61                              ? 0 : FUTEX_PRIVATE_FLAG);
  62
  63   return 0;
  64 }
  65 strong_alias (__pthread_rwlock_init, pthread_rwlock_init)