sem_wait.c

glibc 库, 不仅可以学习使用库函数,还可以学习函数的具体实现,是提高功力的好资料

/* sem_wait -- wait on a semaphore.  Generic futex-using version.
   Copyright (C) 2003 Free Software Foundation, Inc.
   This file is part of the GNU C Library.
   Contributed by Paul Mackerras <[e-mail]>, 2003.

   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, write to the Free
   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
   02111-1307 USA.  */

#include <errno.h>
#include <sysdep.h>
#include <lowlevellock.h>
#include <internaltypes.h>
#include <semaphore.h>

#include <pthreadP.h>
#include <shlib-compat.h>


int
__new_sem_wait (sem_t *sem)
{
  /* First check for cancellation.  */
  CANCELLATION_P (THREAD_SELF);

  int *futex = (int *) sem;
  int err;

  do
    {
      if (atomic_decrement_if_positive (futex) > 0)
	return 0;

      /* Enable asynchronous cancellation.  Required by the standard.  */
      int oldtype = __pthread_enable_asynccancel ();

      err = lll_futex_wait (futex, 0);

      /* Disable asynchronous cancellation.  */
      __pthread_disable_asynccancel (oldtype);
    }
  while (err == 0 || err == -EWOULDBLOCK);

  __set_errno (-err);
  return -1;
}

versioned_symbol (libpthread, __new_sem_wait, sem_wait, GLIBC_2_1);
#if SHLIB_COMPAT (libpthread, GLIBC_2_0, GLIBC_2_1)
strong_alias (__new_sem_wait, __old_sem_wait)
compat_symbol (libpthread, __old_sem_wait, sem_wait, GLIBC_2_0);
#endif