glibc/sysdeps/posix/pread64.c

/* Read block from given position in file without changing file pointer.
   POSIX version.
   Copyright (C) 1997, 1998, 1999, 2000, 2002 Free Software Foundation, Inc.
   This file is part of the GNU C Library.
   Contributed by Ulrich Drepper <drepper@cygnus.com>, 1997.

   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
   <http://www.gnu.org/licenses/>.  */

#include <errno.h>
#include <unistd.h>

/* Note: This implementation of pread64 is not multithread-safe.  */

ssize_t
__libc_pread64 (int fd, void *buf, size_t nbyte, off64_t offset)
{
  /* Since we must not change the file pointer preserve the value so that
     we can restore it later.  */
  int save_errno;
  ssize_t result;
  off64_t old_offset = __libc_lseek64 (fd, 0, SEEK_CUR);
  if (old_offset == (off64_t) -1)
    return -1;

  /* Set to wanted position.  */
  if (__libc_lseek64 (fd, offset, SEEK_SET) == (off64_t) -1)
    return -1;

  /* Write out the data.  */
  result = __libc_read (fd, buf, nbyte);

  /* Now we have to restore the position.  If this fails we have to
     return this as an error.  But if the writing also failed we
     return this error.  */
  save_errno = errno;
  if (__libc_lseek64 (fd, old_offset, SEEK_SET) == (off64_t) -1)
    {
      if (result == -1)
	__set_errno (save_errno);
      return -1;
    }
  __set_errno (save_errno);

  return result;
}

#ifndef __libc_pread64
weak_alias (__libc_pread64, __pread64)
weak_alias (__libc_pread64, pread64)
#endif