sysdeps/posix/pread.c

   1 /* Read block from given position in file without changing file pointer.
   2    POSIX version.
   3    Copyright (C) 1997 Free Software Foundation, Inc.
   4    This file is part of the GNU C Library.
   5    Contributed by Ulrich Drepper <drepper@cygnus.com>, 1997.
   6
   7    The GNU C Library is free software; you can redistribute it and/or
   8    modify it under the terms of the GNU Library General Public License as
   9    published by the Free Software Foundation; either version 2 of the
  10    License, or (at your option) any later version.
  11
  12    The GNU C Library is distributed in the hope that it will be useful,
  13    but WITHOUT ANY WARRANTY; without even the implied warranty of
  14    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  15    Library General Public License for more details.
  16
  17    You should have received a copy of the GNU Library General Public
  18    License along with the GNU C Library; see the file COPYING.LIB.  If not,
  19    write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  20    Boston, MA 02111-1307, USA.  */
  21
  22 #include <errno.h>
  23 #include <unistd.h>
  24
  25 ssize_t
  26 pread (int fd, void *buf, size_t nbyte, off_t offset)
  27 {
  28   /* Since we must not change the file pointer preserve the value so that
  29      we can restore it later.  */
  30   int save_errno;
  31   ssize_t result;
  32   off_t old_offset = lseek (fd, 0, SEEK_CUR);
  33   if (old_offset == (off_t) -1)
  34     return -1;
  35
  36   /* Set to wanted position.  */
  37   if (lseek (fd, offset, SEEK_SET) == (off_t) -1)
  38     return -1;
  39
  40   /* Write out the data.  */
  41   result = read (fd, buf, nbyte);
  42
  43   /* Now we have to restore the position.  If this fails we have to
  44      return this as an error.  But if the writing also failed we
  45      return this error.  */
  46   save_errno = errno;
  47   if (lseek (fd, old_offset, SEEK_SET) == (off_t) -1)
  48     {
  49       if (result == -1)
  50         __set_errno (save_errno);
  51       return -1;
  52     }
  53   __set_errno (save_errno);
  54
  55   return result;
  56 }