sysdeps/posix/pread.c

   1 /* Read block from given position in file without changing file pointer.
   2    POSIX version.
   3    Copyright (C) 1997-2017 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 Lesser General Public
   9    License as published by the Free Software Foundation; either
  10    version 2.1 of the 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    Lesser General Public License for more details.
  16
  17    You should have received a copy of the GNU Lesser General Public
  18    License along with the GNU C Library; if not, see
  19    <http://www.gnu.org/licenses/>.  */
  20
  21 #include <errno.h>
  22 #include <unistd.h>
  23
  24 /* Note: This implementation of pread is not multithread-safe.  */
  25
  26 ssize_t
  27 __libc_pread (int fd, void *buf, size_t nbyte, off_t offset)
  28 {
  29   /* Since we must not change the file pointer preserve the value so that
  30      we can restore it later.  */
  31   int save_errno;
  32   ssize_t result;
  33   off_t old_offset = __libc_lseek (fd, 0, SEEK_CUR);
  34   if (old_offset == (off_t) -1)
  35     return -1;
  36
  37   /* Set to wanted position.  */
  38   if (__libc_lseek (fd, offset, SEEK_SET) == (off_t) -1)
  39     return -1;
  40
  41   /* Write out the data.  */
  42   result = __libc_read (fd, buf, nbyte);
  43
  44   /* Now we have to restore the position.  If this fails we have to
  45      return this as an error.  But if the writing also failed we
  46      return this error.  */
  47   save_errno = errno;
  48   if (__libc_lseek (fd, old_offset, SEEK_SET) == (off_t) -1)
  49     {
  50       if (result == -1)
  51         __set_errno (save_errno);
  52       return -1;
  53     }
  54   __set_errno (save_errno);
  55
  56   return result;
  57 }
  58
  59 #ifndef __libc_pread
  60 strong_alias (__libc_pread, __pread)
  61 weak_alias (__libc_pread, pread)
  62 #endif