lib/pwrite.c

   1 /* Write block to given position in file without changing file pointer.
   2    POSIX version.
   3    Copyright (C) 1997-1999, 2002, 2011-2020 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    This program is free software: you can redistribute it and/or modify
   8    it under the terms of the GNU General Public License as published by
   9    the Free Software Foundation; either version 3 of the License, or
  10    (at your option) any later version.
  11
  12    This program 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
  15    GNU General Public License for more details.
  16
  17    You should have received a copy of the GNU General Public License
  18    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  19
  20 #include <config.h>
  21
  22 /* Specification.  */
  23 #include <unistd.h>
  24
  25 #include <errno.h>
  26
  27 #define __libc_lseek(f,o,w) lseek (f, o, w)
  28 #define __set_errno(Val) errno = (Val)
  29 #define __libc_write(f,b,n) write (f, b, n)
  30
  31 /* Note: This implementation of pwrite is not multithread-safe.  */
  32
  33 ssize_t
  34 pwrite (int fd, const void *buf, size_t nbyte, off_t offset)
  35 {
  36   /* Since we must not change the file pointer preserve the value so that
  37      we can restore it later.  */
  38   int save_errno;
  39   ssize_t result;
  40   off_t old_offset = __libc_lseek (fd, 0, SEEK_CUR);
  41   if (old_offset == (off_t) -1)
  42     return -1;
  43
  44   /* Set to wanted position.  */
  45   if (__libc_lseek (fd, offset, SEEK_SET) == (off_t) -1)
  46     return -1;
  47
  48   /* Write out the data.  */
  49   result = __libc_write (fd, buf, nbyte);
  50
  51   /* Now we have to restore the position.  If this fails we have to
  52      return this as an error.  But if the writing also failed we
  53      return this error.  */
  54   save_errno = errno;
  55   if (__libc_lseek (fd, old_offset, SEEK_SET) == (off_t) -1)
  56     {
  57       if (result == -1)
  58         __set_errno (save_errno);
  59       return -1;
  60     }
  61   __set_errno (save_errno);
  62
  63   return result;
  64 }