pread.c 2 KB
Newer Older
Jim Meyering's avatar
Jim Meyering committed
1
/* replacement pread function
Paul Eggert's avatar
Paul Eggert committed
2
   Copyright (C) 2009-2016 Free Software Foundation, Inc.
Jim Meyering's avatar
Jim Meyering committed
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.

   This program 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 General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */

#include <config.h>

/* Specification.  */
#include <unistd.h>

#include <errno.h>

#define __libc_lseek(f,o,w) lseek (f, o, w)
#define __set_errno(Val) errno = (Val)
26
#define __libc_read(f,b,n) read (f, b, n)
Jim Meyering's avatar
Jim Meyering committed
27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

/* pread substitute for systems that the function, such as mingw32 and BeOS.  */
/* The following is identical to the function from glibc's
   sysdeps/posix/pread.c */

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

ssize_t
pread (int fd, void *buf, size_t nbyte, off_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;
  off_t old_offset = __libc_lseek (fd, 0, SEEK_CUR);
  if (old_offset == (off_t) -1)
    return -1;

  /* Set to wanted position.  */
  if (__libc_lseek (fd, offset, SEEK_SET) == (off_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_lseek (fd, old_offset, SEEK_SET) == (off_t) -1)
    {
      if (result == -1)
59
        __set_errno (save_errno);
Jim Meyering's avatar
Jim Meyering committed
60 61 62 63 64 65
      return -1;
    }
  __set_errno (save_errno);

  return result;
}