lib/pread.c

   1 /* replacement pread function
   2    Copyright (C) 2009-2024 Free Software Foundation, Inc.
   3
   4    This file is free software: you can redistribute it and/or modify
   5    it under the terms of the GNU Lesser General Public License as
   6    published by the Free Software Foundation; either version 2.1 of the
   7    License, or (at your option) any later version.
   8
   9    This file is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12    GNU Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public License
  15    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  16
  17 #include <config.h>
  18
  19 /* Specification.  */
  20 #include <unistd.h>
  21
  22 #include <errno.h>
  23
  24 #define __libc_lseek(f,o,w) lseek (f, o, w)
  25 #define __set_errno(Val) errno = (Val)
  26 #define __libc_read(f,b,n) read (f, b, n)
  27
  28 /* pread substitute for systems that the function, such as mingw32 and BeOS.  */
  29 /* The following is identical to the function from glibc's
  30    sysdeps/posix/pread.c */
  31
  32 /* Note: This implementation of pread is not multithread-safe.  */
  33
  34 ssize_t
  35 pread (int fd, void *buf, size_t nbyte, off_t offset)
  36 {
  37   /* Since we must not change the file pointer preserve the value so that
  38      we can restore it later.  */
  39   int saved_errno;
  40   ssize_t result;
  41   off_t old_offset = __libc_lseek (fd, 0, SEEK_CUR);
  42   if (old_offset == (off_t) -1)
  43     return -1;
  44
  45   /* Set to wanted position.  */
  46   if (__libc_lseek (fd, offset, SEEK_SET) == (off_t) -1)
  47     return -1;
  48
  49   /* Write out the data.  */
  50   result = __libc_read (fd, buf, nbyte);
  51
  52   /* Now we have to restore the position.  If this fails we have to
  53      return this as an error.  But if the writing also failed we
  54      return this error.  */
  55   saved_errno = errno;
  56   if (__libc_lseek (fd, old_offset, SEEK_SET) == (off_t) -1)
  57     {
  58       if (result == -1)
  59         __set_errno (saved_errno);
  60       return -1;
  61     }
  62   __set_errno (saved_errno);
  63
  64   return result;
  65 }