sysdeps/posix/pread.c

   1 /* Read block from given position in file without changing file pointer.
   2    POSIX version.
   3    Copyright (C) 1997-2024 Free Software Foundation, Inc.
   4    This file is part of the GNU C Library.
   5
   6    The GNU C Library is free software; you can redistribute it and/or
   7    modify it under the terms of the GNU Lesser General Public
   8    License as published by the Free Software Foundation; either
   9    version 2.1 of the License, or (at your option) any later version.
  10
  11    The GNU C Library is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14    Lesser General Public License for more details.
  15
  16    You should have received a copy of the GNU Lesser General Public
  17    License along with the GNU C Library; if not, see
  18    <https://www.gnu.org/licenses/>.  */
  19
  20 #include <errno.h>
  21 #include <unistd.h>
  22
  23 /* Note: This implementation of pread is not multithread-safe.  */
  24
  25 ssize_t
  26 __libc_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 = __libc_lseek (fd, 0, SEEK_CUR);
  33   if (old_offset == (off_t) -1)
  34     return -1;
  35
  36   /* Set to wanted position.  */
  37   if (__libc_lseek (fd, offset, SEEK_SET) == (off_t) -1)
  38     return -1;
  39
  40   /* Write out the data.  */
  41   result = __libc_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 (__libc_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 }
  57
  58 #ifndef __libc_pread
  59 strong_alias (__libc_pread, __pread)
  60 weak_alias (__libc_pread, pread)
  61 #endif