1 /* Read block from given position in file without changing file pointer.
2 POSIX version.
3 Copyright (C) 1997-2022 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
__libc_pread(int fd,void * buf,size_t nbyte,off_t offset)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
62