1/* Copyright (C) 2000-2022 Free Software Foundation, Inc.
2 This file is part of the GNU C Library.
3
4 The GNU C Library is free software; you can redistribute it and/or
5 modify it under the terms of the GNU Lesser General Public
6 License as published by the Free Software Foundation; either
7 version 2.1 of the License, or (at your option) any later version.
8
9 The GNU C Library 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 GNU
12 Lesser General Public License for more details.
13
14 You should have received a copy of the GNU Lesser General Public
15 License along with the GNU C Library; if not, see
16 <https://www.gnu.org/licenses/>. */
17
18#include <errno.h>
19#include <fcntl.h>
20#include <unistd.h>
21#include <stdint.h>
22#include <sys/fcntl.h>
23#include <sys/stat.h>
24#include <sys/statfs.h>
25
26/* Reserve storage for the data of the file associated with FD. This
27 emulation is far from perfect, but the kernel cannot do not much
28 better for network file systems, either. */
29
30int
31posix_fallocate (int fd, __off_t offset, __off_t len)
32{
33 struct __stat64_t64 st;
34
35 if (offset < 0 || len < 0)
36 return EINVAL;
37
38 /* Perform overflow check. The outer cast relies on a GCC
39 extension. */
40 if ((__off_t) ((uint64_t) offset + (uint64_t) len) < 0)
41 return EFBIG;
42
43 /* pwrite below will not do the right thing in O_APPEND mode. */
44 {
45 int flags = __fcntl (fd, F_GETFL, 0);
46 if (flags < 0 || (flags & O_APPEND) != 0)
47 return EBADF;
48 }
49
50 /* We have to make sure that this is really a regular file. */
51 if (__fstat64_time64 (fd, &st) != 0)
52 return EBADF;
53 if (S_ISFIFO (st.st_mode))
54 return ESPIPE;
55 if (! S_ISREG (st.st_mode))
56 return ENODEV;
57
58 if (len == 0)
59 {
60 /* This is racy, but there is no good way to satisfy a
61 zero-length allocation request. */
62 if (st.st_size < offset)
63 {
64 int ret = __ftruncate (fd, offset);
65
66 if (ret != 0)
67 ret = errno;
68 return ret;
69 }
70 return 0;
71 }
72
73 /* Minimize data transfer for network file systems, by issuing
74 single-byte write requests spaced by the file system block size.
75 (Most local file systems have fallocate support, so this fallback
76 code is not used there.) */
77
78 unsigned increment;
79 {
80 struct statfs64 f;
81
82 if (__fstatfs64 (fd, &f) != 0)
83 return errno;
84 if (f.f_bsize == 0)
85 increment = 512;
86 else if (f.f_bsize < 4096)
87 increment = f.f_bsize;
88 else
89 /* NFS does not propagate the block size of the underlying
90 storage and may report a much larger value which would still
91 leave holes after the loop below, so we cap the increment at
92 4096. */
93 increment = 4096;
94 }
95
96 /* Write a null byte to every block. This is racy; we currently
97 lack a better option. Compare-and-swap against a file mapping
98 might additional local races, but requires interposition of a
99 signal handler to catch SIGBUS. */
100 for (offset += (len - 1) % increment; len > 0; offset += increment)
101 {
102 len -= increment;
103
104 if (offset < st.st_size)
105 {
106 unsigned char c;
107 ssize_t rsize = __pread (fd, &c, 1, offset);
108
109 if (rsize < 0)
110 return errno;
111 /* If there is a non-zero byte, the block must have been
112 allocated already. */
113 else if (rsize == 1 && c != 0)
114 continue;
115 }
116
117 if (__pwrite (fd, "", 1, offset) != 1)
118 return errno;
119 }
120
121 return 0;
122}
123

source code of glibc/sysdeps/posix/posix_fallocate.c