KCoreAddons

posix_fallocate_mac.h
1 /* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*-
2 
3  SPDX-FileCopyrightText: 2010 Mozilla Foundation
4  SPDX-FileContributor: Taras Glek <[email protected]>
5 
6  SPDX-License-Identifier: MPL-1.1 OR GPL-2.0-or-later OR LGPL-2.1-or-later
7 */
8 
9 #ifndef POSIX_FALLOCATE_MAC_H
10 #define POSIX_FALLOCATE_MAC_H
11 
12 #include <fcntl.h>
13 #include <sys/stat.h>
14 #include <sys/types.h>
15 #include <unistd.h>
16 
17 // created from the OSX-specific code from Mozilla's mozilla::fallocation() function
18 // of which the licensing information is copied above.
19 // Adaptation (C) 2015,2016 R.J.V. Bertin
20 
21 // From Linux `man posix_fallocate`:
22 // DESCRIPTION
23 // The function posix_fallocate() ensures that disk space is allocated for
24 // the file referred to by the descriptor fd for the bytes in the range
25 // starting at offset and continuing for len bytes. After a successful
26 // call to posix_fallocate(), subsequent writes to bytes in the specified
27 // range are guaranteed not to fail because of lack of disk space.
28 //
29 // If the size of the file is less than offset+len, then the file is
30 // increased to this size; otherwise the file size is left unchanged.
31 
32 // From OS X man fcntl:
33 // F_PREALLOCATE Preallocate file storage space. Note: upon success, the space
34 // that is allocated can be the same size or larger than the space
35 // requested.
36 // The F_PREALLOCATE command operates on the following structure:
37 // typedef struct fstore {
38 // u_int32_t fst_flags; /* IN: flags word */
39 // int fst_posmode; /* IN: indicates offset field */
40 // off_t fst_offset; /* IN: start of the region */
41 // off_t fst_length; /* IN: size of the region */
42 // off_t fst_bytesalloc; /* OUT: number of bytes allocated */
43 // } fstore_t;
44 // The flags (fst_flags) for the F_PREALLOCATE command are as follows:
45 // F_ALLOCATECONTIG Allocate contiguous space.
46 // F_ALLOCATEALL Allocate all requested space or no space at all.
47 // The position modes (fst_posmode) for the F_PREALLOCATE command indicate how to use
48 // the offset field. The modes are as follows:
49 // F_PEOFPOSMODE Allocate from the physical end of file.
50 // F_VOLPOSMODE Allocate from the volume offset.
51 
52 // From OS X man ftruncate:
53 // DESCRIPTION
54 // ftruncate() and truncate() cause the file named by path, or referenced by fildes, to
55 // be truncated (or extended) to length bytes in size. If the file size exceeds length,
56 // any extra data is discarded. If the file size is smaller than length, the file
57 // extended and filled with zeros to the indicated length. The ftruncate() form requires
58 // the file to be open for writing.
59 // Note: ftruncate() and truncate() do not modify the current file offset for any open
60 // file descriptions associated with the file.
61 
62 static int posix_fallocate(int fd, off_t offset, off_t len)
63 {
64  off_t c_test;
65  int ret;
66  if (!__builtin_saddll_overflow(offset, len, &c_test)) {
67  fstore_t store = {F_ALLOCATECONTIG, F_PEOFPOSMODE, 0, offset + len};
68  // Try to get a continuous chunk of disk space
69  fcntl(fd, F_PREALLOCATE, &store);
70  if (ret < 0) {
71  // OK, perhaps we are too fragmented, allocate non-continuous
72  store.fst_flags = F_ALLOCATEALL;
73  ret = fcntl(fd, F_PREALLOCATE, &store);
74  if (ret < 0) {
75  return ret;
76  }
77  }
78  ret = ftruncate(fd, offset + len);
79  } else {
80  // offset+len would overflow.
81  ret = -1;
82  }
83  return ret;
84 }
85 
86 #endif
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