1FALLOCATE(2) Linux Programmer's Manual FALLOCATE(2)
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6 fallocate - manipulate file space
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9 #define _GNU_SOURCE /* See feature_test_macros(7) */
10 #include <fcntl.h>
11 int fallocate(int fd, int mode, off_t offset, off_t len);
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15 This is a nonportable, Linux-specific system call. For the portable,
16 POSIX.1-specified method of ensuring that space is allocated for a
17 file, see posix_fallocate(3).
18 fallocate() allows the caller to directly manipulate the allocated disk
20 space for the file referred to by fd for the byte range starting at
21 offset and continuing for len bytes.
22 The mode argument determines the operation to be performed on the given
24 range. Details of the supported operations are given in the subsec‐
25 tions below.
26 Allocating disk space
28 The default operation (i.e., mode is zero) of fallocate() allocates the
29 disk space within the range specified by offset and len. The file size
30 (as reported by stat(2)) will be changed if offset+len is greater than
31 the file size. Any subregion within the range specified by offset and
32 len that did not contain data before the call will be initialized to
33 zero. This default behavior closely resembles the behavior of the
34 posix_fallocate(3) library function, and is intended as a method of op‐
35 timally implementing that function.
36 After a successful call, subsequent writes into the range specified by
38 offset and len are guaranteed not to fail because of lack of disk
39 space.
40 If the FALLOC_FL_KEEP_SIZE flag is specified in mode, the behavior of
42 the call is similar, but the file size will not be changed even if off‐
43 set+len is greater than the file size. Preallocating zeroed blocks be‐
44 yond the end of the file in this manner is useful for optimizing append
45 workloads.
46 If the FALLOC_FL_UNSHARE flag is specified in mode, shared file data
48 extents will be made private to the file to guarantee that a subsequent
49 write will not fail due to lack of space. Typically, this will be done
50 by performing a copy-on-write operation on all shared data in the file.
51 This flag may not be supported by all filesystems.
52 Because allocation is done in block size chunks, fallocate() may allo‐
54 cate a larger range of disk space than was specified.
55 Deallocating file space
57 Specifying the FALLOC_FL_PUNCH_HOLE flag (available since Linux 2.6.38)
58 in mode deallocates space (i.e., creates a hole) in the byte range
59 starting at offset and continuing for len bytes. Within the specified
60 range, partial filesystem blocks are zeroed, and whole filesystem
61 blocks are removed from the file. After a successful call, subsequent
62 reads from this range will return zeros.
63 The FALLOC_FL_PUNCH_HOLE flag must be ORed with FALLOC_FL_KEEP_SIZE in
65 mode; in other words, even when punching off the end of the file, the
66 file size (as reported by stat(2)) does not change.
67 Not all filesystems support FALLOC_FL_PUNCH_HOLE; if a filesystem
69 doesn't support the operation, an error is returned. The operation is
70 supported on at least the following filesystems:
71 * XFS (since Linux 2.6.38)
73 * ext4 (since Linux 3.0)
75 * Btrfs (since Linux 3.7)
77 * tmpfs(5) (since Linux 3.5)
79 * gfs2(5) (since Linux 4.16)
81 Collapsing file space
83 Specifying the FALLOC_FL_COLLAPSE_RANGE flag (available since Linux
84 3.15) in mode removes a byte range from a file, without leaving a hole.
85 The byte range to be collapsed starts at offset and continues for len
86 bytes. At the completion of the operation, the contents of the file
87 starting at the location offset+len will be appended at the location
88 offset, and the file will be len bytes smaller.
89 A filesystem may place limitations on the granularity of the operation,
91 in order to ensure efficient implementation. Typically, offset and len
92 must be a multiple of the filesystem logical block size, which varies
93 according to the filesystem type and configuration. If a filesystem
94 has such a requirement, fallocate() fails with the error EINVAL if this
95 requirement is violated.
96 If the region specified by offset plus len reaches or passes the end of
98 file, an error is returned; instead, use ftruncate(2) to truncate a
99 file.
100 No other flags may be specified in mode in conjunction with FAL‐
102 LOC_FL_COLLAPSE_RANGE.
103 As at Linux 3.15, FALLOC_FL_COLLAPSE_RANGE is supported by ext4 (only
105 for extent-based files) and XFS.
106 Zeroing file space
108 Specifying the FALLOC_FL_ZERO_RANGE flag (available since Linux 3.15)
109 in mode zeros space in the byte range starting at offset and continuing
110 for len bytes. Within the specified range, blocks are preallocated for
111 the regions that span the holes in the file. After a successful call,
112 subsequent reads from this range will return zeros.
113 Zeroing is done within the filesystem preferably by converting the
115 range into unwritten extents. This approach means that the specified
116 range will not be physically zeroed out on the device (except for par‐
117 tial blocks at the either end of the range), and I/O is (otherwise) re‐
118 quired only to update metadata.
119 If the FALLOC_FL_KEEP_SIZE flag is additionally specified in mode, the
121 behavior of the call is similar, but the file size will not be changed
122 even if offset+len is greater than the file size. This behavior is the
123 same as when preallocating space with FALLOC_FL_KEEP_SIZE specified.
124 Not all filesystems support FALLOC_FL_ZERO_RANGE; if a filesystem
126 doesn't support the operation, an error is returned. The operation is
127 supported on at least the following filesystems:
128 * XFS (since Linux 3.15)
130 * ext4, for extent-based files (since Linux 3.15)
132 * SMB3 (since Linux 3.17)
134 * Btrfs (since Linux 4.16)
136 Increasing file space
138 Specifying the FALLOC_FL_INSERT_RANGE flag (available since Linux 4.1)
139 in mode increases the file space by inserting a hole within the file
140 size without overwriting any existing data. The hole will start at
141 offset and continue for len bytes. When inserting the hole inside
142 file, the contents of the file starting at offset will be shifted up‐
143 ward (i.e., to a higher file offset) by len bytes. Inserting a hole
144 inside a file increases the file size by len bytes.
145 This mode has the same limitations as FALLOC_FL_COLLAPSE_RANGE regard‐
147 ing the granularity of the operation. If the granularity requirements
148 are not met, fallocate() fails with the error EINVAL. If the offset is
149 equal to or greater than the end of file, an error is returned. For
150 such operations (i.e., inserting a hole at the end of file), ftrun‐
151 cate(2) should be used.
152 No other flags may be specified in mode in conjunction with FAL‐
154 LOC_FL_INSERT_RANGE.
155 FALLOC_FL_INSERT_RANGE requires filesystem support. Filesystems that
157 support this operation include XFS (since Linux 4.1) and ext4 (since
158 Linux 4.2).
159
161 On success, fallocate() returns zero. On error, -1 is returned and er‐
162 rno is set to indicate the error.
163
165 EBADF fd is not a valid file descriptor, or is not opened for writing.
166 EFBIG offset+len exceeds the maximum file size.
168 EFBIG mode is FALLOC_FL_INSERT_RANGE, and the current file size+len
170 exceeds the maximum file size.
171 EINTR A signal was caught during execution; see signal(7).
173 EINVAL offset was less than 0, or len was less than or equal to 0.
175 EINVAL mode is FALLOC_FL_COLLAPSE_RANGE and the range specified by off‐
177 set plus len reaches or passes the end of the file.
178 EINVAL mode is FALLOC_FL_INSERT_RANGE and the range specified by offset
180 reaches or passes the end of the file.
181 EINVAL mode is FALLOC_FL_COLLAPSE_RANGE or FALLOC_FL_INSERT_RANGE, but
183 either offset or len is not a multiple of the filesystem block
184 size.
185 EINVAL mode contains one of FALLOC_FL_COLLAPSE_RANGE or FALLOC_FL_IN‐
187 SERT_RANGE and also other flags; no other flags are permitted
188 with FALLOC_FL_COLLAPSE_RANGE or FALLOC_FL_INSERT_RANGE.
189 EINVAL mode is FALLOC_FL_COLLAPSE_RANGE or FALLOC_FL_ZERO_RANGE or FAL‐
191 LOC_FL_INSERT_RANGE, but the file referred to by fd is not a
192 regular file.
193 EIO An I/O error occurred while reading from or writing to a
195 filesystem.
196 ENODEV fd does not refer to a regular file or a directory. (If fd is a
198 pipe or FIFO, a different error results.)
199 ENOSPC There is not enough space left on the device containing the file
201 referred to by fd.
202 ENOSYS This kernel does not implement fallocate().
204 EOPNOTSUPP
206 The filesystem containing the file referred to by fd does not
207 support this operation; or the mode is not supported by the
208 filesystem containing the file referred to by fd.
209 EPERM The file referred to by fd is marked immutable (see chattr(1)).
211 EPERM mode specifies FALLOC_FL_PUNCH_HOLE or FALLOC_FL_COLLAPSE_RANGE
213 or FALLOC_FL_INSERT_RANGE and the file referred to by fd is
214 marked append-only (see chattr(1)).
215 EPERM The operation was prevented by a file seal; see fcntl(2).
217 ESPIPE fd refers to a pipe or FIFO.
219 ETXTBSY
221 mode specifies FALLOC_FL_COLLAPSE_RANGE or FALLOC_FL_IN‐
222 SERT_RANGE, but the file referred to by fd is currently being
223 executed.
224
226 fallocate() is available on Linux since kernel 2.6.23. Support is pro‐
227 vided by glibc since version 2.10. The FALLOC_FL_* flags are defined
228 in glibc headers only since version 2.18.
229
231 fallocate() is Linux-specific.
232
234 fallocate(1), ftruncate(2), posix_fadvise(3), posix_fallocate(3)
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237 This page is part of release 5.12 of the Linux man-pages project. A
238 description of the project, information about reporting bugs, and the
239 latest version of this page, can be found at
240 https://www.kernel.org/doc/man-pages/.
241Linux 2019-11-19 FALLOCATE(2)