#define _GNU_SOURCE /* See feature_test_macros(7) */
int fallocate(int fd, int mode, off_t offset, off_t len);
This is a nonportable, Linux-specific system call. For the portable,
POSIX.1-specified method of ensuring that space is allocated for a
file, see posix_fallocate(3).
fallocate() allows the caller to directly manipulate the allocated disk
space for the file referred to by fd for the byte range starting at
offset and continuing for len bytes.
The mode argument determines the operation to be performed on the given
range. Details of the supported operations are given in the subsec-
Allocating disk space
The default operation (i.e., mode is zero) of fallocate() allocates the
disk space within the range specified by offset and len. The file size
(as reported by stat(2)) will be changed if offset+len is greater than
the file size. Any subregion within the range specified by offset and
len that did not contain data before the call will be initialized to
zero. This default behavior closely resembles the behavior of the
posix_fallocate(3) library function, and is intended as a method of
optimally implementing that function.
After a successful call, subsequent writes into the range specified by
offset and len are guaranteed not to fail because of lack of disk
If the FALLOC_FL_KEEP_SIZE flag is specified in mode, the behavior of
the call is similar, but the file size will not be changed even if off-
set+len is greater than the file size. Preallocating zeroed blocks
beyond the end of the file in this manner is useful for optimizing
Because allocation is done in block size chunks, fallocate() may allo-
cate a larger range of disk space than was specified.
Deallocating file space
Specifying the FALLOC_FL_PUNCH_HOLE flag (available since Linux 2.6.38)
in mode deallocates space (i.e., creates a hole) in the byte range
starting at offset and continuing for len bytes. Within the specified
range, partial filesystem blocks are zeroed, and whole filesystem
blocks are removed from the file. After a successful call, subsequent
reads from this range will return zeroes.
The FALLOC_FL_PUNCH_HOLE flag must be ORed with FALLOC_FL_KEEP_SIZE in
mode; in other words, even when punching off the end of the file, the
file size (as reported by stat(2)) does not change.
Not all filesystems support FALLOC_FL_PUNCH_HOLE; if a filesystem
Specifying the FALLOC_FL_COLLAPSE_RANGE flag (available since Linux
3.15) in mode removes a byte range from a file, without leaving a hole.
The byte range to be collapsed starts at offset and continues for len
bytes. At the completion of the operation, the contents of the file
starting at the location offset+len will be appended at the location
offset, and the file will be len bytes smaller.
A filesystem may place limitations on the granularity of the operation,
in order to ensure efficient implementation. Typically, offset and len
must be a multiple of the filesystem logical block size, which varies
according to the filesystem type and configuration. If a filesystem
has such a requirement, fallocate() will fail with the error EINVAL if
this requirement is violated.
If the region specified by offset plus len reaches or passes the end of
file, an error is returned; instead, use ftruncate(2) to truncate a
No other flags may be specified in mode in conjunction with FAL-
As at Linux 3.15, FALLOC_FL_COLLAPSE_RANGE is supported by ext4 (only
for extent-based files) and XFS.
Zeroing file space
Specifying the FALLOC_FL_ZERO_RANGE flag (available since Linux 3.15)
in mode zeroes space in the byte range starting at offset and continu-
ing for len bytes. Within the specified range, blocks are preallocated
for the regions that span the holes in the file. After a successful
call, subsequent reads from this range will return zeroes.
Zeroing is done within the filesystem preferably by converting the
range into unwritten extents. This approach means that the specified
range will not be physically zeroed out on the device (except for par-
tial blocks at the either end of the range), and I/O is (otherwise)
required only to update metadata.
If the FALLOC_FL_KEEP_SIZE flag is additionally specified in mode, the
behavior of the call is similar, but the file size will not be changed
even if offset+len is greater than the file size. This behavior is the
same as when preallocating space with FALLOC_FL_KEEP_SIZE specified.
Not all filesystems support FALLOC_FL_ZERO_RANGE; if a filesystem
doesn't support the operation, an error is returned. The operation is
supported on at least the following filesystems:
* XFS (since Linux 3.15)
* ext4, for extent-based files (since Linux 3.15)
* SMB3 (since Linux 3.17)
Increasing file space
Specifying the FALLOC_FL_INSERT_RANGE flag (available since Linux 4.1)
cate(2) should be used.
No other flags may be specified in mode in conjunction with FAL-
FALLOC_FL_INSERT_RANGE requires filesystem support. Filesystems that
support this operation include XFS (since Linux 4.1) and ext4 (since
On success, fallocate() returns zero. On error, -1 is returned and
errno is set to indicate the error.
EBADF fd is not a valid file descriptor, or is not opened for writing.
EFBIG offset+len exceeds the maximum file size.
EFBIG mode is FALLOC_FL_INSERT_RANGE, and the current file size+len
exceeds the maximum file size.
EINTR A signal was caught during execution.
EINVAL offset was less than 0, or len was less than or equal to 0.
EINVAL mode is FALLOC_FL_COLLAPSE_RANGE and the range specified by off-
set plus len reaches or passes the end of the file.
EINVAL mode is FALLOC_FL_INSERT_RANGE and the range specified by offset
reaches or passes the end of the file.
EINVAL mode is FALLOC_FL_COLLAPSE_RANGE or FALLOC_FL_INSERT_RANGE, but
either offset or len is not a multiple of the filesystem block
EINVAL mode contains one of FALLOC_FL_COLLAPSE_RANGE or FAL-
LOC_FL_INSERT_RANGE and also other flags; no other flags are
permitted with FALLOC_FL_COLLAPSE_RANGE or FAL-
EINVAL mode is FALLOC_FL_COLLAPSE_RANGE or FALLOC_FL_ZERO_RANGE or FAL-
LOC_FL_INSERT_RANGE, but the file referred to by fd is not a
EIO An I/O error occurred while reading from or writing to a
ENODEV fd does not refer to a regular file or a directory. (If fd is a
pipe or FIFO, a different error results.)
ENOSPC There is not enough space left on the device containing the file
referred to by fd.
ENOSYS This kernel does not implement fallocate().
EPERM The operation was prevented by a file seal; see fcntl(2).
ESPIPE fd refers to a pipe or FIFO.
mode specifies FALLOC_FL_COLLAPSE_RANGE or FAL-
LOC_FL_INSERT_RANGE, but the file referred to by fd is currently
fallocate() is available on Linux since kernel 2.6.23. Support is pro-
vided by glibc since version 2.10. The FALLOC_FL_* flags are defined
in glibc headers only since version 2.18.
fallocate() is Linux-specific.
fallocate(1), ftruncate(2), posix_fadvise(3), posix_fallocate(3)
This page is part of release 4.04 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
Linux 2015-07-23 FALLOCATE(2)
Man Pages Copyright Respective Owners. Site Copyright (C) 1994 - 2019
All Rights Reserved.