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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | SEE ALSO | COLOPHON |
FALLOCATE(2) Linux Programmer's Manual FALLOCATE(2)
fallocate - manipulate file space
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <fcntl.h>
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
subsections below.
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
space.
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
offset+len is greater than the file size. Preallocating zeroed
blocks beyond the end of the file in this manner is useful for
optimizing append workloads.
If the FALLOC_FL_UNSHARE flag is specified in mode, shared file data
extents will be made private to the file to guarantee that a
subsequent write will not fail due to lack of space. Typically, this
will be done by performing a copy-on-write operation on all shared
data in the file. This flag may not be supported by all filesystems.
Because allocation is done in block size chunks, fallocate() may
allocate 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
doesn't support the operation, an error is returned. The operation
is supported on at least the following filesystems:
* XFS (since Linux 2.6.38)
* ext4 (since Linux 3.0)
* Btrfs (since Linux 3.7)
* tmpfs(5) (since Linux 3.5)
Collapsing file space
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 file.
No other flags may be specified in mode in conjunction with
FALLOC_FL_COLLAPSE_RANGE.
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
continuing 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
partial 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) in mode increases the file space by inserting a hole within the
file size without overwriting any existing data. The hole will start
at offset and continue for len bytes. When inserting the hole inside
file, the contents of the file starting at offset will be shifted
upward (i.e., to a higher file offset) by len bytes. Inserting a
hole inside a file increases the file size by len bytes.
This mode has the same limitations as FALLOC_FL_COLLAPSE_RANGE
regarding the granularity of the operation. If the granularity
requirements are not met, fallocate() will fail with the error
EINVAL. If the offset is equal to or greater than the end of file,
an error is returned. For such operations (i.e., inserting a hole at
the end of file), ftruncate(2) should be used.
No other flags may be specified in mode in conjunction with
FALLOC_FL_INSERT_RANGE.
FALLOC_FL_INSERT_RANGE requires filesystem support. Filesystems that
support this operation include XFS (since Linux 4.1) and ext4 (since
Linux 4.2).
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; see signal(7).
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
offset 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 size.
EINVAL mode contains one of FALLOC_FL_COLLAPSE_RANGE or
FALLOC_FL_INSERT_RANGE and also other flags; no other flags
are permitted with FALLOC_FL_COLLAPSE_RANGE or
FALLOC_FL_INSERT_RANGE.
EINVAL mode is FALLOC_FL_COLLAPSE_RANGE or FALLOC_FL_ZERO_RANGE or
FALLOC_FL_INSERT_RANGE, but the file referred to by fd is not
a regular file.
EIO An I/O error occurred while reading from or writing to a
filesystem.
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().
EOPNOTSUPP
The filesystem containing the file referred to by fd does not
support this operation; or the mode is not supported by the
filesystem containing the file referred to by fd.
EPERM The file referred to by fd is marked immutable (see
chattr(1)).
EPERM mode specifies FALLOC_FL_PUNCH_HOLE or
FALLOC_FL_COLLAPSE_RANGE or FALLOC_FL_INSERT_RANGE and the
file referred to by fd is marked append-only (see chattr(1)).
EPERM The operation was prevented by a file seal; see fcntl(2).
ESPIPE fd refers to a pipe or FIFO.
ETXTBSY
mode specifies FALLOC_FL_COLLAPSE_RANGE or
FALLOC_FL_INSERT_RANGE, but the file referred to by fd is
currently being executed.
fallocate() is available on Linux since kernel 2.6.23. Support is
provided 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 4.12 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
https://www.kernel.org/doc/man-pages/.
Linux 2016-12-12 FALLOCATE(2)
Pages that refer to this page: fallocate(1), rsync(1), fcntl(2), ioctl_ficlonerange(2), ioctl_fideduperange(2), lseek(2), madvise(2), syscalls(2), posix_fallocate(3), fanotify(7), inotify(7), xfs_io(8)