fallocate(2) System Calls Manual fallocate(2)
NAME
fallocate - manipulate file space
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <fcntl.h>
int fallocate(int fd, int mode, off_t offset, off_t len);
DESCRIPTION
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 off-
set 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 op-
timally 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 off-
set+len is greater than the file size. Preallocating zeroed blocks be-
yond the end of the file in this manner is useful for optimizing append
workloads.
If the FALLOC_FL_UNSHARE_RANGE flag is specified in mode, shared file
data extents will be made private to the file to guarantee that a subse-
quent 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 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 zeros.
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)
• gfs2(5) (since Linux 4.16)
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() fails with the error EINVAL if this re-
quirement 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 FAL-
LOC_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 zeros 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, sub-
sequent reads from this range will return zeros.
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)
• Btrfs (since Linux 4.16)
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 off-
set 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() fails 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), ftrun-
cate(2) should be used.
No other flags may be specified in mode in conjunction with FAL-
LOC_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).
RETURN VALUE
On success, fallocate() returns zero. On error, -1 is returned and er-
rno is set to indicate the error.
ERRORS
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 ex-
ceeds 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 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
size.
EINVAL mode contains one of FALLOC_FL_COLLAPSE_RANGE or FALLOC_FL_IN-
SERT_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, FALLOC_FL_ZERO_RANGE, or FAL-
LOC_FL_INSERT_RANGE, but the file referred to by fd is not a reg-
ular file.
EIO An I/O error occurred while reading from or writing to a filesys-
tem.
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, 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_IN-
SERT_RANGE, but the file referred to by fd is currently being ex-
ecuted.
STANDARDS
Linux.
HISTORY
fallocate()
Linux 2.6.23, glibc 2.10.
FALLOC_FL_*
glibc 2.18.
SEE ALSO
fallocate(1), ftruncate(2), posix_fadvise(3), posix_fallocate(3)
Linux man-pages 6.9.1 2024-05-02 fallocate(2)
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