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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | SEE ALSO | COLOPHON |
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fallocate(2) System Calls Manual fallocate(2)
fallocate - manipulate file space
Standard C library (libc, -lc)
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <fcntl.h>
int fallocate(int fd, int mode, off_t offset, off_t size);
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 size 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
size. The file size (as reported by stat(2)) will be changed if
offset+size is greater than the file size. Any subregion within
the range specified by offset and size 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 size 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+size 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_RANGE 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 size 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 size bytes. At the completion of the operation,
the contents of the file starting at the location offset+size will
be appended at the location offset, and the file will be size
bytes smaller.
A filesystem may place limitations on the granularity of the
operation, in order to ensure efficient implementation.
Typically, offset and size 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 requirement is
violated.
If the region specified by offset plus size 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 zeros space in the byte range starting at offset and
continuing for size 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 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+size 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 offset and continue for size 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 size
bytes. Inserting a hole inside a file increases the file size by
size 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), 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+size 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 size was less than or equal to
0.
EINVAL mode is FALLOC_FL_COLLAPSE_RANGE and the range specified by
offset plus size 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 size 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, 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,
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.
Linux.
fallocate()
Linux 2.6.23, glibc 2.10.
FALLOC_FL_*
glibc 2.18.
fallocate(1), ftruncate(2), posix_fadvise(3), posix_fallocate(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
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⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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2025-08-11. If you discover any rendering problems in this HTML
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part of the original manual page), send a mail to
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Linux man-pages 6.15 2025-05-17 fallocate(2)
Pages that refer to this page: fallocate(1), rsync(1), F_GET_SEALS(2const), FICLONE(2const), FIDEDUPERANGE(2const), io_uring_enter2(2), io_uring_enter(2), lseek(2), madvise(2), syscalls(2), io_uring_prep_fallocate(3), posix_fallocate(3), fanotify(7), inotify(7), xfs_io(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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