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.
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.
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)
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
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.
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, 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+
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)
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() 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+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)