man-pages/man2/fallocate.2

.\" Copyright (c) 2007 Silicon Graphics, Inc. All Rights Reserved
.\" Written by Dave Chinner <dgc@sgi.com>
.\"
.\" %%%LICENSE_START(GPLv2_ONELINE)
.\" May be distributed as per GNU General Public License version 2.
.\" %%%LICENSE_END
.\"
.\" 2011-09-19: Added FALLOC_FL_PUNCH_HOLE
.\" 2011-09-19: Substantial restructuring of the page
.\"
.TH FALLOCATE 2 2014-01-24 "Linux" "Linux Programmer's Manual"
.SH NAME
fallocate \- manipulate file space
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" "             /* See feature_test_macros(7) */"
.B #include <fcntl.h>

.BI "int fallocate(int " fd ", int " mode ", off_t " offset \
", off_t " len ");
.fi
.SH 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
.BR posix_fallocate (3).

.BR fallocate ()
allows the caller to directly manipulate the allocated disk space
for the file referred to by
.I fd
for the byte range starting at
.I offset
and continuing for
.I len
bytes.

The
.I mode
argument determines the operation to be performed on the given range.
Details of the supported operations are given in the subsections below.
.SS Allocating disk space
The default operation (i.e.,
.I mode
is zero) of
.BR fallocate ()
allocates the disk space within the range specified by
.I offset
and
.IR len .
The file size (as reported by
.BR stat (2))
will be changed if
.IR offset + len
is greater than the file size.
Any subregion within the range specified by
.I offset
and
.IR len
that did not contain data before the call will be initialized to zero.
This default behavior closely resembles the behavior of the
.BR 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
.IR offset
and
.IR len
are guaranteed not to fail because of lack of disk space.

If the
.B FALLOC_FL_KEEP_SIZE
flag is specified in
.IR mode ,
the behavior of the call is similar,
but the file size will not be changed even if
.IR 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.
.PP
Because allocation is done in block size chunks,
.BR fallocate ()
may allocate a larger range of disk space than was specified.
.SS Deallocating file space
Specifying the
.BR FALLOC_FL_PUNCH_HOLE
flag (available since Linux 2.6.38) in
.I mode
deallocates space (i.e., creates a hole)
in the byte range starting at
.I offset
and continuing for
.I 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
.BR FALLOC_FL_PUNCH_HOLE
flag must be ORed with
.BR FALLOC_FL_KEEP_SIZE
in
.IR mode ;
in other words, even when punching off the end of the file, the file size
(as reported by
.BR stat (2))
does not change.

Not all filesystems support
.BR 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
.IP * 3
XFS (since Linux 2.6.38)
.IP *
ext4 (since Linux 3.0)
.\" commit a4bb6b64e39abc0e41ca077725f2a72c868e7622
.IP *
Btrfs (since Linux 3.7)
.IP *
tmpfs (since Linux 3.5)
.\" commit 83e4fa9c16e4af7122e31be3eca5d57881d236fe
.SH RETURN VALUE
On success,
.BR fallocate ()
returns zero.
On error, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EBADF
.I fd
is not a valid file descriptor, or is not opened for writing.
.TP
.B EFBIG
.IR offset + len
exceeds the maximum file size.
.TP
.B EINTR
A signal was caught during execution.
.TP
.B EINVAL
.I offset
was less than 0, or
.I len
.\" FIXME (raise a kernel bug) Probably the len==0 case should be
.\" a no-op, rather than an error. That would be consistent with
.\" similar APIs for the len==0 case.
.\" See "Re: [PATCH] fallocate.2: add FALLOC_FL_PUNCH_HOLE flag definition"
.\" 21 Sep 2012
.\" http://thread.gmane.org/gmane.linux.file-systems/48331/focus=1193526
was less than or equal to 0.
.TP
.B EIO
An I/O error occurred while reading from or writing to a filesystem.
.TP
.B ENODEV
.I fd
does not refer to a regular file or a directory.
(If
.I fd
is a pipe or FIFO, a different error results.)
.TP
.B ENOSPC
There is not enough space left on the device containing the file
referred to by
.IR fd .
.TP
.B ENOSYS
This kernel does not implement
.BR fallocate ().
.TP
.B EOPNOTSUPP
The filesystem containing the file referred to by
.I fd
does not support this operation;
or the
.I mode
is not supported by the filesystem containing the file referred to by
.IR fd .
.TP
.B EPERM
The file referred to by
.I fd
is marked immutable (see
.BR chattr (1)).
Or:
.I mode
specifies
.BR FALLOC_FL_PUNCH_HOLE
and
the file referred to by
.I fd
is marked append-only
(see
.BR chattr (1)).
.TP
.B ESPIPE
.I fd
refers to a pipe or FIFO.
.SH VERSIONS
.BR fallocate ()
is available on Linux since kernel 2.6.23.
Support is provided by glibc since version 2.10.
The
.BR FALLOC_FL_*
flags are defined in glibc headers only since version 2.18.
.\" See http://sourceware.org/bugzilla/show_bug.cgi?id=14964
.SH CONFORMING TO
.BR fallocate ()
is Linux-specific.
.SH SEE ALSO
.BR fallocate (1),
.BR ftruncate (2),
.BR posix_fadvise (3),
.BR posix_fallocate (3)