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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 and initializes to
zero 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. 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.
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 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.
On success, fallocate()
returns zero. On error, −1 is returned and errno is set to indicate the error.
fd is not a
valid file descriptor, or is not opened for
writing.
offset+len exceeds the maximum
file size.
A signal was caught during execution.
offset was
less than 0, or len was less than or
equal to 0.
An I/O error occurred while reading from or writing to a filesystem.
fd does not
refer to a regular file or a directory. (If fd is a pipe or FIFO, a
different error results.)
There is not enough space left on the device
containing the file referred to by fd.
This kernel does not implement fallocate().
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.
The file referred to by fd is marked immutable
(see chattr(1)). Or:
mode specifies
FALLOC_FL_PUNCH_HOLE and
the file referred to by fd is marked append-only
(see chattr(1)).
fd refers to
a pipe or FIFO.
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.
This page is part of release 3.54 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man−pages/.
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Copyright (c) 2007 Silicon Graphics, Inc. All Rights Reserved Written by Dave Chinner <dgcsgi.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 |