#define _GNU_SOURCE /* See feature_test_macros(7) */
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 subsec-
Allocating disk space
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 func-
tion, and is intended as a method of optimally implementing that func-
After a successful call, subsequent writes into the range specified by
offset and len are guaranteed not to fail because of lack of disk
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
beyond the end of the file in this manner is useful for optimizing
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 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.
EINTR A signal was caught during execution.
EINVAL offset was less than 0, or len was less than or equal to 0.
EIO An I/O error occurred while reading from or writing to a
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().
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)).
Or: mode specifies FALLOC_FL_PUNCH_HOLE and the file referred to
by fd is marked append-only (see chattr(1)).
ESPIPE fd refers to a pipe or FIFO.
fallocate() is available on Linux since kernel 2.6.23. Support is pro-
vided 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)
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/.
Linux 2013-06-10 FALLOCATE(2)
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