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FALLOCATE(2) Linux Programmer's Manual FALLOCATE(2)

NAME

fallocate - manipulate file space

SYNOPSIS

#define _GNU_SOURCE             /* See feature_test_macros(7) */
#include <fcntl.h>
int fallocate(int fd, int mode, off_t offset, off_t len);

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 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.

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 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.

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 file system blocks are zeroed, and whole file system 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 file systems support FALLOC_FL_PUNCH_HOLE; if a file system doesn't support the operation, an error is returned.

RETURN VALUE

On success, fallocate() returns zero. On error, -1 is returned and errno is set to indicate the error.

ERRORS

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 file system.
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 file system containing the file referred to by fd does not support this operation; or the mode is not supported by the file system 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.

VERSIONS

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.

CONFORMING TO

fallocate() is Linux-specific.

SEE ALSO

fallocate(1), ftruncate(2), posix_fadvise(3), posix_fallocate(3)

COLOPHON

This page is part of release 3.53 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/.

2013-06-10 Linux