FALLOCATE(2) Linux Programmer's Manual FALLOCATE(2)
fallocate - manipulate file space
#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 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 func-
tion, 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 opti-
mizing 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 deallo-
cates 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.
On success, fallocate() returns zero. On error, -1 is returned and errno is set to indi-
cate the error.
EBADF fd is not a valid file descriptor, or is not opened for writing.
EFBIG offset+len exceeds the maximum file size.
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 file system.
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 file system containing the file referred to by fd does not support this opera-
tion; or the mode is not supported by the file system 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
ESPIPE 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.
fallocate() is Linux-specific.
fallocate(1), ftruncate(2), posix_fadvise(3), posix_fallocate(3)
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Linux 2013-06-10 FALLOCATE(2)