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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 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.
RETURN VALUE
On success, fallocate() returns zero. On error, -1 is returned and errno is set to indicate the error.
ERRORS
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 filesystem.
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().
EOPNOTSUPP
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.
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
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information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.