ioctl_ficlonerange, ioctl_ficlone - share some the data of one file with another
file
#include <sys/ioctl.h>
#include <linux/fs.h>
int ioctl(int dest_fd, FICLONERANGE, struct file_clone_range
*arg);
int ioctl(int dest_fd, FICLONE, int src_fd);
If a filesystem supports files sharing physical storage between multiple files
("reflink"), this
ioctl(2) operation can be used to make some
of the data in the
src_fd file appear in the
dest_fd file by
sharing the underlying storage, which is faster than making a separate
physical copy of the data. Both files must reside within the same filesystem.
If a file write should occur to a shared region, the filesystem must ensure
that the changes remain private to the file being written. This behavior is
commonly referred to as "copy on write".
This ioctl reflinks up to
src_length bytes from file descriptor
src_fd at offset
src_offset into the file
dest_fd at
offset
dest_offset, provided that both are files. If
src_length
is zero, the ioctl reflinks to the end of the source file. This information is
conveyed in a structure of the following form:
struct file_clone_range {
__s64 src_fd;
__u64 src_offset;
__u64 src_length;
__u64 dest_offset;
};
Clones are atomic with regards to concurrent writes, so no locks need to be
taken to obtain a consistent cloned copy.
The
FICLONE ioctl clones entire files.
On error, -1 is returned, and
errno is set to indicate the error.
Error codes can be one of, but are not limited to, the following:
- EBADF
- src_fd is not open for reading; dest_fd is not open for
writing or is open for append-only writes; or the filesystem which
src_fd resides on does not support reflink.
- EINVAL
- The filesystem does not support reflinking the ranges of the given files.
This error can also appear if either file descriptor represents a device,
FIFO, or socket. Disk filesystems generally require the offset and length
arguments to be aligned to the fundamental block size. XFS and Btrfs do
not support overlapping reflink ranges in the same file.
- EISDIR
- One of the files is a directory and the filesystem does not support shared
regions in directories.
- EOPNOTSUPP
- This can appear if the filesystem does not support reflinking either file
descriptor, or if either file descriptor refers to special inodes.
- EPERM
- dest_fd is immutable.
- ETXTBSY
- One of the files is a swap file. Swap files cannot share storage.
- EXDEV
- dest_fd and src_fd are not on the same mounted
filesystem.
These ioctl operations first appeared in Linux 4.5. They were previously known
as
BTRFS_IOC_CLONE and
BTRFS_IOC_CLONE_RANGE, and were private
to Btrfs.
This API is Linux-specific.
Because a copy-on-write operation requires the allocation of new storage, the
fallocate(2) operation may unshare shared blocks to guarantee that
subsequent writes will not fail because of lack of disk space.
ioctl(2)