man-pages/man2/ioctl_ficlonerange.2

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.TH IOCTL-FICLONERANGE 2 2016-02-10 "Linux" "Linux Programmer's Manual"
.SH NAME
ioctl_ficlonerange, ioctl_ficlone \- share some the data of one file with another file
.SH SYNOPSIS
.br
.B #include <sys/ioctl.h>
.br
.B #include <linux/fs.h>
.sp
.BI "int ioctl(int " dest_fd ", FICLONERANGE, struct file_clone_range * " arg );
.br
.BI "int ioctl(int " dest_fd ", FICLONE, int " src_fd );
.SH DESCRIPTION
If a filesystem supports files sharing physical storage between multiple
files ("reflink"), this
.BR ioctl (2)
operation can be used to make some of the data in the
.I src_fd
file appear in the
.I dest_fd
file by sharing the underlying storage, which is faster than making a separate
physical copy of the data.
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
.IR src_length
bytes from file descriptor
.IR src_fd
at offset
.IR src_offset
into the file
.IR dest_fd
at offset
.IR dest_offset ",
provided that both are files.
This information is conveyed in a structure of
the following form:
.in +4n
.nf

struct file_clone_range {
    __s64 src_fd;
    __u64 src_offset;
    __u64 src_length;
    __u64 dest_offset;
};

.fi
.in
Clones are atomic with regards to concurrent writes, so no locks need to be
taken to obtain a consistent cloned copy.

The
.B FICLONE
ioctl clones entire files.
.SH RETURN VALUE
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.PP
.SH ERRORS
Error codes can be one of, but are not limited to, the following:
.TP
.B EXDEV
.IR dest_fd " and " src_fd
are not on the same mounted filesystem.
.TP
.B EISDIR
One of the files is a directory and the filesystem does not support shared
regions in directories.
.TP
.B 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.
.TP
.B EBADF
.IR src_fd
is not open for reading;
.IR dest_fd
is not open for writing or is open for append-only writes;
or the filesystem which
.IR src_fd
resides on does not support reflink.
.TP
.B EPERM
.IR dest_fd
is immutable.
.TP
.B ETXTBSY
One of the files is a swap file.
Swap files cannot share storage.
.TP
.B EOPNOTSUPP
This can appear if the filesystem does not support reflinking either file
descriptor.
.SH VERSIONS
These ioctl operations first appeared in Linux 4.5.
They were previously known as
.B BTRFS_IOC_CLONE
and
.BR BTRFS_IOC_CLONE_RANGE ,
and were private to Btrfs.
.SH CONFORMING TO
This API is Linux-specific.
.SH NOTES
Because a copy-on-write operation requires the allocation of new storage, the
.BR fallocate (2)
operation may unshare shared blocks to guarantee that subsequent writes will
not fail because of lack of disk space.
.SH SEE ALSO
.BR ioctl (2)