IOCTL-FICLONERANGE(2)      Linux Programmer's Manual     IOCTL-FICLONERANGE(2)

       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  multi-
       ple 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,  pro-
       vided  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 descrip-
              tor represents a device, FIFO, or socket.  Disk filesystems gen-
              erally  require the offset and length arguments to be aligned to
              the fundamental block size.  XFS and Btrfs do not support  over-
              lapping reflink ranges in the same file.

       EISDIR One of the files is a directory and the filesystem does not sup-
              port shared regions in directories.

              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.

              One of the files is a swap file.  Swap files cannot share  stor-

       EXDEV  dest_fd and src_fd are not on the same mounted filesystem.

       These  ioctl  operations first appeared in Linux 4.5.  They were previ-
       ously known as BTRFS_IOC_CLONE and BTRFS_IOC_CLONE_RANGE, and were pri-
       vate to Btrfs.

       This API is Linux-specific.

       Because  a copy-on-write operation requires the allocation of new stor-
       age, the fallocate(2) operation may unshare shared blocks to  guarantee
       that subsequent writes will not fail because of lack of disk space.


       This  page  is  part of release 4.15 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at

Linux                             2017-09-15             IOCTL-FICLONERANGE(2)
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