IOCTL_FICLONERANGE(2)     (2017-09-15)      IOCTL_FICLONERANGE(2)

     NAME
          ioctl_ficlonerange, ioctl_ficlone - share some the data of
          one file with another file

     SYNOPSIS
          #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);

     DESCRIPTION
          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 stor-
          age, which is faster than making a separate physical copy of
          the data.  Both files must reside within the same filesys-
          tem.  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.

     RETURN VALUE
          On error, -1 is returned, and errno is set to indicate the
          error.

     ERRORS
          Error codes can be one of, but are not limited to, the fol-
          lowing:

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          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 filesys-
               tem.

     VERSIONS
          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.

     CONFORMING TO
          This API is Linux-specific.

     NOTES
          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.

     SEE ALSO
          ioctl(2)

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     COLOPHON
          This page is part of release 5.10 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 https://www.kernel.org/doc/man-pages/.

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