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


       #include <linux/fs.h>        /* Definition of FICLONE* constants */
       #include <sys/ioctl.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

       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.

              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 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.




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