FICLONE(2const) - Linux manual page (original) (raw)

FICLONE(2const) FICLONE(2const)

NAME top

   FICLONE, FICLONERANGE - share some the data of one file with
   another file

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

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

   **int ioctl(int** _destfd_**, FICLONERANGE, struct file_clone_range ***_arg_**);**
   **int ioctl(int** _destfd_**, FICLONE, int** _srcfd_**);**

DESCRIPTION top

   If a filesystem supports files sharing physical storage between
   multiple files ("reflink"), this [ioctl(2)](../man2/ioctl.2.html) operation can be used to
   make some of the data in the _srcfd_ file appear in the _destfd_
   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 _srclength_ bytes from file descriptor
   _srcfd_ at offset _srcoffset_ into the file _destfd_ at offset
   _destoffset_, provided that both are files.  If _srclength_ 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 top

   On error, -1 is returned, and _[errno](../man3/errno.3.html)_ is set to indicate the error.

ERRORS top

   Error codes can be one of, but are not limited to, the following:

   **EBADF** _srcfd_ is not open for reading; _destfd_ is not open for
          writing or is open for append-only writes; or the
          filesystem which _srcfd_ 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** _destfd_ is immutable.

   **ETXTBSY**
          One of the files is a swap file.  Swap files cannot share
          storage.

   **EXDEV** _destfd_ and _srcfd_ are not on the same mounted filesystem.

STANDARDS top

   Linux.

HISTORY top

   Linux 4.5.

   They were previously known as **BTRFS_IOC_CLONE** and
   **BTRFS_IOC_CLONE_RANGE**, and were private to Btrfs.

CAVEATS top

   Because a copy-on-write operation requires the allocation of new
   storage, the [fallocate(2)](../man2/fallocate.2.html) operation may unshare shared blocks to
   guarantee that subsequent writes will not fail because of lack of
   disk space.

SEE ALSO top

   [ioctl(2)](../man2/ioctl.2.html)

COLOPHON top

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Linux man-pages 6.10 2024-07-23 FICLONE(2const)

Pages that refer to this page:ioctl_fs(2)