man2/ioctl_ficlonerange.2

   1 .\" Copyright (c) 2016, Oracle.  All rights reserved.
   2 .\"
   3 .\" %%%LICENSE_START(GPLv2+_DOC_FULL)
   4 .\" This is free documentation; you can redistribute it and/or
   5 .\" modify it under the terms of the GNU General Public License as
   6 .\" published by the Free Software Foundation; either version 2 of
   7 .\" the License, or (at your option) any later version.
   8 .\"
   9 .\" The GNU General Public License's references to "object code"
  10 .\" and "executables" are to be interpreted as the output of any
  11 .\" document formatting or typesetting system, including
  12 .\" intermediate and printed output.
  13 .\"
  14 .\" This manual is distributed in the hope that it will be useful,
  15 .\" but WITHOUT ANY WARRANTY; without even the implied warranty of
  16 .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  17 .\" GNU General Public License for more details.
  18 .\"
  19 .\" You should have received a copy of the GNU General Public
  20 .\" License along with this manual; if not, see
  21 .\" <http://www.gnu.org/licenses/>.
  22 .\" %%%LICENSE_END
  23 .TH IOCTL_FICLONERANGE 2 2017-09-15 "Linux" "Linux Programmer's Manual"
  24 .SH NAME
  25 ioctl_ficlonerange, ioctl_ficlone \- share some the data of one file with another file
  26 .SH SYNOPSIS
  27 .nf
  28 .B #include <sys/ioctl.h>
  29 .B #include <linux/fs.h>
  30 .PP
  31 .BI "int ioctl(int " dest_fd ", FICLONERANGE, struct file_clone_range *" arg );
  32 .BI "int ioctl(int " dest_fd ", FICLONE, int " src_fd );
  33 .fi
  34 .SH DESCRIPTION
  35 If a filesystem supports files sharing physical storage between multiple
  36 files ("reflink"), this
  37 .BR ioctl (2)
  38 operation can be used to make some of the data in the
  39 .I src_fd
  40 file appear in the
  41 .I dest_fd
  42 file by sharing the underlying storage, which is faster than making a separate
  43 physical copy of the data.
  44 Both files must reside within the same filesystem.
  45 If a file write should occur to a shared region,
  46 the filesystem must ensure that the changes remain private to the file being
  47 written.
  48 This behavior is commonly referred to as "copy on write".
  49 .PP
  50 This ioctl reflinks up to
  51 .IR src_length
  52 bytes from file descriptor
  53 .IR src_fd
  54 at offset
  55 .IR src_offset
  56 into the file
  57 .IR dest_fd
  58 at offset
  59 .IR dest_offset ,
  60 provided that both are files.
  61 If
  62 .IR src_length
  63 is zero, the ioctl reflinks to the end of the source file.
  64 This information is conveyed in a structure of
  65 the following form:
  66 .PP
  67 .in +4n
  68 .EX
  69 struct file_clone_range {
  70     __s64 src_fd;
  71     __u64 src_offset;
  72     __u64 src_length;
  73     __u64 dest_offset;
  74 };
  75 .EE
  76 .in
  77 .PP
  78 Clones are atomic with regards to concurrent writes, so no locks need to be
  79 taken to obtain a consistent cloned copy.
  80 .PP
  81 The
  82 .B FICLONE
  83 ioctl clones entire files.
  84 .SH RETURN VALUE
  85 On error, \-1 is returned, and
  86 .I errno
  87 is set to indicate the error.
  88 .SH ERRORS
  89 Error codes can be one of, but are not limited to, the following:
  90 .TP
  91 .B EBADF
  92 .IR src_fd
  93 is not open for reading;
  94 .IR dest_fd
  95 is not open for writing or is open for append-only writes;
  96 or the filesystem which
  97 .IR src_fd
  98 resides on does not support reflink.
  99 .TP
 100 .B EINVAL
 101 The filesystem does not support reflinking the ranges of the given files.
 102 This error can also appear if either file descriptor represents
 103 a device, FIFO, or socket.
 104 Disk filesystems generally require the offset and length arguments
 105 to be aligned to the fundamental block size.
 106 XFS and Btrfs do not support
 107 overlapping reflink ranges in the same file.
 108 .TP
 109 .B EISDIR
 110 One of the files is a directory and the filesystem does not support shared
 111 regions in directories.
 112 .TP
 113 .B EOPNOTSUPP
 114 This can appear if the filesystem does not support reflinking either file
 115 descriptor, or if either file descriptor refers to special inodes.
 116 .TP
 117 .B EPERM
 118 .IR dest_fd
 119 is immutable.
 120 .TP
 121 .B ETXTBSY
 122 One of the files is a swap file.
 123 Swap files cannot share storage.
 124 .TP
 125 .B EXDEV
 126 .IR dest_fd " and " src_fd
 127 are not on the same mounted filesystem.
 128 .SH VERSIONS
 129 These ioctl operations first appeared in Linux 4.5.
 130 They were previously known as
 131 .B BTRFS_IOC_CLONE
 132 and
 133 .BR BTRFS_IOC_CLONE_RANGE ,
 134 and were private to Btrfs.
 135 .SH CONFORMING TO
 136 This API is Linux-specific.
 137 .SH NOTES
 138 Because a copy-on-write operation requires the allocation of new storage, the
 139 .BR fallocate (2)
 140 operation may unshare shared blocks to guarantee that subsequent writes will
 141 not fail because of lack of disk space.
 142 .SH SEE ALSO
 143 .BR ioctl (2)