ioctl_ficlone(2)

1ioctl_ficlonerange(2)         System Calls Manual        ioctl_ficlonerange(2)
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NAME

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

10       Standard C library (libc, -lc)
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SYNOPSIS

13       #include <linux/fs.h>        /* Definition of FICLONE* constants */
14       #include <sys/ioctl.h>
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16       int ioctl(int dest_fd, FICLONERANGE, struct file_clone_range *arg);
17       int ioctl(int dest_fd, FICLONE, int src_fd);
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DESCRIPTION

20       If a filesystem supports files sharing physical storage between  multi‐
21       ple files ("reflink"), this ioctl(2) operation can be used to make some
22       of the data in the src_fd file appear in the dest_fd  file  by  sharing
23       the underlying storage, which is faster than making a separate physical
24       copy of the data.  Both files must reside within the  same  filesystem.
25       If  a  file  write should occur to a shared region, the filesystem must
26       ensure that the changes remain private to the file being written.  This
27       behavior is commonly referred to as "copy on write".
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29       This  ioctl reflinks up to src_length bytes from file descriptor src_fd
30       at offset src_offset into the file dest_fd at offset dest_offset,  pro‐
31       vided  that  both are files.  If src_length is zero, the ioctl reflinks
32       to the end of the source file.   This  information  is  conveyed  in  a
33       structure of the following form:
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35           struct file_clone_range {
36               __s64 src_fd;
37               __u64 src_offset;
38               __u64 src_length;
39               __u64 dest_offset;
40           };
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42       Clones  are  atomic with regards to concurrent writes, so no locks need
43       to be taken to obtain a consistent cloned copy.
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45       The FICLONE ioctl clones entire files.
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RETURN VALUE

48       On error, -1 is returned, and errno is set to indicate the error.
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ERRORS

51       Error codes can be one of, but are not limited to, the following:
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53       EBADF  src_fd is not open for reading; dest_fd is not open for  writing
54              or  is  open  for  append-only  writes;  or the filesystem which
55              src_fd resides on does not support reflink.
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57       EINVAL The filesystem does not support reflinking  the  ranges  of  the
58              given files.  This error can also appear if either file descrip‐
59              tor represents a device, FIFO, or socket.  Disk filesystems gen‐
60              erally  require the offset and length arguments to be aligned to
61              the fundamental block size.  XFS and Btrfs do not support  over‐
62              lapping reflink ranges in the same file.
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64       EISDIR One of the files is a directory and the filesystem does not sup‐
65              port shared regions in directories.
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67       EOPNOTSUPP
68              This can appear if the filesystem does  not  support  reflinking
69              either  file  descriptor, or if either file descriptor refers to
70              special inodes.
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72       EPERM  dest_fd is immutable.
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74       ETXTBSY
75              One of the files is a swap file.  Swap files cannot share  stor‐
76              age.
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78       EXDEV  dest_fd and src_fd are not on the same mounted filesystem.
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STANDARDS

81       Linux.
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HISTORY

84       Linux 4.5.
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86       They     were     previously     known     as    BTRFS_IOC_CLONE    and
87       BTRFS_IOC_CLONE_RANGE, and were private to Btrfs.
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NOTES

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