lib/libsys/copy_file_range.2

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.Dd August 16, 2025
.Dt COPY_FILE_RANGE 2
.Os
.Sh NAME
.Nm copy_file_range
.Nd kernel copy of a byte range from one regular file to another
or within one regular file
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In unistd.h
.Ft ssize_t
.Fo copy_file_range
.Fa "int infd"
.Fa "off_t *inoffp"
.Fa "int outfd"
.Fa "off_t *outoffp"
.Fa "size_t len"
.Fa "unsigned int flags"
.Fc
.Sh DESCRIPTION
The
.Fn copy_file_range
system call
copies up to
.Fa len
bytes from
.Fa infd
to
.Fa outfd
in the kernel.
It may do this using a file system specific technique if
.Fa infd
and
.Fa outfd
are on the same file system.
If
.Fa infd
and
.Fa outfd
refer to the same file, the byte ranges defined by
the input file offset, output file offset and
.Fa len
cannot overlap.
The
.Fa infd
argument must be opened for reading and the
.Fa outfd
argument must be opened for writing, but not
.Dv O_APPEND .
.Pp
If
.Fa inoffp
or
.Fa outoffp
is
.Dv NULL ,
the file offset for
.Fa infd
or
.Fa outfd
respectively will be used and updated by
the number of bytes copied.
If
.Fa inoffp
or
.Fa outoffp
is not
.Dv NULL ,
the byte offset pointed to by
.Fa inoffp
or
.Fa outoffp
respectively will be used/updated and the file offset for
.Fa infd
or
.Fa outfd
respectively will not be affected.
.Pp
The only
.Fa flags
argument currently defined is
.Dv COPY_FILE_RANGE_CLONE .
When this flag is set,
.Fn copy_file_range
will return
.Er EOPNOTSUPP
if the copy cannot be done via
block cloning.
When
.Fa flags
is 0, a file system may do the copy via block cloning
or by data copying.
Block cloning is only possible when the offsets (plus
.Fa len
if not to EOF on the input file) are block
aligned.
The correct block alignment can normally be acquired via the
.Dv _PC_CLONE_BLKSIZE
query for
.Xr pathconf 2 .
.Pp
This system call attempts to maintain holes in the output file for
the byte range being copied.
However, this does not always work well.
It is recommended that sparse files be copied in a loop using
.Xr lseek 2
with
.Dv SEEK_HOLE ,
.Dv SEEK_DATA
arguments and this system call for the
data ranges found.
.Pp
For best performance, call
.Fn copy_file_range
with the largest
.Fa len
value possible.
It is interruptible on most file systems,
so there is no penalty for using very large len values, even SSIZE_MAX.
.Pp
.Sh RETURN VALUES
If it succeeds, the call returns the number of bytes copied, which can be fewer
than
.Fa len .
Returning fewer bytes than
.Fa len
does not necessarily indicate that EOF was reached.
However, a return of zero for a non-zero
.Fa len
argument indicates that the offset for
.Fa infd
is at or beyond EOF.
.Fn copy_file_range
should be used in a loop until copying of the desired byte range has been
completed.
If an error has occurred, a \-1 is returned and the error code is placed in
the global variable
.Va errno .
.Sh ERRORS
The
.Fn copy_file_range
system call
will fail if:
.Bl -tag -width Er
.It Bq Er EBADF
If
.Fa infd
is not open for reading or
.Fa outfd
is not open for writing, or opened for writing with
.Dv O_APPEND ,
or if
.Fa infd
and
.Fa outfd
refer to the same file.
.It Bq Er EFBIG
If the copy exceeds the process's file size limit or the maximum file size
for the file system
.Fa outfd
resides on.
.It Bq Er EINTR
A signal interrupted the system call
before it could be completed.
This may happen for files on some NFS mounts.
When this happens, the values pointed to by
.Fa inoffp
and
.Fa outoffp
are reset to the initial values for the system call.
.It Bq Er EINVAL
.Fa infd
and
.Fa outfd
refer to the same file and the byte ranges overlap.
.It Bq Er EINVAL
The
.Fa flags
argument is not zero.
.It Bq Er EINVAL
Either
.Fa infd
or
.Fa outfd
refers to a file object that is not a regular file.
.It Bq Er EIO
An I/O error occurred while reading/writing the files.
.It Bq Er EINTEGRITY
Corrupted data was detected while reading from a file system.
.It Bq Er EISDIR
If either
.Fa infd
or
.Fa outfd
refers to a directory.
.It Bq Er ENOSPC
File system that stores
.Fa outfd
is full.
.It Bq Er EOPNOTSUPP
Cannot do the copy via block cloning and the
.Dv COPY_FILE_RANGE_CLONE
.Fa flags
argument is specified.
.El
.Sh SEE ALSO
.Xr lseek 2 ,
.Xr pathconf 2
.Sh STANDARDS
The
.Fn copy_file_range
system call is expected to be compatible with the Linux system call of
the same name.
.Sh HISTORY
The
.Fn copy_file_range
function appeared in
.Fx 13.0 .