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.Dd March 30, 2020
.Dt COPY_FILE_RANGE 2
.Os
.Sh NAME
.Nm copy_file_range
.Nd kernel copy of a byte range from one file to another
or within one 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 .
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.
The
.Fa flags
argument must be 0.
.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
.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 or
.Fa
flags
is not zero.
.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.
.El
.Sh SEE ALSO
.Xr lseek 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 .