.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH copy_file_range 2 2025-09-21 "Linux man-pages (unreleased)"
.SH NAME
copy_file_range \- Copy a range of data from one file to another
.SH LIBRARY
Standard C library
.RI ( libc ,\~ \-lc )
.SH SYNOPSIS
.nf
.B #define _GNU_SOURCE
.B #define _FILE_OFFSET_BITS 64
.B #include <unistd.h>
.P
.BI "ssize_t copy_file_range(int " fd_in ", off_t *_Nullable " off_in ,
.BI "                        int " fd_out ", off_t *_Nullable " off_out ,
.BI "                        size_t " size ", unsigned int " flags );
.fi
.SH DESCRIPTION
The
.BR copy_file_range ()
system call performs an in-kernel copy between two file descriptors
without the additional cost of transferring data from the kernel to user space
and then back into the kernel.
It copies up to
.I size
bytes of data from the source file descriptor
.I fd_in
to the target file descriptor
.IR fd_out ,
overwriting any data that exists within the requested range of the target file.
.P
The following semantics apply for
.IR off_in ,
and similar statements apply to
.IR off_out :
.IP \[bu] 3
If
.I off_in
is NULL, then bytes are read from
.I fd_in
starting from the file offset, and the file offset is
adjusted by the number of bytes copied.
.IP \[bu]
If
.I off_in
is not NULL, then
.I off_in
must point to a buffer that specifies the starting
offset where bytes from
.I fd_in
will be read.
The file offset of
.I fd_in
is not changed, but
.I off_in
is adjusted appropriately.
.P
.I fd_in
and
.I fd_out
can refer to the same file.
If they refer to the same file, then the source and target ranges are not
allowed to overlap.
.P
The
.I flags
argument is provided to allow for future extensions
and currently must be set to 0.
.SH RETURN VALUE
Upon successful completion,
.BR copy_file_range ()
will return the number of bytes copied between files.
This could be less than the size originally requested.
If the file offset of
.I fd_in
is at or past the end of file, no bytes are copied, and
.BR copy_file_range ()
returns zero.
.P
On error,
.BR copy_file_range ()
returns \-1 and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EBADF
One or more file descriptors are not valid.
.TP
.B EBADF
.I fd_in
is not open for reading;
or
.I fd_out
is not open for writing.
.TP
.B EBADF
The
.B O_APPEND
flag is set for the open file description (see
.BR open (2))
referred to by the file descriptor
.IR fd_out .
.TP
.B EFBIG
An attempt was made to write at a position past the maximum file offset the
kernel supports.
.TP
.B EFBIG
An attempt was made to write a range that exceeds the allowed maximum file size.
The maximum file size differs between filesystem implementations and can be
different from the maximum allowed file offset.
.TP
.B EFBIG
An attempt was made to write beyond the process's file size resource limit.
This may also result in the process receiving a
.B SIGXFSZ
signal.
.TP
.B EINVAL
The
.I flags
argument is not 0.
.TP
.B EINVAL
.I fd_in
and
.I fd_out
refer to the same file and the source and target ranges overlap.
.TP
.B EINVAL
Either
.I fd_in
or
.I fd_out
is not a regular file.
.TP
.B EIO
A low-level I/O error occurred while copying.
.TP
.B EISDIR
Either
.I fd_in
or
.I fd_out
refers to a directory.
.TP
.B ENOMEM
Out of memory.
.TP
.B ENOSPC
There is not enough space on the target filesystem to complete the copy.
.TP
.BR EOPNOTSUPP " (since Linux 5.19)"
.\" commit 868f9f2f8e004bfe0d3935b1976f625b2924893b
The filesystem does not support this operation.
.TP
.B EOVERFLOW
The requested source or destination range is too large to represent in the
specified data types.
.TP
.B EPERM
.I fd_out
refers to an immutable file.
.TP
.B ETXTBSY
Either
.I fd_in
or
.I fd_out
refers to an active swap file.
.TP
.BR EXDEV " (before Linux 5.3)"
.\" commit 5dae222a5ff0c269730393018a5539cc970a4726
The files referred to by
.IR fd_in " and " fd_out
are not on the same filesystem.
.TP
.BR EXDEV " (since Linux 5.19)"
.\" commit 868f9f2f8e004bfe0d3935b1976f625b2924893b
The files referred to by
.IR fd_in " and " fd_out
are not on the same filesystem,
and the source and target filesystems are not of the same type,
or do not support cross-filesystem copy.
.SH VERSIONS
A major rework of the kernel implementation occurred in Linux 5.3.
Areas of the API that weren't clearly defined were clarified and the API bounds
are much more strictly checked than on earlier kernels.
.P
Since Linux 5.19,
cross-filesystem copies can be achieved
when both filesystems are of the same type,
and that filesystem implements support for it.
See BUGS for behavior prior to Linux 5.19.
.P
Applications should target the behaviour and requirements of Linux 5.19,
that was also backported to earlier stable kernels.
.SH STANDARDS
Linux, GNU.
.SH HISTORY
Linux 4.5,
but glibc 2.27 provides a user-space
emulation when it is not available.
.\" https://sourceware.org/git/?p=glibc.git;a=commit;f=posix/unistd.h;h=bad7a0c81f501fbbcc79af9eaa4b8254441c4a1f
.SH NOTES
If
.I fd_in
is a sparse file, then
.BR copy_file_range ()
may expand any holes existing in the requested range.
Users may benefit from calling
.BR copy_file_range ()
in a loop, and using the
.BR lseek (2)
.B SEEK_DATA
and
.B SEEK_HOLE
operations to find the locations of data segments.
.P
.BR copy_file_range ()
gives filesystems an opportunity to implement "copy acceleration" techniques,
such as the use of reflinks (i.e., two or more inodes that share
pointers to the same copy-on-write disk blocks)
or server-side-copy (in the case of NFS).
.P
.B _FILE_OFFSET_BITS
should be defined to be 64 in code that uses non-null
.I off_in
or
.I off_out
or that takes the address of
.BR copy_file_range ,
if the code is intended to be portable
to traditional 32-bit x86 and ARM platforms where
.BR off_t 's
width defaults to 32 bits.
.SH BUGS
In Linux 5.3 to Linux 5.18,
cross-filesystem copies were implemented by the kernel,
if the operation was not supported by individual filesystems.
However, on some virtual filesystems,
the call failed to copy, while still reporting success.
.SH EXAMPLES
.\" SRC BEGIN (copy_file_range.c)
.EX
#define _GNU_SOURCE
#define _FILE_OFFSET_BITS 64
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
\&
int
main(int argc, char *argv[])
{
    int          fd_in, fd_out;
    off_t        size, ret;
    struct stat  stat;
\&
    if (argc != 3) {
        fprintf(stderr, "Usage: %s <source> <destination>\[rs]n", argv[0]);
        exit(EXIT_FAILURE);
    }
\&
    fd_in = open(argv[1], O_RDONLY);
    if (fd_in == \-1) {
        perror("open (argv[1])");
        exit(EXIT_FAILURE);
    }
\&
    if (fstat(fd_in, &stat) == \-1) {
        perror("fstat");
        exit(EXIT_FAILURE);
    }
\&
    size = stat.st_size;
\&
    fd_out = open(argv[2], O_CREAT | O_WRONLY | O_TRUNC, 0644);
    if (fd_out == \-1) {
        perror("open (argv[2])");
        exit(EXIT_FAILURE);
    }
\&
    do {
        ret = copy_file_range(fd_in, NULL, fd_out, NULL, size, 0);
        if (ret == \-1) {
            perror("copy_file_range");
            exit(EXIT_FAILURE);
        }
\&
        size \-= ret;
    } while (size > 0 && ret > 0);
\&
    close(fd_in);
    close(fd_out);
    exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR lseek (2),
.BR sendfile (2),
.BR splice (2)