copy_file_range man page

copy_file_range — Copy a range of data from one file to another


#include <sys/syscall.h>
#include <unistd.h>

ssize_t copy_file_range(int fd_in, loff_t *off_in,
 int fd_out, loff_t *off_out,
 size_t len, unsigned int flags);


The 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 len bytes of data from file descriptor fd_in to file descriptor fd_out, overwriting any data that exists within the requested range of the target file.

The following semantics apply for off_in, and similar statements apply to off_out:

If off_in is NULL, then bytes are read from fd_in starting from the file offset, and the file offset is adjusted by the number of bytes copied.
If off_in is not NULL, then off_in must point to a buffer that specifies the starting offset where bytes from fd_in will be read. The file offset of fd_in is not changed, but off_in is adjusted appropriately.

The flags argument is provided to allow for future extensions and currently must be to 0.

Return Value

Upon successful completion, copy_file_range() will return the number of bytes copied between files. This could be less than the length originally requested.

On error, copy_file_range() returns -1 and errno is set to indicate the error.


One or more file descriptors are not valid; or fd_in is not open for reading; or fd_out is not open for writing; or the O_APPEND flag is set for the open file description referred to by fd_out.
Requested range extends beyond the end of the source file; or the flags argument is not 0.
A low-level I/O error occurred while copying.
Out of memory.
There is not enough space on the target filesystem to complete the copy.
The files referred to by file_in and file_out are not on the same mounted filesystem.


The copy_file_range() system call first appeared in Linux 4.5.

Conforming to

The copy_file_range() system call is a nonstandard Linux extension.


If file_in is a sparse file, then copy_file_range() may expand any holes existing in the requested range. Users may benefit from calling copy_file_range() in a loop, and using the lseek(2) SEEK_DATA and SEEK_HOLE operations to find the locations of data segments.

copy_file_range() gives filesystems an opportunity to implement "copy acceleration" techniques, such as the use of reflinks (i.e., two or more i-nodes that share pointers to the same copy-on-write disk blocks) or server-side-copy (in the case of NFS).


#define _GNU_SOURCE
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <unistd.h>

static loff_t
copy_file_range(int fd_in, loff_t *off_in, int fd_out,
                loff_t *off_out, size_t len, unsigned int flags)
    return syscall(__NR_copy_file_range, fd_in, off_in, fd_out,
                   off_out, len, flags);

main(int argc, char **argv)
    int fd_in, fd_out;
    struct stat stat;
    loff_t len, ret;

    if (argc != 3) {
        fprintf(stderr, "Usage: %s <source> <destination>\n", argv[0]);

    fd_in = open(argv[1], O_RDONLY);
    if (fd_in == -1) {
        perror("open (argv[1])");

    if (fstat(fd_in, &stat) == -1) {

    len = stat.st_size;

    fd_out = open(argv[2], O_CREAT | O_WRONLY | O_TRUNC, 0644);
    if (fd_out == -1) {
        perror("open (argv[2])");

    do {
        ret = copy_file_range(fd_in, NULL, fd_out, NULL, len, 0);
        if (ret == -1) {

        len -= ret;
    } while (len > 0);


See Also

lseek(2), sendfile(2), splice(2)

Referenced By

sendfile(2), splice(2), syscalls(2), xfs_io(8).

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