COPY_FILE_RANGE(2)        Linux Programmer's Manual       COPY_FILE_RANGE(2)

NAME         top

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

SYNOPSIS         top

       #define _GNU_SOURCE
       #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);

DESCRIPTION         top

       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

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

RETURN VALUE         top

       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.

ERRORS         top

       EBADF  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 (see
              open(2)) referred to by the file descriptor fd_out.

       EFBIG  An attempt was made to write a file that exceeds the
              implementation-defined maximum file size or the process's file
              size limit, or to write at a position past the maximum allowed

       EINVAL Requested range extends beyond the end of the source file; or
              the flags argument is not 0.

       EIO    A low-level I/O error occurred while copying.

       EISDIR fd_in or fd_out refers to a directory.

       ENOMEM Out of memory.

       ENOSPC There is not enough space on the target filesystem to complete
              the copy.

       EXDEV  The files referred to by file_in and file_out are not on the
              same mounted filesystem.

VERSIONS         top

       The copy_file_range() system call first appeared in Linux 4.5, but
       glibc 2.27 provides a user-space emulation when it is not available.

CONFORMING TO         top

       The copy_file_range() system call is a nonstandard Linux and GNU

NOTES         top

       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 inodes that share pointers to the same copy-on-write disk
       blocks) or server-side-copy (in the case of NFS).

EXAMPLE         top

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

       /* On versions of glibc before 2.27, we must invoke copy_file_range()
          using syscall(2) */

       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         top

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

COLOPHON         top

       This page is part of release 5.01 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2019-03-06               COPY_FILE_RANGE(2)

Pages that refer to this page: sendfile(2)splice(2)syscalls(2)xfs_io(8)