copy_file_range(2) — Linux manual page


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 the source file descriptor fd_in
       to the target 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

       fd_in and 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

       The flags argument is provided to allow for future extensions and
       currently must be set 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.  If the file offset of fd_in is at or past the
       end of file, no bytes are copied, and copy_file_range() returns zero.

       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.

       EBADF  fd_in is not open for reading; or fd_out is not open for

       EBADF  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 at a position past the maximum
              file offset the kernel supports.

       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.

       EFBIG  An attempt was made to write beyond the process's file size
              resource limit.  This may also result in the process receiving
              a SIGXFSZ signal.

       EINVAL The flags argument is not 0.

       EINVAL fd_in and fd_out refer to the same file and the source and
              target ranges overlap.

       EINVAL Either fd_in or fd_out is not a regular file.

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

       EISDIR Either 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.

              The requested source or destination range is too large to
              represent in the specified data types.

       EPERM  fd_out refers to an immutable file.

              Either fd_in or fd_out refers to an active swap file.

       EXDEV  The files referred to by fd_in and fd_out are not on the same
              mounted filesystem (pre Linux 5.3).

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.

       A major rework of the kernel implementation occurred in 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.
       Applications should target the behaviour and requirements of 5.3

       First support for cross-filesystem copies was introduced in Linux
       5.3.  Older kernels will return -EXDEV when cross-filesystem copies
       are attempted.

CONFORMING TO         top

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

NOTES         top

       If fd_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).

EXAMPLES         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 && ret > 0);


SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.08 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                            2020-06-09               COPY_FILE_RANGE(2)

Pages that refer to this page: sendfile(2)sendfile64(2)splice(2)syscalls(2)xfs_io(8)