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

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

       #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);

       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

       *  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

       *  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 overlap.

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

       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

       EBADF  One or more file descriptors are not valid.

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

       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

              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).

       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 kernels.

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

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

       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).

       #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);


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

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Linux                              2020-06-09                 COPY_FILE_RANGE(2)