NAME

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

SYNOPSIS

       #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

       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.

ERRORS

       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 referred to by 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 offset.

       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

       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

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

NOTES

       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).

EXAMPLE

       #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);
       }

       int
       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]);
               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);
           }

           len = 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, len, 0);
               if (ret == -1) {
                   perror("copy_file_range");
                   exit(EXIT_FAILURE);
               }

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

           close(fd_in);
           close(fd_out);
           exit(EXIT_SUCCESS);
       }

SEE ALSO

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

COLOPHON

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       information  about  reporting  bugs,  and  the  latest  version  of  this   page,   can   be   found   at
       https://www.kernel.org/doc/man-pages/.