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COPY_FILE_RANGE(2)         Linux Programmer's Manual        COPY_FILE_RANGE(2)
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 set 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
              either fd_in or fd_out is not  a  regular  file;  or  the  flags
              argument is not 0.
       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.
       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
       This  page  is  part of release 4.15 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
       https://www.kernel.org/doc/man-pages/.
Linux                             2018-02-02                COPY_FILE_RANGE(2)