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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 (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 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 inodes 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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