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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 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 appropriately.

       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.

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.  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

       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 copy.

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

       EPERM  fd_out refers to an immutable file.

       ETXTBSY
              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

       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 attempted.

CONFORMING TO

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

NOTES

       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

       #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 && ret > 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 5.10 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/.