COPY_FILE_RANGE(2)        (2020-06-09)         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 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 state-
          ments apply to off_out:

          *  If off_in is NULL, then bytes are read from fd_in start-
             ing 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 exten-
          sions 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 off-
          set 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.

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     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 dif-
               fers between filesystem implementations and can be dif-
               ferent 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 pro-
               cess 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

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               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 bene-
          fit 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 imple-
          ment "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>

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          /* 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);

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              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/.

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