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 descrip-
       tor 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>
", 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)

Linux								    2019-03-06							COPY_FILE_RANGE(2)