copy_file_range

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 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 spec-
          ifies 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 appro-
          priately.

       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 cur-
       rently 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 origi-
       nally 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  writ-
              ing.

       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  re-
              source  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  tar-
              get 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 repre-
              sent 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 file_in and file_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 at-
       tempted.

CONFORMING TO
       The copy_file_range() system call is a nonstandard Linux and GNU exten-
       sion.

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 && 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 5.05 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                             2019-10-10                COPY_FILE_RANGE(2)