copy_file_range(2) — Linux manual page

copy_file_range(2)         System Calls Manual         copy_file_range(2)

NAME         top

       copy_file_range - Copy a range of data from one file to another

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #define _GNU_SOURCE
       #define _FILE_OFFSET_BITS 64
       #include 

       ssize_t copy_file_range(int fd_in, off_t *_Nullable off_in,
                               int fd_out, off_t *_Nullable off_out,
                               size_t size, unsigned int flags);

DESCRIPTION         top

       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 size 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         top

       Upon successful completion, copy_file_range() will return the
       number of bytes copied between files.  This could be less than the
       size 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         top

       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.

       EOPNOTSUPP (since Linux 5.19)
              The filesystem does not support this operation.

       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 (before Linux 5.3)
              The files referred to by fd_in and fd_out are not on the
              same filesystem.

       EXDEV (since Linux 5.19)
              The files referred to by fd_in and fd_out are not on the
              same filesystem, and the source and target filesystems are
              not of the same type, or do not support cross-filesystem
              copy.

VERSIONS         top

       A major rework of the kernel implementation occurred in Linux 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.

       Since Linux 5.19, cross-filesystem copies can be achieved when
       both filesystems are of the same type, and that filesystem
       implements support for it.  See BUGS for behavior prior to Linux
       5.19.

       Applications should target the behaviour and requirements of Linux
       5.19, that was also backported to earlier stable kernels.

STANDARDS         top

       Linux, GNU.

HISTORY         top

       Linux 4.5, but glibc 2.27 provides a user-space emulation when it
       is not available.

NOTES         top

       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).

       _FILE_OFFSET_BITS should be defined to be 64 in code that uses
       non-null off_in or off_out or that takes the address of
       copy_file_range, if the code is intended to be portable to
       traditional 32-bit x86 and ARM platforms where off_t's width
       defaults to 32 bits.

BUGS         top

       In Linux 5.3 to Linux 5.18, cross-filesystem copies were
       implemented by the kernel, if the operation was not supported by
       individual filesystems.  However, on some virtual filesystems, the
       call failed to copy, while still reporting success.

EXAMPLES         top

       #define _GNU_SOURCE
       #define _FILE_OFFSET_BITS 64
       #include 
       #include 
       #include 
       #include 
       #include 
       #include 

       int
       main(int argc, char *argv[])
       {
           int          fd_in, fd_out;
           off_t        size, ret;
           struct stat  stat;

           if (argc != 3) {
               fprintf(stderr, "Usage: %s  \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);
           }

           size = 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, size, 0);
               if (ret == -1) {
                   perror("copy_file_range");
                   exit(EXIT_FAILURE);
               }

               size -= ret;
           } while (size > 0 && ret > 0);

           close(fd_in);
           close(fd_out);
           exit(EXIT_SUCCESS);
       }

SEE ALSO         top

       lseek(2), sendfile(2), splice(2)

COLOPHON         top

       This page is part of the man-pages (Linux kernel and C library
       user-space interface documentation) project.  Information about
       the project can be found at 
       ⟨https://www.kernel.org/doc/man-pages/⟩.  If you have a bug report
       for this manual page, see
       ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
       This page was obtained from the tarball man-pages-6.10.tar.gz
       fetched from
       ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
       2025-02-02.  If you discover any rendering problems in this HTML
       version of the page, or you believe there is a better or more up-
       to-date source for the page, or you have corrections or
       improvements to the information in this COLOPHON (which is not
       part of the original manual page), send a mail to
       [email protected]

Linux man-pages 6.10            2024-11-17             copy_file_range(2)

Pages that refer to this page: sendfile(2),  splice(2),  syscalls(2),  off_t(3type),  proc_pid_io(5),  feature_test_macros(7),  dracut(8),  xfs_io(8)

HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH.