copy_file_range(2) — Linux manual page


COPY_FILE_RANGE(2)      Linux Programmer's Manual     COPY_FILE_RANGE(2)

NAME         top

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

SYNOPSIS         top

       #define _GNU_SOURCE
       #include <unistd.h>

       ssize_t copy_file_range(int fd_in, off64_t *off_in,
                               int fd_out, off64_t *off_out,
                               size_t len, 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 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         top

       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         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

       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.

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

       EPERM  fd_out refers to an immutable file.

              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         top

       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

       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         top

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

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

       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         top

       #define _GNU_SOURCE
       #include <fcntl.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/stat.h>
       #include <unistd.h>

       main(int argc, char *argv[])
           int fd_in, fd_out;
           struct stat stat;
           off64_t len, ret;

           if (argc != 3) {
               fprintf(stderr, "Usage: %s <source> <destination>\n", argv[0]);

           fd_in = open(argv[1], O_RDONLY);
           if (fd_in == -1) {
               perror("open (argv[1])");

           if (fstat(fd_in, &stat) == -1) {

           len = stat.st_size;

           fd_out = open(argv[2], O_CREAT | O_WRONLY | O_TRUNC, 0644);
           if (fd_out == -1) {
               perror("open (argv[2])");

           do {
               ret = copy_file_range(fd_in, NULL, fd_out, NULL, len, 0);
               if (ret == -1) {

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


SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.13 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

Linux                          2021-08-27             COPY_FILE_RANGE(2)

Pages that refer to this page: sendfile(2)splice(2)syscalls(2)system_data_types(7)xfs_io(8)