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 <unistd.h>

       ssize_t copy_file_range(int fd_in, off_t *_Nullable off_in,
                               int fd_out, off_t *_Nullable 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.

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

              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 (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

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

       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

       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 <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;
           off_t        len, ret;
           struct stat  stat;

           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)

Linux man-pages (unreleased)     (date)               copy_file_range(2)

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