copy_file_range(2)

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

LIBRARY
Standard C library (libc, -lc)

SYNOPSIS
#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
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
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

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
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
Linux, GNU.

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

NOTES
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
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
    #define _GNU_SOURCE
    #define _FILE_OFFSET_BITS 64
    #include <fcntl.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <sys/stat.h>
    #include <unistd.h>

    int
    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]);
            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)