man-pages/man2/copy_file_range.2

.\"This manpage is Copyright (C) 2015 Anna Schumaker <Anna.Schumaker@Netapp.com>
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of
.\" this manual under the conditions for verbatim copying, provided that
.\" the entire resulting derived work is distributed under the terms of
.\" a permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume
.\" no responsibility for errors or omissions, or for damages resulting
.\" from the use of the information contained herein.  The author(s) may
.\" not have taken the same level of care in the production of this
.\" manual, which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\" %%%LICENSE_END
.\"
.TH COPY_FILE_RANGE 2 2019-03-06 "Linux" "Linux Programmer's Manual"
.SH NAME
copy_file_range \- Copy a range of data from one file to another
.SH SYNOPSIS
.nf
.B #define _GNU_SOURCE
.B #include <unistd.h>
.PP
.BI "ssize_t copy_file_range(int " fd_in ", loff_t *" off_in ,
.BI "                        int " fd_out ", loff_t *" off_out ,
.BI "                        size_t " len ", unsigned int " flags );
.fi
.SH DESCRIPTION
The
.BR 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
.I len
bytes of data from file descriptor
.I fd_in
to file descriptor
.IR fd_out ,
overwriting any data that exists within the requested range of the target file.
.PP
The following semantics apply for
.IR off_in ,
and similar statements apply to
.IR off_out :
.IP * 3
If
.I off_in
is NULL, then bytes are read from
.I fd_in
starting from the file offset, and the file offset is
adjusted by the number of bytes copied.
.IP *
If
.I off_in
is not NULL, then
.I off_in
must point to a buffer that specifies the starting
offset where bytes from
.I fd_in
will be read.
The file offset of
.I fd_in
is not changed, but
.I off_in
is adjusted appropriately.
.PP
.PP
The
.I flags
argument is provided to allow for future extensions
and currently must be to 0.
.SH RETURN VALUE
Upon successful completion,
.BR copy_file_range ()
will return the number of bytes copied between files.
This could be less than the length originally requested.
.PP
On error,
.BR copy_file_range ()
returns \-1 and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EBADF
One or more file descriptors are not valid; or
.I fd_in
is not open for reading; or
.I fd_out
is not open for writing; or
the
.B O_APPEND
flag is set for the open file description (see
.BR open (2))
referred to by the file descriptor
.IR fd_out .
.TP
.B EFBIG
An attempt was made to write a file that exceeds the implementation-defined
maximum file size or the process's file size limit,
or to write at a position past the maximum allowed offset.
.TP
.B EINVAL
Requested range extends beyond the end of the source file; or the
.I flags
argument is not 0.
.TP
.B EIO
A low-level I/O error occurred while copying.
.TP
.B EISDIR
.I fd_in
or
.I fd_out
refers to a directory.
.TP
.B ENOMEM
Out of memory.
.TP
.B ENOSPC
There is not enough space on the target filesystem to complete the copy.
.TP
.B EXDEV
The files referred to by
.IR file_in " and " file_out
are not on the same mounted filesystem.
.SH VERSIONS
The
.BR 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.
.\" https://sourceware.org/git/?p=glibc.git;a=commit;f=posix/unistd.h;h=bad7a0c81f501fbbcc79af9eaa4b8254441c4a1f
.SH CONFORMING TO
The
.BR copy_file_range ()
system call is a nonstandard Linux and GNU extension.
.SH NOTES
If
.I file_in
is a sparse file, then
.BR copy_file_range ()
may expand any holes existing in the requested range.
Users may benefit from calling
.BR copy_file_range ()
in a loop, and using the
.BR lseek (2)
.BR SEEK_DATA
and
.BR SEEK_HOLE
operations to find the locations of data segments.
.PP
.BR 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).
.SH EXAMPLE
.EX
#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>\en", 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);

    close(fd_in);
    close(fd_out);
    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.BR lseek (2),
.BR sendfile (2),
.BR splice (2)