copy_file_range.2

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.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 the source file descriptor
.I fd_in
to the target 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
.I fd_in
and
.I 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.
.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.
If the file offset of
.I fd_in
is at or past the end of file, no bytes are copied, and
.BR copy_file_range ()
returns zero.
.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.
.TP
.B EBADF
.I fd_in
is not open for reading; or
.I fd_out
is not open for writing.
.TP
.B EBADF
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 at a position past the maximum file offset the
kernel supports.
.TP
.B 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.
.TP
.B EFBIG
An attempt was made to write beyond the process's file size resource limit.
This may also result in the process receiving a
.I SIGXFSZ
signal.
.TP
.B EINVAL
The
.I flags
argument is not 0.
.TP
.B EINVAL
.I fd_in
and
.I fd_out
refer to the same file and the source and target ranges overlap.
.TP
.B EINVAL
Either
.I fd_in
or
.I fd_out
is not a regular file.
.TP
.B EISDIR
Either
.I fd_in
or
.I fd_out
refers to a directory.
.TP
.B EOVERFLOW
The requested source or destination range is too large to represent in the
specified data types.
.TP
.B EIO
A low-level I/O error occurred while copying.
.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 (pre Linux 5.3).
.TP
.B TXTBSY
Either
.I fd_in
or
.I fd_out
refers to an active swap file.
.TP
.B EPERM
.I fd_out
refers to an immutable file.
.TP
.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
.PP
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.
.PP
First support for cross-filesystem copies was introduced in Linux 5.3.
Older kernels will return -EXDEV when cross-filesystem copies are attempted.
.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 && ret > 0);

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