Implement copy_file_range() system call for Linux

[ArniDagur]

copy_file_range() has yet to be implemented.

[asomers]

If you're interested, feel free to submit a patch. Nix works 99% on user contributions.

[ArniDagur]

How I should handle the flags argument of copy_file_range? The man page says that "the flags argument is provided to allow for future extensions and currently must be to 0".

Here's what I've got at the moment (haven't tested it yet but it compiles):

#[cfg(any(target_os = "linux", target_os = "android"))]
pub fn copy_file_range(fd_in: RawFd, off_in: Option<&mut libc::loff_t>,
          fd_out: RawFd, off_out: Option<&mut libc::loff_t>,
          len: usize) -> Result<usize> {
    let off_in = off_in.map(|offset| offset as *mut _).unwrap_or(ptr::null_mut());
    let off_out = off_out.map(|offset| offset as *mut _).unwrap_or(ptr::null_mut());

    let ret = unsafe {
        libc::syscall(libc::SYS_copy_file_range, fd_in, off_in, fd_out, off_out, len, 0 )
    };
    Errno::result(ret).map(|r| r as usize)
}

I just leave out the flags argument, but this is not good for forwards compatibility.

[asomers]

There are two choices:

1. Leave out the flags argument as you did.
2. Create a CopyFileRangeFlags enum with a single None variant.