Re: OT: why no file copy() libc/syscall ??

[Jakub Jelinek <jakub@redhat.com>]

On Tue, Nov 11, 2003 at 02:38:59PM +0100, Rogier Wolff wrote:
> On Mon, Nov 10, 2003 at 08:05:11PM -0500, Albert Cahalan wrote:
> > So open the file, change context, and then:
> > 
> > long copy_fd_to_file(int fd, const char *name, ...)
> > 
> > (if you can no longer read from the OPEN fd,
> > either we override that or we just don't care
> > about such mostly-fictional cases)
> 
> 
> Actually, I think we should have a: 
> 
> 	long copy_fd_to_fd (int src, int dst, int len)
> 
> type of systemcall. 

We have one, sendfile(2).

> It should do something like:
> 
> 	while ((nbytes = read (src, buf, BUFSIZE)) >= 0) {
> 		if (write (dst, buf, nbytes) < 0) 
> 			return totbytes; 
> 		totbytes += nbytes;
> 	}
> 
> but it allows kernel-space to optimize this whenever possible. Kernel
> then becomes responsible for detecting and handling the optimizable
> cases. 
> 
> The kernel then becomes something
> 
> 	if (islocalfile (src) && issocket (dst)) 
> 		/* Call the old sendfile */ 
> 		return sendfile (....);
> 
> 	if (isCIFS (src), isCIFS(dst))
> 		/* Tell remote host to copy the file. */
> 		return CIFS_copy_file (....); 
> 
> 	...

Can you explain why this cannot be in sys_sendfile?
It doesn't make much sense to provide any default in the kernel,
that's something the userland can handle equally well.
But e.g. the CIFS copy can be done as sendfile hook.

	Jakub