[LC++]Copying files and deleting directories

[Paul Gearon]

On 10/09/2004, at 7:33 AM, Krishna Monian wrote:

> Hi,
> So what is the best way to implement my own version of
> copy?
>
> Currently, I open the file, read a certain number of
> bytes into a buffer and then write it into the file
> until I am done. This is a very lame way I would say.

No, that's just fine.  Just make sure that the size of your buffer 
isn't tiny, else you'll call read and write too often.  IIRC cp uses 
this method for small files.  Whatever you do, don't use fread and 
fwrite.

For larger files cp uses mmap.  To do this, you just map the source 
file, then you create, expand, and map the destination file, and 
finally you do a memcpy from one map to the other.  It's actually quite 
easy to do.  This method has more overhead to set up, but skips a 
memory copy of the data (which is why it's faster for large files).

> I might have several files to copy at a time (1000s).
> From your discussion it seems that the system function
> will be relatively slow for this purpose.

If you have lots of files, then set up time is important.  If the files 
are small, then using read/write would probably offer the best option 
here.

> Is there some better way to go about doing this? It
> doesn't have to faster than cp. Anything faster than
> my current method is acceptable.

Just use a read/write loop on a decent sized buffer.  If you discover 
that it's too slow for what you need, then you can look at optimising 
with mmap.  Don't fall into the pre-optimising trap!

Regards,
Paul Gearon

Software Engineer
Tucana Technologies
http://www.tucanatech.com

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