Re: [Qemu-devel] [QEMU : VVFAT] vvfat.c - help required for understanding/modification

[Johannes Schindelin]

Hi Pintu,

On Sat, 22 Oct 2011, Pintu Kumar wrote:

> I already did some work for scanning only top level directory in vvfat. 
> Using the following logic in read_directory()
> 
> if(parent_index >= 0 & (!dot & !dotdot))
> {
>        free(buffer);
>        break;
> }

Sorry, this is way too deep in the code (and I'd have to guess because you 
did not grace me with exact locations), I cannot afford to dive into it 
that far. But what I can say is that it is not all that easy; you'll need 
to refactor the code.

You _will_ need to reserve sectors for future directory parsing. You 
_will_ need to add code that intercepts reads to those sectors. You _will_ 
have to read the directories in question when those sectors are accessed 
and thereby commit the sectors to a final state.

BTW both Kevin and I said that before.

> Hope this is correct logic for skipping sub-directories content by just 
> scanning only dot and dotdot entries inside it.

It would be, if all you want are empty directories.

> But as per the recent analysis it was observed that there is some 
> problem in skipping sub-directory scanning itself. Because when I issue 
> "df" command on VVFAT partition I get read_cluster errors during these 
> sector location.

When you issue "df", you need to commit to which sectors are reserved. By 
that, you take almost all the advantage of lazy directory parsing away.

> Mr. Johannes, can you clarify on this first.
> 
> Then we will look into the dynamic mapping part later.

I am very sorry, but I do get the impression that the intent is to lure me 
into doing all the work, including the investigation what it would take to 
turn the code into a version that reads just the top-level first and only 
later the subdirectories.

This will not happen. I have enough projects that I want to tackle myself, 
and I am extremely unwilling to do the work for somebody else when that 
somebody else is paid to do the work. For the things I _want_ to do I 
already have too little time, given that my day job is quite demanding and 
asks software design and architecture of me already.

So: you _will_ have to acquaint yourself with the FAT and then the VFAT 
specification. You _will_ have to make a proper plan how the client may be 
fooled into believing that there is a fixed filesystem when there is 
actually just directories of files. And you _will_ have to write that code 
yourself.

I hope this clarified it?

Ciao,
Johannes