[Toybox] fstype/blkid

[Rob Landley]

I'm a few days behind on email but this came in while I was looking,  
so...

On 09/11/2013 05:32:22 PM, Conroy, Bradley Quentin wrote:
> I have a semi-functional (getting ntfs device name eludes me)  
> implementation of
> blkid that I would be willing to toybox if there is any interest (Its  
> not on the
> list AFAICT, so I will be brief)

I'm happy to look at an implementation, sure.

> The implementation is as follows:
> 1.  read 65k buffer from file (reiserfs and others have superblock  
> starting
> at 64k)

Reserving space for a bootloader. :)

> 2.  perform switch cases @ 0 offset to compare to many magics
> (0 is a fairly common offset)
> **Note: the switch case really provides no benefit over if-else in  
> this situation
> since the Magic values are too far apart to be optimized into a jump  
> table.
>  I can change this for uniformity if desired.

I've noticed that if/else tends to produce smaller code, both in the  
binary and in the source. (It's the label/statement/break; pattern  
eating 3 lines. Plus the jump table is a speed optimization but not a  
size optimization, and given that outside of tight loops the dominant  
performance factor on modern architectures is cache line loading,  
smaller code _is_ faster code until proven otherwise...)

So I lean towards if/else unless there's a good reason not to. :)

> 3. Then go through the rest of the fs types and check for #defined  
> magic @ #defined offset(s)

Woot.

At some point, I need to do file. It's a posix command, but all the  
brains of the thing are in a file of match heuristics, and so far  
toybox doesn't depend on external data files.

   http://pubs.opengroup.org/onlinepubs/9699919799/utilities/file.html

(I'll probably hardwire in the "example" magic file.)

> 4. If a match is found it returns the result of print_fsinfo(char  
> *fstype, int UUID_offset, int UUID_type, int name_offset, int  
> name_len) where the UUID_offset, UUID_type, name_offset, name_len are  
> also #defined in the header for that fs type.
> 
> I know many people only want to get the fs type (for mount) instead  
> of the full blkid output.  Does toybox have some kind of facility for  
> setting function pointers based on the name it is called with or a  
> way to get the current toy-command name to use for conditional code?

There's a global array in toys.h:

   struct toy_list toy_list[];

And a global structure of command-independent context, called "this"  
(also in toys.h).

The current command's name is this.which->name (because this.which is a  
pointer to our entry in toylist[], and then name is the command name  
field, main is the function pointer, and so on).

The function toy_find("name") returns a pointer to the toy_list entry  
for that command, or NULL if it wasn't found.

Rob
 1378940385.0