Re: [AD] implementing seek

[Thomas Fjellstrom <tfjellstrom@xxxxxxxxxx>]

On April 7, 2010, Peter Wang wrote:
> On 2010-04-07, Matthew Leverton <meffer@xxxxxxxxxx> wrote:
> > I agree. But what about:
> > 
> > ALLEGRO_FILE *f = fopen("images.blob", "rb");
> > int pos = al_fread32le(f);
> > ALLEGRO_BITMAP *bmp1 = al_load_bmp_f(f);
> > 
> > al_fseek(f, pos, ALLEGRO_SEEK_SET);
> > pos += al_fread32le(f);
> > ALLEGRO_BITMAP *bmp2 = al_load_bmp_f(f);
> > 
> > al_fseek(f, pos, ALLEGRO_SEEK_SET);
> > pos += al_fread32le(f);
> > ALLEGRO_BITMAP *bmp3 = al_load_bmp_f(f);
> > 
> > where 'pos' is always pointing to the next position because the file
> > format stored the length?
> 
> Intuitively, I would expect it to work.  But, actually, it should NOT be
> guaranteed.  (You might get away with it if you know exactly the loaders
> you are dealing with.)
> 
> It imposes too many constraints on the loader implementation.
> You couldn't use SEEK_END at all, or even SEEK_SET with an absolute
> offset (as you mention).
> 
> For a robust solution, the user should pass an ALLEGRO_FILE handle that
> provides a view of exactly one file.
> 
> > So one system-wide solution would be something like al_fopen_f or even
> > maybe more simply:
> > 
> > al_fseek(f, 1000);  // seeks to position 1000
> > al_set_fseek_origin(f, al_ftell(f));  // consider the current position
> > 1000 to be 0.
> > al_fseek(f, 0);  // actually seeks to position 1000.
> > 
> > ALLEGRO_BITMAP *bmp2 = al_load_bmp_f(f); // guaranteed to work now
> > 
> > Basically it's just an offset that gets added to the offset value to
> > SEEK_SET. Now when loading functions want to seek from the "beginning"
> > of the file, the proper location is preserved.
> 
> The function you alluded to earlier was cleaner:
> 
>     ALLEGRO_FILE *al_fopen_slice(ALLEGRO_FILE *f, size_t length);
> 
> perhaps combined with memory buffering as an option, for underlying
> streams that don't understand backward seeks.  Maybe also mix in
> read/write permissions.

That could work. Could also think about sub files, like A4 had.

> Peter
> 
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-- 
Thomas Fjellstrom
tfjellstrom@xxxxxxxxxx