[AD] implementing seek

[Matthew Leverton <meffer@xxxxxxxxxx>]

How does seeking work with Allegro file streams? For instance, say you
have a binary blob file that contains three consecutive images. Is the
following supposed to work (guaranteed)? (Let's assume that byte size
of each bitmap can be determined exactly while reading.)

ALLEGRO_FILE *f = fopen("images.blob", "rb");
ALLEGRO_BITMAP *bmp1 = al_load_bmp_f(f);
ALLEGRO_BITMAP *bmp2 = al_load_bmp_f(f);
ALLEGRO_BITMAP *bmp3 = al_load_bmp_f(f);

I ask because the GDI+ load routines do a lot of seeking forward and
backward. When loading mysha2.bmp, it appears to load it from end of
file to beginning of file with a seek from set before every read.

I haven't tested it, but I assume that with my crude GDI+ loader, it
would actually load the first image three times. (I know how I could
work around it at the gdi loading level... that's not an issue.)

So, basically, is there any type of function like:

al_fopen_f(ALLEGRO_FILE *f, size_t length)  ???

And secondly, should a call to al_load_whatever_f() always set the
current position to the end of the data chunk (if it can be
determined)? e.g., I know that in the DUMB readme, Ben specifically
mentions that he cannot (or does not) determine the actual length of
the mod file, so if you want to consecutively load them from packfiles
(allegro 4), you have to manually seek to the next spot after
finishing reading.

And finally, do I (as a writer of an image / audio loader) need to be
concerned about any forward-only seeking files?

Maybe I'm just over analyzing things, but it seems like people
wouldn't expect an image loader to fail due to it seeking backward on
a handle that only can seek forward.

--
Matthew Leverton