[fpc-devel] zstream, fpc and lazarus

Daniël Mantione daniel.mantione at freepascal.org
Sun Dec 2 11:02:38 CET 2007

Op Sun, 2 Dec 2007, schreef Sergei Gorelkin:

> Daniël Mantione wrote:
> > 
> > Okay, that explains it. The memory stream is the only solution to seek in
> > a sane way. Now we could switch to Delphi behaviour, but to be honest, I
> > don't want to enforce the memory stream to everyone who wants to use a
> > compressed stream; it should be usable on plain files.
> > 
> > I'm going to interpret the Delphi docs liberally; "the beginning" is the
> > beginning of the zstream, not the beginning of the file. If you want to
> > reset the source stream, you will have to reset the source stream :) Ok,
> > it is a Delphi incompatibility, but really a corner case, and doing so
> > has many advantages.
> > 
> I believe that fpreadpng.pp must be fixed, too. At least it should reset
> memory stream to zero position itself and don't assume that zstream will do it
> instead. At maximum everything can be done without memory stream at all - PNG
> format is designed to be sequential.
> As for zstream behaviour, I fully agree: it should remember source stream
> position at creation time and seek to it at reset. This way, compatibility
> existing with Delphi code won't be broken, because its authors supply streams
> in which compression data starts from the beginning.

We actually don't need remember the start position, I simply to a 
backwards seek, which does the trick.

> Furthermore (and I issued a corresponding patch for the previous version), at
> destruction decompression stream should adjust source position to the end of
> actually decompressed data. This way makes it possible to process streams that
> have compressed data followed by some other data.

I agree, but I would like to be carefull about one thing: zstreams read 
from pipes or sockets. If someone closes a socket zstream early, a seek to 
end would result in an exception. Is there a way we can detect a stream to 
be seekable?


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