how to make sync demos?

Any time I'm recording a demo *.lmp it goes out of sync badly after playing it. Even If I recorded the demo under ZDoom and replayed it in the same program. Sometimes I take items on the demo I didn't realy take, because I stopped in front of 'em... some mosters are doing things different than before, and so on, and so on.

Do you know what should I do not to make the demo go out of sync?

Many versions of Zdoom have failed to support demo playback, though the recent betas do. Make sure you have an up-to-date version (from here).

I don't know whether future versions of Zdoom will be compatible with the current version.

Note that most demos are recorded with Doom2.exe or with Doom2.exe compatibility, and some people use Legacy. Zdoom demos are pretty rare.

If you insist on using an old version of ZDoom, there is a reason it went out of sync: it would play it back on a low skill level. I'm not sure if it was skill 1 or 2 but when I made Grind2 I successfully made a demo that plays back with older versions of ZDoom simply by recording on the easy skill level.

Although if you get versions too very old I'm not sure how far back that "bug" goes and where the just plain no-playback starts.

zdoom1.22 has pretty reasonable demo_support. note that every release since then has been a BETA version.
I have never seen a 1.22 demo fail to playback with 1.22.

I have, usually in situations involving lost souls.

Little Faith said:

I have, usually in situations involving lost souls.

Huh? Lost souls don't count as kills.

Lost souls, the revenent fireball, etc... each rely on random number generation of the OS. I have no idea how this was addressed in PrBoom.
Some of the difference comes from the monster behaviour was changed for the Lostsouls with the patches... so the source that Zdoom was built on is not the same functionality as the patched real game.

DEMOn: your post confuses me a little. Are you referring to the issue of some Zdoom versions not supporting playback of their own demos, or why demo compatibility with doom2.exe is better in some ports than in others?

I'd always assumed that the reason for prboom and eternity's compatibility was just that the programmers had put a lot of effort into emulating the original game's algorithms. But you seem to be hinting that the fact that the source upon which they were based (i.e. via Boom(->MBF(->SMMU)), rather than the Linux source) gives them a better chance to start with.

This is question for Lee, Proff, and Heit, but I am assuming that the random behaviors are generated in real time(or the random algorithm choose from a listing instead of a single number -- like current clock millisecond + gametic = 23 or 90 or 134 instead of just 23)... thus demos could desync?
Doom was only coded for dos. one would assume that other platforms would create unexpected calculations.
in short, I don't know.