--stepSize=0 is accepted and then hangs (or aborts) instead of being rejected

[adrpo]

--stepSize=0 is accepted and then hangs (or aborts) instead of being rejected

What happens

Passing --stepSize=0 makes OMSimulator accept the value, print maximum step size ... 0.000000, and then never finish. It writes a result file header and nothing else. On some FMUs it does not hang but aborts with pure virtual method called and dumps core.

A step size of zero cannot produce a simulation, so it seems better to reject it with a message than to accept it and stop making progress.

How to reproduce

model HelloWorld
  Real x(start = 1, fixed = true);
equation
  der(x) = -x;
  annotation(experiment(StartTime = 0, StopTime = 1));
end HelloWorld;

omc -q build.mos      # buildModelFMU(HelloWorld, version="2.0", fmuType="me", platforms={"static"})

# fine: 502 steps, 12350 byte result
OMSimulator -r=ok.mat  --startTime=0 --stopTime=1 --tolerance=1e-6 HelloWorld.fmu

# hangs until killed, 350 byte result
OMSimulator -r=bad.mat --startTime=0 --stopTime=1 --stepSize=0 --tolerance=1e-6 HelloWorld.fmu

The second command produces:

info:    *** FMU Simulation Info ***
          - model:     HelloWorld (model exchange)
          - path:      HelloWorld.fmu
          - startTime: 0.000000
          - stopTime:  1.000000
          - tolerance: 0.000001
          - stepSize:  0.000000
info:    maximum step size for 'HelloWorld.root': 0.000000
info:    Result file: bad.mat (bufferSize=1)

and then makes no further progress; killed after 60 s.

On an FMU generated from ExternData.Examples.INITest the same argument gives, instead of the hang:

pure virtual method called
terminate called without an active exception

with a core dump and a truncated result file.

Version

OMSimulator 3.0.0~154-g1c852587-linux-debug
OpenModelica 1.28.0~dev-284-g5fb7305
Ubuntu 24.04, x86_64

Why it came up

The OpenModelicaLibraryTesting scripts omit --stepSize when the experiment annotation gives no step size, precisely because passing 0 behaves this way (the guard is if stepSize != 0.0). A model whose StopTime equals its StartTime produces stepSize = 0, so a caller that passes the flag unconditionally hits this. Rejecting the value would make the guard unnecessary.

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Generated by Claude Code.