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Structs vs. single parameters in C API #667

t-sommer opened this issue Nov 8, 2019 · 2 comments

Structs vs. single parameters in C API #667

t-sommer opened this issue Nov 8, 2019 · 2 comments


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@t-sommer t-sommer commented Nov 8, 2019

Some functions of the C API uses single parameters e.g.

fmi3Status fmi3SetupExperiment(fmi3Instance instance,
                                           fmi3Boolean toleranceDefined,
                                           fmi3Float64 tolerance,
                                           fmi3Float64 startTime,
                                           fmi3Boolean stopTimeDefined,
                                           fmi3Float64 stopTime);

others use structs e.g.

fmi3Status fmi3NewDiscreteStates(fmi3Instance instance,
                                             fmi3EventInfo* eventInfo);

and some use a combination of both e.g.

fmi3Instance fmi3Instantiate(fmi3String        instanceName,
                                 fmi3InterfaceType fmuType,
                                 fmi3String        fmuInstantiationToken,
                                 fmi3String        fmuResourceLocation,
                                 const fmi3CallbackFunctions* functions,
                                 fmi3Boolean       visible,
                                 fmi3Boolean       loggingOn,
                                 const fmi3CoSimulationConfiguration* fmuCoSimulationConfiguration);

Both approaches have their advantages:


  • can be extended in later versions
  • reduce the number of parameters

single arguments

  • make clear what "goes in" and what "comes out" of the function
  • are consistent (no mix of structs and single parameters)
  • are easier to implement (esp. in languages that have no C interface, e.g. Java or Python)

IMHO we should consider whether we really need structures as parameters and if so, come up with a rule when to use (and how to extend) them.

@t-sommer t-sommer added the discussion label Nov 8, 2019
@t-sommer t-sommer added this to the v3.0 milestone Nov 8, 2019

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@chrbertsch chrbertsch commented Jan 10, 2020

Regular Design meeting:

Pierre: I do not have a strong opinion. If I would be redoing it, I would try to avoid structs. Structs have the benefit, that one could add elements in the future.
Torsten S: for EventInfo, we have In- and Out-arguments which is not nice

--> Discuss at F2F meeting


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@andreas-junghanns andreas-junghanns commented Feb 21, 2020

The initial reason for structs was that we could simply extend without changing the function signature. This turned out to be a hollow reason - we never really used this to prevent API changes.

I would suggest to be consistent and remove all structures.

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