FMI API Call Sequence during container_do_step()¶

Audience: Developers working on the FMU Container C runtime.

This page documents the sequence of FMI API calls that the container performs on each embedded FMU during a single call to container_do_step().

The container alternates between STEP MODE (continuous-time advancement via fmi*DoStep) and EVENT MODE (discrete state updates and clock handling). The diagram below illustrates the sequence for the sequential execution mode; the parallel modes (mono-thread and multi-thread) follow the same logical order but interleave the Set / DoStep / Get phases across all FMUs.

sequenceDiagram
   autonumber
    participant C as Container
    participant F1 as FMU #1
    participant F2 as other FMU #2 (… #N)

    Note over C, F2: [STEP MODE]
    C->>C: datalog_log()
    C->>F1: fmi*SetReal/Int/Bool/...<br/>(set inputs)
    C->>F2: fmi*SetReal/Int/Bool/...<br/>(set inputs)

    C->>F1: fmi*DoStep(time, next_step)
    F1-->>C: status + need_event_update

    C->>F2: fmi*DoStep(time, next_step)
    F2-->>C: status + need_event_update

    C->>F1: fmi*GetReal/Int/Bool/...<br/>(get outputs)
        C->>F2: fmi*GetReal/Int/Bool/...<br/>(get outputs)
    C->>C: time += next_step

    Note over C, F2: [EVENT MODE]    
    C->>F1: fmi*EnterEventMode()
    C->>F2: fmi*EnterEventMode()

    Note Over C, F2: Activate time based clocks
    C->>F1: fmi3SetClock(vr, true) <br/>for time based clocks 
    C->>F2: fmi3SetClock(vr, true) <br/>for time based clocks

    C->>F1: fmi*SetReal/Int/Bool/...<br/>(corresponding clocked inputs)
    C->>F2: fmi*SetReal/Int/Bool/...<br/>(corresponding clocked inputs)

    C->>F1: fmi*GetClock <br/>(output clocks)
    C->>F1: fmi*GetReal/Int/Bool <br/>(clocked outputs)

    C->>F2: fmi*GetClock <br/>(output clocks)
    C->>F2: fmi*GetReal/Int/Bool <br/>(clocked outputs)

    loop Do While more_event
        C->>F1: fmi*SetClock(vr, true) <br/>(activated clocks)
        C->>F1: fmi*SetReal/Int/Bool (corresponding clocked inputs)
        C->>F2: fmi*SetClock(vr, true) <br/>(activated clocks)
        C->>F2: fmi*SetReal/Int/Bool (corresponding clocked inputs)

        C->>C: datalog_log()

        C->>F1: fmi*GetClock <br/>(triggered clocks)
        C->>F1: fmi*GetReal/Int/Bool (corresponding clocked outputs)
        C->>F2: fmi*GetClock <br/>(triggered clocks)
        C->>F2: fmi*GetReal/Int/Bool (corresponding clocked outputs)

        C->>F1: fmi*UpdateDiscreteStates()
        F1-->>C: nextEventTime, more_event

        C->>F2: fmi*UpdateDiscreteStates()
        F2-->>C: nextEventTime, more_event
    end

    Note over C, F2: Prepare next scheduled event
    C->>F1: fmi3GetIntervalDecimal()
    C->>F2: fmi3GetIntervalDecimal()
    C->>C: compute next_step

    C->>F1: fmi*EnterStepMode()
    C->>F2: fmi*EnterStepMode()

Notes¶

• fmu_set_inputs / fmu_get_outputs in the C runtime expand into a series of typed FMI calls (fmi2SetReal, fmi3SetFloat64, fmi3SetBoolean, …) according to the ports declared in fmu_io.
• In the multi-thread parallel mode, fmi*DoStep is executed concurrently on each FMU via per-FMU worker threads, synchronized through mutex_container / mutex_fmu.
• The EVENT MODE phase is entered only when at least one FMU has reported need_event_update, or when clocks are declared in clocks_list.
• fmi3GetIntervalDecimal() is invoked only for FMUs that own scheduled clocks.