Writing S-Functions

Function-Call Subsystems

You can create a triggered subsystem whose execution is determined by logic internal to an S-function instead of by the value of a signal. A subsystem so configured is called a function-call subsystem. To implement a function-call subsystem:

• In the Trigger block, select function-call as the Trigger type parameter.
• In the S-function, use the ssCallSystemWithTid macro to call the triggered subsystem.
• In the model, connect the S-Function block output directly to the trigger port.

Note    Function-call connections can only be performed on the first output port.

Function-call subsystems are not executed directly by Simulink; rather, the S-function determines when to execute the subsystem. When the subsystem completes execution, control returns to the S-function. This figure illustrates the interaction between a function-call subsystem and an S-function.

In this figure, ssCallSystemWithTid executes the function-call subsystem that is connected to the first output port element. ssCallSystemWithTid returns 0 if an error occurs while executing the function-call subsystem or if the output is unconnected. After the function-call subsystem executes, control is returned to your S-function.

Function-call subsystems can only be connected to S-functions that have been properly configured to accept them.

To configure an S-function to call a function-call subsystem:

1. Specify which elements are to execute the function-call system in mdlInitializeSampleTimes. For example,

   ssSetCallSystemOutput(S,0);  /* call on 1st element */
   ssSetCallSystemOutput(S,2);  /* call on 3rd element */
   

2. Execute the subsystem in the appropriate mdlOutputs or mdlUpdates S-function routines. For example,

   static void mdlOutputs(...)
   {
       if (((int)*uPtrs[0]) % 2 == 1) {
         if (!ssCallSystemWithTid(S,0,tid)) {
           /* Error occurred, which will be reported by Simulink */
           return;
         }
       } else {
         if (!ssCallSystemWithTid(S,2,tid)) {
           /* Error occurred, which will be reported by Simulink */
           return;
         }
       }
       ...
   }
   

See simulink/src/sfun_fcncall.c for an example.

Function-call subsystems are a powerful modeling construct. You can configure Stateflow® blocks to execute function-call subsystems, thereby extending the capabilities and integration of state logic (Stateflow) with dataflow (Simulink). For more information on their use in Stateflow, see the Stateflow documentation.

Memory Allocation

Handling Errors