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 ssEnableSystemWithTid and ssDisableSystemWithTid to enable or disable the triggered subsystem and 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:

• Specify the elements that 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 */
  

• Speify in mdlInitializeSampleTimes whether you want the S-function to be able to enable or disable the function-call subsystem. For example:

• ssSetExplicitFCSSCtrl(SimStruct *S, TRUE);
  

• Execute the subsystem in the appropriate mdlOutputs or mdlUpdate S-function routine. 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 of the blocks. For more information on their use in Stateflow, see the Stateflow documentation.

Memory Allocation

Handling Errors