Writing S-Functions

Handling Errors

When working with S-functions, it is important to handle unexpected events correctly such as invalid parameter values.

If your S-function has parameters whose contents you need to validate, use the following technique to report errors encountered.

ssSetErrorStatus(S,"error encountered due to ...");
return;

Note that the second argument to ssSetErrorStatus must be persistent memory. It cannot be a local variable in your procedure. For example, the following will cause unpredictable errors.

mdlOutputs()
{
    char msg[256];   {ILLEGAL: to fix use "static char 
msg[256];"}
    sprintf(msg,"Error due to %s", string);
    ssSetErrorStatus(S,msg);
    return;
}

The ssSetErrorStatus error handling approach is the suggested alternative to using the mexErrMsgTxt function. The function mexErrMsgTxt uses exception handling to immediately terminate S-function execution and return control to Simulink. In order to support exception handling inside of S-functions, Simulink must set up exception handlers prior to each S-function invocation. This introduces overhead into simulation.

Function-Call Subsystems

Exception Free Code