Initialize the state vectors of this C MEX S-function




C, C++


void mdlInitializeConditions(SimStruct *S)



SimStruct representing an S-Function block.


The Simulink® engine invokes this optional method at the beginning of a simulation. It should initialize the continuous and discrete states, if any, of this S-Function block. In a C MEX S-function, use ssGetContStates and/or ssGetDiscStates to access the states. This method can also perform any other initialization activities that this S-function requires.


If you have Simulink Coder™ and you need to ensure that the initialization code in the mdlInitializeConditions function is run only once, then move this initialization code into the mdlStart method. MathWorks recommends this code change as a best practice.

If this S-function resides in an enabled subsystem configured to reset states, the Simulink engine also calls this method when the enabled subsystem restarts execution. C MEX S-functions can use the ssIsFirstInitCond macro to determine whether the time at which mdlInitializeCondition is called is equal to the simulation start time.


If you have Simulink Coder, when generating code for a noninlined C MEX S-function that contains this method, make sure the method is not wrapped in a #if defined(MATLAB_MEX_FILE) statement. For example:

static void mdlInitializeConditions(SimStruct *S) 
   /* Add mdlInitializeConditions code here */

The define statement makes the mdlInitializeConditions method available only to a MATLAB® MEX file. If the S-function is not inlined, the Simulink Coder product cannot use this method, resulting in link or run-time errors.

The Simulink engine calls mdlInitializeConditions prior to calculating the S-function's input signals. Therefore, since the input signal values are not yet available, mdlInitializeConditions should not use the input signal values to set initial conditions. If your S-function needs to initialize internal values using the block's input signals, perform the initialization in mdlOutputs.

For example, in a C MEX S-function, initializes an IWork vector with one element in the mdlInitializeSizes method.

ssSetNumIWork(S, 1);

The IWork vector holds a flag indicating if initial values have been specified. Initialize the flag's value in the mdlInitializeCondition method.

static void mdlInitializeConditions(SimStruct *S)
  /* The mdlInitializeConditions method is called when the simulation
     start and every time an enabled subsystem is re-enabled.
     Reset the IWork flag to 1 when values need to be reinitialized.*/

  ssSetIWorkValue(S, 0, 1);

Check the value of the IWork vector flag in the mdlOutputs method, to determine if initial values need to be set. Since the engine has calculated input values at this point in the simulation, the mdlOutputs method can use them to initialize internal values.

static void mdlOutputs(SimStruct *S, int_T tid)
    // Initialize values if the IWork vector flag is true. //
    if (ssGetIWorkValue(S, 0) == 1) {
             // Enter initialization code here //

    // Remainder of mdlOutputs function //

For a Level-2 MATLAB S-function, use a DWork vector instead of an IWork vector in the previous example.


This example initializes both a continuous and discrete state to 1.0.

#define MDL_INITIALIZE_CONDITIONS   /*Change to #undef to remove */

static void mdlInitializeConditions(SimStruct *S)
  int i;
  real_T *xcont    = ssGetContStates(S);
  int_T   nCStates = ssGetNumContStates(S);
  real_T *xdisc    = ssGetRealDiscStates(S);
  int_T   nDStates = ssGetNumDiscStates(S);

  for (i = 0; i < nCStates; i++) {
    *xcont++ = 1.0;

  for (i = 0; i < nDStates; i++) {
    *xdisc++ = 1.0;


For another example that initializes only the continuous states, see resetint.c.

Introduced before R2006a

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