This is machine translation

Translated by Microsoft
Mouseover text to see original. Click the button below to return to the English verison of the page.

Note: This page has been translated by MathWorks. Please click here
To view all translated materals including this page, select Japan from the country navigator on the bottom of this page.


Update a block's states




C, C++


#define MDL_UPDATE
void mdlUpdate(SimStruct *S, int_T tid)



SimStruct representing an S-Function block.


Task ID.


The Simulink® engine invokes this optional method at each major simulation time step. The method should compute the S-function's states at the current time step and store the states in the S-function's state vector. The method can also perform any other tasks that the S-function needs to perform at each major time step.

Use this code if your S-function has one or more discrete states or does not have direct feedthrough.

The reason for this is that most S-functions that do not have discrete states but do have direct feedthrough do not have update functions. Therefore, the engine is able to eliminate the need for the extra call in these circumstances.

If your C MEX S-function needs to have its mdlUpdate routine called and it does not satisfy either of the above two conditions, specify that it has a discrete state, using the ssSetNumDiscStates macro in the mdlInitializeSizes function.

In C MEX S-functions, the tid (task ID) argument specifies the task running when the mdlOutputs routine is invoked. You can use this argument in the mdlUpdate routine of a multirate S-Function block to encapsulate task-specific blocks of code (see Multirate S-Function Blocks).

Use the UNUSED_ARG macro if your C MEX S-function does not contain task-specific blocks of code to indicate that the tid input argument is required but not used in the body of the callback. To do this, insert the line


after the declarations in mdlUpdate.

    Note   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:

    #define MDL_UPDATE 
    #if defined(MDL_UPDATE) && defined(MATLAB_MEX_FILE) 
    static void mdlUpdate(SimStruct *S, int_T tid) 
       /* Add mdlUpdate code here *

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


For an example that uses this function to update discrete states, see dsfunc.c. For an example that uses this function to update the transfer function coefficients of a time-varying continuous transfer function, see stvctf.c.

Introduced before R2006a

Was this topic helpful?