Writing S-Functions

Defining S-Function Block Characteristics

For Simulink to recognize an M-file S-function, you must provide it with specific information about the S-function. This information includes the number of inputs, outputs, states, and other block characteristics.

To give Simulink this information, call the simsizes function at the beginning of mdlInitializeSizes.

sizes = simsizes;

This function returns an uninitialized sizes structure. You must load the sizes structure with information about the S-function. The table below lists the sizes structure fields and describes the information contained in each field.

Table 2-2: Fields in the sizes Structure 

Field Name	Description
sizes.NumContStates	Number of continuous states
sizes.NumDiscStates	Number of discrete states
sizes.NumOutputs	Number of outputs
sizes.NumInputs	Number of inputs
sizes.DirFeedthrough	Flag for direct feedthrough
sizes.NumSampleTimes	Number of sample times

After you initialize the sizes structure, call simsizes again.

sys = simsizes(sizes);

This passes the information in the sizes structure to sys, a vector that holds the information for use by Simulink.

S-Function Outputs

A Simple M-File S-Function Example