What Is an S-Function?
S-functions (system-functions) provide a powerful mechanism
for extending the capabilities of the Simulink® environment.
An S-function is a computer language description
of a Simulink block written in MATLAB®,
C, C++, or Fortran. C, C++, and Fortran S-functions are compiled as
MEX-files using the mex utility (see Building MEX-Files in MATLAB External Interfaces).
As with other MEX-files, S-functions are dynamically linked subroutines
that the MATLAB interpreter can automatically load and execute.
S-functions use a special calling syntax called the S-function
API that enables you to interact with the Simulink engine. This
interaction is very similar to the interaction that takes place between
the engine and built-in Simulink blocks.
S-functions follow a general form and can accommodate continuous,
discrete, and hybrid systems. By following a set of simple rules,
you can implement an algorithm in an S-function and use the S-Function
block to add it to a Simulink model. After you write your S-function
and place its name in an S-Function block (available in the User-Defined
Functions block library), you can customize the user interface using
masking (see Creating Custom Blocks).
 | Overview of S-Functions | | Using S-Functions in Models |  |
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