Writing S-Functions

Basic Files Required for Implementation

This section briefly describes what files and functions you'll need to create noninlined, wrapper, and fully inlined S-functions.

• Noninlined S-functions require the C-MEX S-function source code sfunction.c.
• Wrapper S-functions that inline a call to your algorithm (your C function) require an sfunction.tlc file.
• Fully inlined S-functions require an sfunction.tlc file. Fully inlined S-functions produce the optimal code for a parameterized S-function. This is an S-function that operates in a specific mode dependent upon fixed S-function parameters that do not change during model execution. For a given operating mode, the sfunction.tlc file specifies the exact code that is generated to implement the algorithm for that mode. For example, the direct-index lookup table S-function at the end of this chapter contains two operating modes -- one for evenly spaced x-data and one for unevenly spaced x-data.
• Fully inlined S-functions might require the placement of the mdlRTW routine in your S-function MEX-file sfunction.c. The mdlRTW routine lets you place information in model.rtw, which is the file that is processed by the Target Language Compiler prior to executing sfunction.tlc when generating code. This is useful when you want to introduce nontunable parameters into your TLC file.

For S-functions to work correctly in the Simulink environment, a certain amount of overhead code is necessary. When the Real-Time Workshop generates code from models that contain S-functions (without sfunction.tlc files), it embeds some of this overhead code in the generated C code. If you want to optimize your real-time code and eliminate some of the overhead code, you must inline (or embed) your S-functions. This involves writing a TLC (sfunction.tlc) file that directs the Real-Time Workshop to eliminate all overhead code from the generated code. The Target Language Compiler, which is part of the Real-Time Workshop, processes sfunction.tlc files to define how to inline your S-function algorithm in the generated code.

Note    The term inline should not be confused with the C++ inline keyword. In MathWorks terminology, inline means to specify a textual string in place of the call to the general S-function API routines (e.g., mdlOutputs). For example, when we say that a TLC file is used to inline an S-function, we mean that the generated code contains the appropriate C code that would normally appear within the S-function routines and the S-function itself has been removed from the build process.

Types of S-Functions

Noninlined S-Functions