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Use MATLAB Engine to Execute a Function Call During Fixed-Point Algorithm Acceleration

When processing a call to a function foo in your MATLAB® code, the code generator finds the definition of foo and generates code for its body. In some cases, you might want to bypass code generation and instead use the MATLAB engine to execute the call. Use coder.extrinsic('foo') to declare that calls to foo do not generate code and instead use the MATLAB engine for execution. In this context, foo is referred to as an extrinsic function. This functionality is available only when the MATLAB engine is available during execution. Examples of such situations include execution of MEX functions, Simulink® simulations, or function calls at the time of code generation (also known as compile time).

If you generate standalone code for a function that calls foo and includes coder.extrinsic('foo'), the code generator attempts to determine whether foo affects the output. If foo does not affect the output, the code generator proceeds with code generation, but excludes foo from the generated code. Otherwise, the code generator produces a compilation error.

Including the coder.extrinsic('foo') directive inside a certain MATLAB function declares all calls to foo inside that MATLAB function as extrinsic. Alternatively, you might want to narrow the scope of extrinsic declaration to just one call to foo. See Call MATLAB Functions Using feval (MATLAB Coder).

This image shows the flowchart on what you should do when using the MATLAB engine to execute the call to a function.

When To Declare a Function as Extrinsic

These are some common situations in which you might consider declaring a MATLAB function as extrinsic:

  • The function performs display or logging actions. Such functions are useful primarily during simulation and are not used in embedded systems.

  • In your MEX execution or Simulink simulation, you want to use a MATLAB function that is not supported for code generation. This workflow does not apply to non-simulation targets.

  • You instruct the code generator to constant fold a function call by using coder.const. In such situations, the function is called only during code generation when the MATLAB engine is available for executing the call.

Use the coder.extrinsic Construct

To declare a function foo as extrinsic, include this statement in your MATLAB code.

coder.extrinsic('foo')

When declaring functions as extrinsic for code generation, adhere to these rules:

  • Declare the function as extrinsic before you call it.

  • Do not use the extrinsic declaration in conditional statements.

  • Assign the return value of an extrinsic function to a known type. See Working with mxArrays (MATLAB Coder).

For additional information and examples, see coder.extrinsic.

The code generator automatically treats many common MATLAB visualization functions, such as plot, disp, and figure, as extrinsic. You do not have to explicitly declare them as extrinsic functions by using coder.extrinsic. For example, you might want to call plot to visualize your results in the MATLAB environment. If you generate a MEX function from a function that calls plot, and then run the generated MEX function, the code generator dispatches calls to the plot function to the MATLAB engine. If you generate a library or executable, the generated code does not contain calls to the plot function.

If you generate MEX or standalone C/C++ code by using MATLAB Coder™, the code generation report highlights calls from your MATLAB code to extrinsic functions. By inspecting the report, you can determine which functions are supported only in the MATLAB environment.

This image shows the report for the function mystats. The cursor points to the extrinsic function, plot, and the report displays the function properties. The extrinsic function is purple.

Scope of Extrinsic Function Declarations

The coder.extrinsic construct has function scope. For example, consider the following code:

function y = foo %#codegen
coder.extrinsic('rat','min');
[N D] = rat(pi);
y = 0;
y = min(N, D);

In this example, rat and min as treated as extrinsic every time they are called in the main function foo. There are two ways to narrow the scope of an extrinsic declaration inside the main function:

  • Declare the MATLAB function extrinsic in a local function, as in this example:

    function y = foo %#codegen
coder.extrinsic('rat');
[N D] = rat(pi);
y = 0;
y = mymin(N, D);
 
function y = mymin(a,b)
coder.extrinsic('min');
y = min(a,b);

    Here, the function rat is extrinsic every time it is called inside the main function foo, but the function min is extrinsic only when called inside the local function mymin.

  • Instead of using the coder.extrinsic construct, call the MATLAB function using feval. This approach is described in the next section.

Extrinsic Declaration for Nonstatic Methods

Suppose that you define a class myClass that has a nonstatic method foo, and then create an instance obj of this class. If you want to declare the method obj.foo as extrinsic in your MATLAB code that you intend for code generation, follow these rules:

  • Write the call to foo as a function call. Do not write the call by using the dot notation.

  • Declare foo to be extrinsic by using the syntax coder.extrinsic('foo').

For example, define myClass as:

classdef myClass
    properties
        prop = 1
    end
    methods
        function y = foo(obj,x)
            y = obj.prop + x;
        end
    end
end

Here is an example MATLAB function that declares foo as extrinsic.

function y = myFunction(x) %#codegen
coder.extrinsic('foo');
obj = myClass;
y = foo(obj,x);
end

Nonstatic methods are also known as ordinary methods. See Method Syntax.

Additional Uses

Use the coder.extrinsic construct to:

  • Call MATLAB functions that do not produce output during simulation without generating unnecessary code.

  • Make your code self-documenting and easier to debug. You can scan the source code for coder.extrinsic statements to isolate calls to MATLAB functions, which can potentially create and propagate mxArrays. See Working with mxArrays (MATLAB Coder).

Call MATLAB Functions Using feval

To narrow the scope of extrinsic declaration to just one function call, use the function feval. feval is automatically interpreted as an extrinsic function during code generation. So, you can use feval to call functions that you want to execute in the MATLAB environment, rather than compile to generated code.

Consider this example:

function y = foo 
coder.extrinsic('rat');
[N D] = rat(pi);
y = 0;
y = feval('min',N,D);

Because feval is extrinsic, the statement feval('min',N,D) is evaluated by MATLAB, not compiled, which has the same result as declaring the function min extrinsic for just this one call. By contrast, the function rat is extrinsic throughout the function foo.

The code generator does not support the use of feval to call local functions or functions that are located in a private folder.

Working with mxArrays

The run-time output of an extrinsic function is an mxArray, also known as a MATLAB array. The only valid operations for mxArrays are:

  • Storing an mxArray in a variable.

  • Passing an mxArray to an extrinsic function.

  • Returning an mxArray from a function back to MATLAB.

  • Converting an mxArray to a known type at run time. Assign the mxArray to a variable whose type is already defined by a prior assignment. See the following example.

To use an mxArray returned by an extrinsic function in other operations (for example, returning it from a MATLAB Function block to Simulink execution), you must first convert it to a known type.

If the input arguments of a function are mxArrays, the code generator automatically treats the function as extrinsic.

Convert mxArrays to Known Types

To convert an mxArray to a known type, assign the mxArray to a variable whose type is defined. At run time, the mxArray is converted to the type of the variable that it is assigned to. If the data in the mxArray is not consistent with the type of the variable, you get a run-time error.

For example, consider this code:

function y = foo %#codegen
coder.extrinsic('rat');
[N D] = rat(pi);
y = min(N,D);

Here, the top-level function foo calls the extrinsic MATLAB function rat, which returns two mxArrays representing the numerator N and denominator D of the rational fraction approximation of pi. You can pass these mxArrays to another MATLAB function, in this case, min. Because the inputs passed to min are mxArrays, the code generator automatically treats min as an extrinsic function. As a result, min returns an mxArray.

While generating a MEX function by using MATLAB Coder, you can directly assign this mxArray returned by min to the output y because the MEX function returns its output to MATLAB.

codegen foo
Code generation successful.

But if you put foo in a MATLAB Function block in a Simulink model and then update or run the model, you get this error:

Code generation does not support mxArray output from this function in this context. 
Preinitialize output variable 'y' with a known type.

This error occurs because returning an mxArray back to Simulink is not supported. To fix this issue, define y to be the type and size of the value that you expect min to return, in this case, a scalar double:

function y = foo %#codegen
coder.extrinsic('rat');
[N D] = rat(pi);
y = 0; % Define y as a scalar of type double
y = min(N,D);

In this example, the output of the extrinsic function min affects the output y of the entry-point function foo for which you are generating code. If you attempt to generate standalone code (for example, a static library) for foo, the code generator is unable to ignore the extrinsic function call and produces a code generation error.

codegen -config:lib foo
??? The extrinsic function 'min' is not available for 
standalone code generation. It must be eliminated for 
stand-alone code to be generated. It could not be 
eliminated because its outputs appear to influence the 
calling function. Fix this error by not using 'min' 
or by ensuring that its outputs are unused.

Error in ==> foo Line: 4 Column: 5
Code generation failed: View Error Report

Error using codegen

Restrictions on Using Extrinsic Functions

The full MATLAB run-time environment is not supported during code generation. Therefore, the following restrictions apply when calling MATLAB functions extrinsically:

  • Some MATLAB functions that inspect the caller, or that read or write to the caller workspace, are not supported for code generation. Such functions include:

  • Functions in generated code can produce unpredictable results if your extrinsic function performs these actions at run time:

    • Changes folders

    • Changes the MATLAB path

    • Deletes or adds MATLAB files

    • Changes warning states

    • Changes MATLAB preferences

    • Changes Simulink parameters

  • The code generator does not support the use of coder.extrinsic to call functions that are located in a private folder.

  • The code generator does not support the use of coder.extrinsic to call local functions.

  • Code generation does not support values passed to or returned from an extrinsic function that are or contain:

    • handle classes

    • function handles

    • opaque values (see coder.opaque (MATLAB Coder))

  • You can call extrinsic functions with up to 64 inputs and 64 outputs.

See Also

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