Use MATLAB Engine to Execute a Function Call in Generated Code
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,
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
coder.extrinsic('foo'), the code generator attempts
to determine whether
affects the output. If
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.
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.
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.
To declare a function
foo as extrinsic, include this statement
in your MATLAB code.
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.
For additional information and examples, see
The code generator automatically treats many common MATLAB visualization functions, such as
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
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.
Scope of Extrinsic Function Declarations
coder.extrinsic construct has function scope. For
example, consider the following
function y = foo %#codegen coder.extrinsic('rat','min'); [N D] = rat(pi); y = 0; y = min(N, D);
In this example,
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
ratis extrinsic every time it is called inside the main function
foo, but the function
minis extrinsic only when called inside the local function
Instead of using the
coder.extrinsicconstruct, 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
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
fooas a function call. Do not write the call by using the dot notation.
footo be extrinsic by using the syntax
For example, define
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.
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.extrinsicstatements to isolate calls to MATLAB functions, which can potentially create and propagate
mxArrays. See Working with mxArrays.
Call MATLAB Functions Using
To narrow the scope of extrinsic declaration to just one function call, use
feval is automatically interpreted as an extrinsic
function during code generation. So, you can use
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);
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
rat is extrinsic throughout the function
The code generator does not support the use of
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
also known as a MATLAB array. The only valid operations for
mxArrayin a variable.
mxArrayto an extrinsic function.
mxArrayfrom a function back to MATLAB.
mxArrayto a known type at run time. Assign the
mxArrayto 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
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
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
code generator automatically treats
min as an extrinsic
function. As a result,
min returns an
While generating a MEX function by using MATLAB
Coder, you can directly assign this
min to the output
because the MEX function returns its output to MATLAB.
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
Function output 'y' cannot be an mxArray in this context. Consider preinitializing the output variable 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
min to return, in this case, a scalar
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
foo for which you are generating
code. If you attempt to generate standalone code (for example, a static
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:
MATLAB functions that inspect the caller, or read or write to the caller workspace, do not work during code generation. Such functions include:
Functions in generated code can produce unpredictable results if your extrinsic function performs these actions at run time:
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.extrinsicto call functions that are located in a private folder.
The code generator does not support the use of
coder.extrinsicto call local functions.
You can call extrinsic functions with up to 64 inputs and 64 outputs.