Datatype mismatch with arduino and coder.ceval

Question

I'm writing a simulink device driver block for Arduino Due with the Matlab Function Approach as explained in the Simulink Device Driver guide. When I want to pass a double from the C++ code to the matlab function code with coder.ceval() I get a wrong number. Here is an example:

C++ function:

#include <Arduino.h>
extern "C" double dout_output()
{
return 50.5;
}

Matlab function in Simulink:

function x = example()
x = 0.0;
if strcmp(coder.target,'rtw'),
x = coder.ceval('dout_output');
end

When I run this code in external mode on an adruino due I don't get the 50.5 but a large number like 1113794816. Any sugestions?

Answer 1

Is the header file containing the declaration of dout_output being included in the C code generated from the MATLAB Function code? If not, you'll likely see compiler warnings that say something about implicit int return type.

When C compilers don't have a declaration for a function, they may assume that the return type is int. Typically sizeof(int) != sizeof(double). So this mismatch can cause surprising results. This answer discusses that more.

Try to add:

coder.cinclude('dout_output.h');

to your MATLAB code where dout_output.h is replaced with the name of the header containing the declaration of dout_output.

You may also need to add an include directory to the custom code settings in:

"Configuration Parameters->Simulation Target->Custom Code->Include Directories"

and possibly:

"Configuration Parameters->Code Generation->Custom Code->Include Directories"

Alternatively, you can use the coder.ExternalDependency approach to encapsulate external code dependencies for your MATLAB code.