How can I write to either the MATLAB Command Window or the Simulink Diagnostic Viewer from code called by a C Function block?

I have a C Function Block that calls a C wrapper of a long C++ method defined in an external file. I would like to periodically write messages from that C++ code and have the messages appear at runtime either in the MATLAB command window or the Simulink Diagnostic Viewer. I have tried redirecting stdout with

if (AllocConsole() == 0) {
    freopen("CONOUT$", "w", stdout);
    freopen("CONOUT$", "w", stderr);
    }

But I still see nothing from either std::cout or fprintf calls. Any help would be appriciated!

Answer 1

[Pramil]

Hello Jeff,

You can use the "mexPrintf" function for printing messages during runtime on Simulink Diagnostic Viewer from C++ code. Here is a link to the documentation for the same:

https://www.mathworks.com/help/matlab/apiref/mexprintf.html

https://www.mathworks.com/help/matlab/matlab_external/user-messages.html

Here is how you can achieve this:

• Include the header files "mex.h" in the C++ code.
• Declare a "print_val" or any other function for printing in the header file with input set to "const char*".
• Implement the "print_val" function like below:

void className::print_val(const char* msg)
{
    mexPrintf(msg);
}

• Call the '"print_val" function in the C++ code with the message for printing the messages at runtime.

Hope it helps.

[Jeff]

Thank you for your detailed answer. I have attempted to use mexPrintf(), but am still not seeing output. I have simplified my code to show the behavior.

I start with a new Simulink model. I have a C Function block set up like:

logwriter.cpp consists of:

#include "logwriter.h"

#include "mex.h"

#include <string>

EXTERNC int write()

{

return mexPrintf("This is a test\n");

}

I also have "logwriter.h" included under the Headers section of the C Function Block. The header consists of:

#ifdef __cplusplus

#define EXTERNC extern "C"

#else

#define EXTERNC

#endif

EXTERNC int write();

I can run this simulation in Simulink, and it shows that mexPrintf() thinks it printed 15 characters, but I don't see anything in either the Diagnostic Viewer or the Matlab Command window.

Any pointers on what might be going wrong would be appriciated! Thanks.

[Pramil]

I tried your code and was able to get output in Simulink Diagnostic Viewer, I am attaching my model in this comment so you can refer the same.

[Jeff]

Ok, with your model I was able to figure out the problem.

Your model, which uses Model Settings->Simulation Target->Custom Code to include "logwriter.h" and "logwriter.cpp", works just fine to display information in the Diagnostic Viewer.

If I move the links to the external "logwriter.cpp" and "logwriter.h" from Simulation Target->Custom Code to the C Function Block Paramaters->Simulation Custom Code (as I had in my model, above), the code compiles just fine, but no output is displayed in the Diagnostic Viewer.

Interesting result, but I can work with this. Thanks for the help in debugging it!