c-mex S-Function error in desktop real-time external mode: Error occurred while executing External Mode MEX-file 'sldrtext': Error loading real-time image: undefined symbol

I have encountered some issues with an S-Function I developed using c-mex. The purpose of my S-Function is to add two inputs together, and I have implemented this functionality using external c source code and header files. Here are the contents of these files:

myadd.h

int myadd(int a, int b);

myadd.c

#include "myadd.h"
int myadd(int a, int b)
{
    return a+b;
}

and my S-Function is mysfun.c, its output function is:

static void mdlOutputs(SimStruct *S, int_T tid)
{
     const real_T *u = (const real_T*) ssGetInputPortSignal(S,0);
     real_T       *y = ssGetOutputPortSignal(S,0);
     y[0] = myadd((int)u[0],10);
}

I compiled them along with my S-Function using the following command:

mex mysfun.c myadd.c

The compilation process showed no errors. Next, I tested the S-Function in normal mode in Simulink, and it indeed worked properly. However, when I switched to Simulink Desktop Real-Time's external mode, I encountered the following error message:

'''

Error occurred while executing External Mode MEX-file 'sldrtext': Error loading real-time image: undefined symbol "myadd"

'''

But when I implement myadd() function directly in mysfun.c, like:

int myadd(int a, int b)
{
    return a+b;
}
static void mdlOutputs(SimStruct *S, int_T tid)
{
     const real_T *u = (const real_T*) ssGetInputPortSignal(S,0);
     real_T       *y = ssGetOutputPortSignal(S,0);
     y[0] = myadd((int)u[0],10);
}

the execution in external mode worked properly. I have no idea what is the problem.

Could someone please help me understand this error message? Thank you!

Answer 1

[Jan Houska]

Hi You-Cheng,

this is because the external mode build system does not know that it should include the file myadd.c to the list of files needed to build the real-time executable. Please see here how to add custom files to the build process, especially see the Specify Additional Source Files for an S-Function section and the S-function modules parameter of the S-function block.

Good Luck, Jan

[You-Cheng]

Hi, Jan

Thank you very much for your answer. Following the method provided in the link, I have successfully resolved the issue.