Matlab engine function in C returns zero

Question

I was wondering if anyone could help me understand the syntax of Matlab engine in C. I am a beginner in C and am trying to call a custom Matlab function in C using Matlab Engine. Research that I have done includes reading the documentation for Matlab API, watching the lecture video from Mathworks, researching on Stack Overflow, reading the example eng.c file in Matlab, and Google.

I have come up with this bit of code that compiles but the output returns zero. The input array also doesn't return an array but an int. I can't find a comprehensive walk through video of how to construct a C script that

1. takes in a vector
2. feeds it into a Matlab function and
3. returns output.

The docs explain making an array very clearly, but I can't find information that walks through in detail reading the array into Matlab, then getting output.

Below, please see the code including comments for what I understand each section of code is doing. The example function add_up just calls the sum function on an array. Any help would be great as I am stuck on why this is returning zero.

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include "engine.h"
#define BUFSIZE 256

int main() {

    //Open call to matlab engine
    Engine *ep;

    //Use this conjunction with define BUFSIZe 256 to create double
    //extData is a variable to read external data
    //Number in brackets refer to size
    //We are using double in this case and create the external data using initialization
    double extData[10]={1.0,4.0,7.0,2.0,5.0,8.0,3.0,6.0,9.0,10.0};

    //Next step is to make a pointer of type mxArray 
    //These are pointers to an array of any size or type
    mxArray *pVarNum;
    double *outp;

    //After we make a matrix for the double data initialized above
    //Initialized to 0
    pVarNum=mxCreateDoubleMatrix(1,10,mxREAL);

    //Our array needs to be assigned a variable name for Matlab
    //Workspace
    //const char *myDouble = "T";

    //Our matlab matrix is initialized to zero. We need to use
    //The C memcpy function to get the data from extData to
    //Get the array data using the pointer to pVarNum
    //Use mxGetPr, a mxGet function
    memcpy((void *)(mxGetPr(pVarNum)), (void *)extData,sizeof(extData));

    //Place the variable T into the Matlab workspace
    engPutVariable(ep,"T",pVarNum);

    //Evalute test function
    engEvalString(ep, "out=T+1");

    //Make a pointer to the matlab variable
    outp=engGetVariable(ep,"out");
    //Now make a pointer to the C variable
    printf("%d\n",outp);

    printf("Done!\n");
    mxDestroyArray(pVarNum);

    engClose(ep);

    return EXIT_SUCCESS;

}

Answer 1

The engGetVariable function returns an mxArray*, not a double*:

double *outp;
/* ... */
outp = engGetVariable(ep, "out");
printf("%d\n", outp);

This should be:

mxArray *out;
double *outp;
int ii;
/* ... */
out = engGetVariable(ep, "out");
outp = mxGetPr(out);
for (ii = 0; ii < 9; ii++) {
   printf("%f, ", outp[ii]);
}
printf("%f\n", outp[9]);

Note also that the %d formatter in printf prints an int, you need to use %f for a double.

Answer 2

The problem was that engputVariable was returning 1, so Matlab engine wasn't taking in the array. I ended up copying and pasting the exact top segment of code from the Matlab engdemo.c (up to the call for engPutVariable), then continuing with my code. Encoding was in ASCII for the working code file. I think the crucial piece is the initial call to open Matlab with a Null string, although I placed this exact piece of code in the non-working script and it did not lead to engPutVariable working.

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include "engine.h"
#define  BUFSIZE 256

int main()

{
    Engine *ep;
    mxArray *T = NULL, *outp = NULL;
    double *out;
    int ii;
    char buffer[BUFSIZE+1];
    double time[10] = { 0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 };


    if (!(ep = engOpen(""))) {
        fprintf(stderr, "\nCan't start MATLAB engine\n");
        return EXIT_FAILURE;
    }

    T = mxCreateDoubleMatrix(1, 10, mxREAL);
    memcpy((void *)mxGetPr(T), (void *)time, sizeof(time));

    engPutVariable(ep, "T", T);


    //Evalute test function. This is new

    engEvalString(ep, "D=add_up(T);");

    //Make a pointer to the matlab variable
    if((outp=engGetVariable(ep,"D"))==NULL){
        printf("Oops!");
    }
    out= mxGetPr(outp);

    //Now make a pointer to the C variable
    for (ii=0; ii<10; ii++) {
        printf("%f\n", out[ii]);
    }
    printf("%f\n",out[2]);

    printf("Done!\n");
    mxDestroyArray(T);
    mxDestroyArray(outp);
    engClose(ep);

    return EXIT_SUCCESS;

}