Error retrieving a struct in MATLAB R2014a with engGetVariable

Question

#include <iostream>
#include "engine.h"
using namespace std;

int main(){
    cout << "Start" << endl;
    Engine *ep;
    //Loading Matlab Version R2014a
    if (!(ep = engOpen("/usr/local/MATLAB/R2014a/bin/matlab"))) {
        cout << "\nCan't start MATLAB engine\n";
        return 1;
    } 
    engEvalString( ep, "load('error.mat')" );
    // PHOTOSS_TEMP3 is a struct
    mxArray *K = engGetVariable(ep, "PHOTOSS_TEMP3");
    if(K !=NULL)
        cout << "Loading variable was successful" << endl;
    cout << "End";
    return 0;
}

With this sample C++ program I am trying to read a struct out of MATLAB. But the program freezes in line mxArray *K = engGetVariable(ep, "PHOTOSS_TEMP3"); and i get the error message:

Error using save
Can't write file stdio.

Here is also the makefile:

g++ -I/usr/local/MATLAB/R2014a/extern/include -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"matlab_error.d" -MT"matlab_error.d" -o "matlab_error.o" "matlab_error.cpp"
g++ -Wl,-rpath,/usr/local/MATLAB/R2014a/bin/glnxa64 -L/usr/local/MATLAB/R2014a/bin/glnxa64 -Xlinker -rpath-link -Xlinker /usr/local/MATLAB/R2014a/bin/glnx64 -o "matlab_error"  matlab_error.o   -leng -lmx

and the "whos" of the MATLAB file:

Name                       Size            Bytes  Class     Attributes

Custom_Parameters          0x0                 0  double    global    
MultiSignal                1x1             83110  struct              
PHOTOSS_TEMP1              1x1                 8  double              
PHOTOSS_TEMP3              1x1              8210  struct              
Results                    0x0                 0  double    global    
alpha                      1x1                 8  double    global    
beta2                      1x1                 8  double    global    
current_block_no           1x1                 8  double              
fiber_length               1x1                 8  double    global    
main_parameters            1x1              4644  struct    global    
message                    0x0                 0  char      global    
simulation_parameters      1x1              4644  struct

This error only appears, when I use the MATLAB version R2014a Linux. I have tried version 2013a/b and it worked fine. It happens also when I try to read another struct of the sample file like main_parameters or MultiSignal. So where is my mistake?

sample program and matlab file

Have you found an answer? I'm getting this behavior with a scalar logical. It's happening in two separate programs, whereas before it was working perfectly fine. Other parts of the code have changed, but this part didn't. It just seems to have spontaneously stopped working. I'm also on R2014a Linux. — Neil Traft

Clarissa G Clarissa G · Accepted Answer · 2015-09-10T15:51:10

You may be having the same issue that this user had, in which case you should use the MATLAB command cell2mat to save the data in your .m file. It is otherwise not possible to read from a cell data type.

Error retrieving a struct in MATLAB R2014a with engGetVariable

1 Answers