mex compilation not working?

I have a very simple C++ program "readBinFile.cpp" for reading the binary file bin, but mex compiles with an error.How do I fix it to compile correctly？（I have successfully paired matlab with mingw64 and verified the runtime environment）

The matlab call syntax is：

outImg = readBinFile(''myBinFile.bin");

readBinFile.cpp is following:

#include "mex.hpp"
#include "mexAdapter.hpp"
#include "MatlabEngine.hpp"

#include <iostream>
#include <string>
#include <stdio.h>
#include <stdlib.h>

using matlab::mex::ArgumentList;
using namespace matlab::data;

class MexFunction : public matlab::mex::Function {
    // Pointer to MATLAB engine
    std::shared_ptr<matlab::engine::MATLABEngine> matlabPtr = getEngine();

    // Factory to create MATLAB data arrays
    ArrayFactory factory;

    typedef struct {
        uint8_t B, G, R, A;
        uint32_t AlignmentDummy;
    } FColorStruct;

    typedef struct {
        uint32_t InSizeX;
        uint32_t InSizeY;
        FColorStruct imgUreal[540 * 480];
    } imgBinStruct;

   public:

    void operator()(ArgumentList outputs, ArgumentList inputs) {
        size_t resolutionX = 540;
        size_t resolutionY = 480;
        // read my file
        FILE* pFile;
        long lSize;
        imgBinStruct* buffer;
        size_t result;

        std::string in1 = inputs[0][0]; // bin file name
        // matlab::data::TypedArrayRef<MATLABString> in = in1;
        // std::string cmdString = std::string(in1);
        pFile = fopen(in1.c_str(), "rb");
        if (pFile == NULL) {
            std::ostringstream stream;
            stream << "Failed to open " + in1 << std::endl;
            displayOnMATLAB(stream);
            exit(1);
        }

        // obtain file size:
        fseek(pFile, 0, SEEK_END);
        lSize = ftell(pFile);
        rewind(pFile);

        // allocate memory to contain the whole file:
        buffer = (imgBinStruct*)malloc(sizeof(imgBinStruct));
        if (buffer == NULL) {
            std::ostringstream stream;
            stream << "Memory error " << std::endl;
            displayOnMATLAB(stream);
            exit(2);
        }

        // copy the file into the buffer:
        result = fread(buffer, 1, lSize, pFile);
        if (result != lSize) {
            std::ostringstream stream;
            stream << "Reading error " << std::endl;
            displayOnMATLAB(stream);
            exit(3);
        }

        // matlab mat
        matlab::data::TypedArray<uint8_t> outImg = factory.createArray<uint8_t>({resolutionY, resolutionX, 3});
        for (size_t i = 0; i < resolutionY; i++) {
            for (size_t j = 0; j < resolutionX; j++) {
                outImg[i][j][2] = (uint8_t)(buffer->imgUreal[i * resolutionX + j]).B;
                outImg[i][j][1] = (uint8_t)(buffer->imgUreal[i * resolutionX + j]).G;
                outImg[i][j][0] = (uint8_t)(buffer->imgUreal[i * resolutionX + j]).R;
            }
        }
        outputs[0] = outImg;

        // terminate
        fclose(pFile);
        free(buffer);
    }

    void displayOnMATLAB(const std::ostringstream& stream) {
        matlabPtr->feval(u"fprintf", 0,
                         std::vector<Array>({factory.createScalar(stream.str())}));
    }

    void checkArguments(matlab::mex::ArgumentList outputs, matlab::mex::ArgumentList inputs) {
        if (inputs.size() != 1) {
            matlabPtr->feval(u"error",
                             0, std::vector<matlab::data::Array>({factory.createScalar("One inputs bin FileName required")}));
        }

        if (inputs[0].getType() != matlab::data::ArrayType::MATLAB_STRING) {
            matlabPtr->feval(u"error",
                             0, std::vector<matlab::data::Array>({factory.createScalar("Input binFileName must a string scalar")}));
        }

        if (outputs.size() != 1) {
            matlabPtr->feval(u"error", 0, std::vector<matlab::data::Array>({factory.createScalar("输出参数必须为1个")}));
        }
    }
};

I then compile the mex using the following command, (run matlab 2022a, windows10)

mex readBinFile.cpp

Error using mex

In file included from C:\Program Files\MATLAB\R2022a/extern/include/MatlabEngine.hpp:11,

from D:\matlab_files\mytest\readBinFile.cpp:6:

C:\Program Files\MATLAB\R2022a/extern/include/MatlabEngine/engine_util.hpp:22:20: error:

multiple definition of 'enum class matlab::engine::WorkspaceType'

enum class WorkspaceType {

^~~~~~~~~~~~~

In file included from C:\Program Files\MATLAB\R2022a/extern/include/mex.hpp:81,

from D:\matlab_files\mytest\readBinFile.cpp:4:

C:\Program Files\MATLAB\R2022a/extern/include/cppmex/mexMatlabEngine.hpp:25:20: note:

previous definition here

enum class WorkspaceType {

^~~~~~~~~~~~~

In file included from C:\Program Files\MATLAB\R2022a/extern/include/MatlabEngine.hpp:12,

from D:\matlab_files\mytest\readBinFile.cpp:6:

...

[cui,xingxing]

I have experimented and after removing the header file “MatlabEngine.hpp”, I managed to get my .mexw64 file through mex compilation, I don't know why this happened.

Answer 1

[Madheswaran]

Hi @cui,xingxing

The error you are facing is because both mex.hpp and MatlabEngine.hpp define the same enumeration matlab::engine::WorkspaceType, leading to multiple definition errors during compilation as shown in the error message:

multiple definition of 'enum class matlab::engine::WorkspaceType'
         enum class WorkspaceType {
                             ^~~~~~~~~~~~~

In general, "mex.hpp" is used when you are writing a MEX function that will be called directly from MATLAB. "MatlabEngine.hpp" is used for writing standalone C++ applications that needs to interact with MATLAB.

Since in your case, you are creating a MEX function to read binary files and interface with MATLAB, "mex.hpp" (which includes "mexMatlabEngine.hpp") already provides all the necessary MATLAB engine functionality you need. This is why removing "MatlabEngine.hpp" solved the compilation issue without affecting your program's functionality.

For more information, refer to the following MathWorks documentation:

1. Write C++ Functions Callable from MATLAB (MEX Files) - https://mathworks.com/help/matlab/cpp-mex-file-applications.html
2. Call MATLAB from C++ (MATLAB Engine) - https://mathworks.com/help/matlab/calling-matlab-engine-from-cpp-programs.html

Hope this helps!

[cui,xingxing]

Thank you