MEX error LNK2019:unresolved symbol

Getting this error and I cannot figure out why,

Determining peak list...
Creating summarisation matrices...
Building with 'Microsoft Windows SDK 7.1 (C)'.
Error using mex
   Creating library symmeig.lib and object symmeig.exp
symmeig.obj : error LNK2019: unresolved external symbol mxGetDoubles
referenced in function mexFunction
symmeig.mexw64 : fatal error LNK1120: 1 unresolved externals



Error in memoryEfficientPCA (line 169)
               mex('-largeArrayDims', 'symmeig.c', lapacklib)
 

Here is the text where the error arises:

                 % Attempt to update Q using the mex file update
        try
            Q = updateQ(Q, counts(indicesList));
        catch exception
            % Failed to call updateQ so compile all functions if a compiler
            % is installed on the system
            
   
                compiler = mex.getCompilerConfigurations().Manufacturer;
                
                if(strcmp(compiler, 'LCC'))
                    folder = 'lcc';
                elseif(strcmp(compiler, 'Microsoft'))
                    folder = 'Microsoft';
                else
                    error('No compiler selected?');
                end
       
          
           lapacklib = fullfile(matlabroot, 'extern', 'lib', 'win64', 'microsoft', 'libmwlapack.lib');
               mex ('-largeArrayDims', 'symmeig.c', lapacklib)
               mex('-largeArrayDims', 'updateQ.c')
               mex('-largeArrayDims', 'calculateE.c')  
             

And the code associated wtih symmeig.c

               #include "mex.h"

void mexFunction( int nlhs, mxArray *plhs[], 
		  int nrhs, const mxArray*prhs[] )

    double *AP, *D, *E, *tau, *Q, *work;
    int n, info;
    char buffer[256];
    
    /* Create the vectors required as output and intermediate variables */
    mxArray *DArray, *EArray, *TAUArray, *QArray, *workArray;

    if (nrhs!=2)
        mexErrMsgTxt("Must have exactly two input arguments.");
    if (nlhs>2)
        mexErrMsgTxt("Too many output arguments.");
    if (!mxIsDouble(prhs[0]) || !mxIsDouble(prhs[1]))
        mexErrMsgTxt("Both inputs must be double arrays.");
    if (mxIsComplex(prhs[0]) || mxIsComplex(prhs[1]))
        mexErrMsgTxt("Both inputs must be real.");
    if (mxGetM(prhs[0])!=1 && mxGetN(prhs[0])!=1)
        mexErrMsgTxt("First input must be a vector.");
    if (mxGetM(prhs[1]) == 1 && mxGetN(prhs[1]) == 0)
        mexErrMsgTxt("Second input must be a scalar.");
  
    /*  */
    n = mxGetPr(prhs[1])[0];
  
    if(mxGetNumberOfElements(prhs[0]) != (n*(n+1)/2))
        mexErrMsgTxt("The vector is the wrong size for the N specified");
  
    DArray = mxCreateDoubleMatrix(n, 1, mxREAL);
    EArray = mxCreateDoubleMatrix(n-1, 1, mxREAL);
    TAUArray = mxCreateDoubleMatrix(n-1, 1, mxREAL);
    QArray = mxCreateDoubleMatrix(n, n, mxREAL);
    workArray = mxCreateDoubleMatrix(2*n-2, 1, mxREAL);

    
    D = mxGetDoubles(DArray);
    E = mxGetDoubles(EArray);
    tau = mxGetDoubles(TAUArray);
    Q = mxGetDoubles(QArray);
    work = mxGetDoubles(workArray);
    
    /* Return D (eigenvalues) and Q (eigenvectors) */
    plhs[0] = DArray;
    plhs[1] = QArray;
    
    /* Assign the input vector to AP */
    AP = mxGetPr(prhs[0]);

    /* Reduce symmetric matrix to tridiagonal form */
#if defined _WIN32 || defined _WIN64
    dsptrd("U", &n, AP, D, E, tau, &info);
#else
    dsptrd_("U", &n, AP, D, E, tau, &info);
#endif
  
    if (info < 0) {
        sprintf(buffer, "Error using DSPTRD with input: %d", -1*info);
      
        /* Cleanup */
        mxDestroyArray(TAUArray);
        mxDestroyArray(EArray);
        mxDestroyArray(workArray);
      
        mexErrMsgTxt(buffer);
    }
  
    /* Generate orthogonal matrix Q */
#if defined _WIN32 || defined _WIN64   
    dopgtr("U", &n, AP, tau, Q, &n, work, &info);
#else
    dopgtr_("U", &n, AP, tau, Q, &n, work, &info);
#endif
        
    /* Free up the unneeded vector */
    mxDestroyArray(TAUArray);
    
    if (info < 0) {
        sprintf(buffer, "Error using DOPGTR with input: %d", -1*info);      
        
        /* Cleanup */
        mxDestroyArray(EArray);
        mxDestroyArray(workArray);
        
        mexErrMsgTxt(buffer);
    }

    /* Compute all eigenvalues and eigenvectors using implicit QL or QR method. */
#if defined _WIN32 || defined _WIN64 
    dsteqr("V", &n, D, E, Q, &n, work, &info);
#else
    dsteqr_("V", &n, D, E, Q, &n, work, &info);
#endif
  
    /* Free up unneeded vectors */
    mxDestroyArray(EArray);
    mxDestroyArray(workArray);

    if (info < 0) {
        sprintf(buffer, "Error using DSTEQR with input: %d", -1*info); 
      
        mexErrMsgTxt(buffer);
    } else if(info > 0) {
        sprintf(buffer, "Error using DSTEQR: failed to find all eigenvalues in a total of %d iterations. %d elements of E have not converged to zero.", 30*n, info); 
      
        mexErrMsgTxt(buffer);
   

What does this error mean and how do I resolve?

Answer 1

[James Tursa]

mxGetDoubles( ) is a new API routine that only exists in the R2018a+ API memory model. You have to compile with the -R2018a flag. E.g.,

mex('-R2018a', 'symmeig.c', lapacklib)

You may also run into problems with these definitions:

int n, info;

For 64-bit code the BLAS and LAPACK libraries probably use 64-bit integers, but int is likely only a 32-bit integer. To be safe, always use mwSignedIndex when using integers that are passed into BLAS or LAPACK library routines. E.g.,

mwSignedIndex n, info;

And this check

    if (nlhs>2)
        mexErrMsgTxt("Too many output arguments.");

should be this instead

    if (nlhs != 2)
        mexErrMsgTxt("Need exactly two output arguments.");

otherwise this line can bomb MATLAB if less than two outputs are requested:

        plhs[1] = QArray;

Answer 2

[Harsha Priya Daggubati]

Hi,

The unresolved external symbols errors indicate that during the link-stage of compiling the MEX-file, these symbols are not available.

Your error points that mxGetDoubles is undefined, which is a basic function available. Can you try building the shipped 'explore.c' file and let me know if the same error pops-up.

Refer to the following link:

https://www.mathworks.com/help/matlab/matlab_external/standalone-example.html