I have gotten my s-function to compile in Visual Studio .NET 2003 but when I run the mex command on it in MATLAB to create the dll for the s-function it gives me this error when attempting to link:

--> "link /out:"regICPMAT.dll" /debug /dll /export:mexFunction /MAP /LIBPATH:"C:\MATLAB\extern\lib\win32\microsoft\msvc71" libmx.lib libmex.lib libmat.lib /implib:_lib9555.x  @9555_tmp.rsp "
 
Microsoft (R) Incremental Linker Version 7.10.3077
Copyright (C) Microsoft Corporation.  All rights reserved.
 
regICPMAT.obj  
   Creating library _lib9555.x and object _lib9555.exp
regICPMAT.obj : error LNK2019: unresolved external symbol "void __cdecl registration(double *,int,double *,int,double *,int,double * const)" (?registration@@YAXPANH0H0HQAN@Z) referenced in function _mdlOutputs
regICPMAT.dll : fatal error LNK1120: 1 unresolved externals
 
C:\MATLAB\BIN\WIN32\MEX.PL: Error: Link of 'regICPMAT.dll' failed.

Can anyone suggest why this is happening when it calls the registration function (registration is a seperate c++ class code that I have written but I included it as a header so i don't understand the problem).

HERE IS MY S_FUNCTION:

#define S_FUNCTION_LEVEL 2
#define MATLAB_MEX_FILE
#define S_FUNCTION_NAME  regICPMAT


#include "mex.h"
#include "registration.h"

#ifdef __cplusplus
extern "C" { // use the C fcn-call standard for all functions  
#endif       // defined within this scope

#include "simstruc.h"

static void mdlInitializeSizes(SimStruct *S)
{
    /* See sfuntmpl_doc.c for more details on the macros below */

    ssSetNumSFcnParams(S, 0);  /* Number of expected parameters */
    if (ssGetNumSFcnParams(S) != ssGetSFcnParamsCount(S)) {
        /* Return if number of expected != number of actual parameters */
        return;
    }

    ssSetNumContStates(S, 0);
    ssSetNumDiscStates(S, 0);

    if (!ssSetNumInputPorts(S, 6)) return;
    
    ssSetInputPortDataType( S, 0, SS_DOUBLE);
    ssSetInputPortWidth(S, 0, DYNAMICALLY_SIZED);
    
    ssSetInputPortDataType(S, 1, SS_INT32);
    ssSetInputPortWidth(S, 1, 1);
    
    ssSetInputPortDataType( S, 2, SS_DOUBLE);
    ssSetInputPortWidth(S, 2, DYNAMICALLY_SIZED);
    
    ssSetInputPortDataType(S, 3, SS_INT32);
    ssSetInputPortWidth(S, 3, 1);
    
    ssSetInputPortDataType( S, 4, SS_DOUBLE);
    ssSetInputPortWidth(S, 4, DYNAMICALLY_SIZED);
    
    ssSetInputPortDataType(S, 5, SS_INT32);
    ssSetInputPortWidth(S, 5, 1);
    
    ssSetInputPortRequiredContiguous(S, 0, true); /*direct input signal access*/
    ssSetInputPortRequiredContiguous(S, 1, true);
    ssSetInputPortRequiredContiguous(S, 2, true);
    ssSetInputPortRequiredContiguous(S, 3, true);
    ssSetInputPortRequiredContiguous(S, 4, true);
    ssSetInputPortRequiredContiguous(S, 5, true);
    ssSetInputPortRequiredContiguous(S, 6, true);

    //i think this should be zero for all since they do not directly enter into output
    ssSetInputPortDirectFeedThrough(S, 0, 0);
    ssSetInputPortDirectFeedThrough(S, 1, 0);
    ssSetInputPortDirectFeedThrough(S, 2, 0);
    ssSetInputPortDirectFeedThrough(S, 3, 0);
    ssSetInputPortDirectFeedThrough(S, 4, 0);
    ssSetInputPortDirectFeedThrough(S, 5, 0);

    if (!ssSetNumOutputPorts(S, 1)) return;
    ssSetOutputPortMatrixDimensions(S, 0, 4, 4);

    ssSetNumSampleTimes(S, 1);
    ssSetNumRWork(S, 0);
    ssSetNumIWork(S, 0);
    ssSetNumPWork(S, 0);
    ssSetNumModes(S, 0);
    ssSetNumNonsampledZCs(S, 0);

    ssSetOptions(S, 0);
}

static void mdlInitializeSampleTimes(SimStruct *S)
{
    //what kind of sample time should be used
    //ssSetSampleTime(S, 0, CONTINUOUS_SAMPLE_TIME);
    ssSetSampleTime(S, 0, INHERITED_SAMPLE_TIME);
    ssSetOffsetTime(S, 0, 0.0);

}

#define MDL_INITIALIZE_CONDITIONS   /* Change to #undef to remove function */
#if defined(MDL_INITIALIZE_CONDITIONS)

  static void mdlInitializeConditions(SimStruct *S)
  {
  }
#endif /* MDL_INITIALIZE_CONDITIONS */

#define MDL_START  /* Change to #undef to remove function */
#if defined(MDL_START)

  static void mdlStart(SimStruct *S)
  {

  }
#endif /*  MDL_START */


static void mdlOutputs(SimStruct *S, int_T tid)
{
    
    //set input from input ports
    double *in1 = (double*) ssGetInputPortSignal(S,0);
    int in2 = (int) ssGetInputPortSignal(S,1);
    double *in3 = (double*) ssGetInputPortSignal(S,2);
    int in4 = (int) ssGetInputPortSignal(S,3);
    double *in5 = (double*) ssGetInputPortSignal(S,4);
    int in6 = (int) ssGetInputPortSignal(S,5);

    //set output port
    double *output = (double*)ssGetOutputPortSignal(S,0); //this is fine
    
    double matrix[16];
    registration(in1,in2,in3,in4,in5,in6,matrix);
    output = matrix;
}


#define MDL_UPDATE  /* Change to #undef to remove function */
#if defined(MDL_UPDATE)

  static void mdlUpdate(SimStruct *S, int_T tid)
  {
  }
#endif /* MDL_UPDATE */



#define MDL_DERIVATIVES  /* Change to #undef to remove function */
#if defined(MDL_DERIVATIVES)

  static void mdlDerivatives(SimStruct *S)
  {
  }
#endif /* MDL_DERIVATIVES */


static void mdlTerminate(SimStruct *S)
{
}


#ifdef  MATLAB_MEX_FILE    /* Is this file being compiled as a MEX-file? */
#include "simulink.c"      /* MEX-file interface mechanism */
#else
#include "cg_sfun.h"       /* Code generation registration function */
#endif

#ifdef __cplusplus
} // end of extern "C" scope
#endif