MathLink c++ passing lists of real values simple example

Question

So this may come down to me not being a very competent programmer but I am struggling with passing list from Mathematica via MathLink that aren't of integers. The final code I am hoping to get out of this will pass a list of values to a C++ function I have written and return a different list of values. I decided to learn slowly using the examples given in the Wolfram documentation center. I was able to write a simple function that received a list of integers...did some math on it...and returned a different list of integers. This worked great. If I tried to do this with a list of real values I kept getting a linker error. To simply things I removed the returning of a list of values and just decided to use the list as input and return a value. This resulted in the same linker error. I used the addtwo code from the MathLink examples supplied by Wolfram. The relevant portion of the code is:

#include "mathlink.h"
extern int sumlist(int *a, long alen);

int tot = 0;

int sumlist(int *a, long alen)
{   
        for(int i = 0;i<alen;i++)
        {
            tot = a[i] + tot;       
        }   
        return tot; 
}

with a template file (*.tm)

:Begin:
:Function:       sumlist
:Pattern:        SumList[a_List]
:Arguments:      {a}
:ArgumentTypes:  {IntegerList}
:ReturnType:     Integer
:End:

This code works great and gives me my desired result. Now if instead I wish to pass a list of Real numbers as in the following.

#include "mathlink.h"
extern double sumlist(double *a, long alen);

double tot = 0.0;

double sumlist(double *a, long alen)
{   
        for(int i = 0;i<alen;i++)
        {
            tot = a[i] + tot;       
        }   
        return tot; 
}

with a template file

:Begin:
:Function:       sumlist
:Pattern:        SumList[a_List]
:Arguments:      {a}
:ArgumentTypes:  {Real32List}
:ReturnType:     Real32
:End:

I am following the directions for the Command Line compiling for Microsoft visual studio from this page. I am using Microsoft visual studio 2013(which may be the problem as it technically isn't supported yet)

Everything is fine until I run:

Link sumlist.obj sumlisttm.obj ml32i3m.lib /OUT:sumlist.exe

Which results in the follow error

sumlisttm.obj : error LNK2019: unresolved external symbol "float __cdecl     sumlist(float *,int)" (?sumlist@@YAMPAMH@Z) referenced in function "int __cdecl     _tr0(struct MLink *)"(?_tr0@@YAHPAUMLink@@@Z)
sumlist.exe : fatal error LNK1120: 1 unresolved externals

I have done some testing and determined that I get this error just by trying to pass the list of real values rather than integers regardless of what the function actually does. I have looked for examples of Mathematica/MathLink code and most of them only deal with lists of integers.

Answer 1

The problem is that Real32 corresponds to a float (32-bit floating point number), not a double (64 bit floating point number).

The linker error message tells you that it can't find

sumlist(float *, int)

That's because you don't have a function with such a prototype. What you have is

sumlist(double *, long)

which is something different.

Make sure you use matching types. First, use int where int is required (not long), then either change the double to float on the C++ side or alternatively use Real64List and Real64 instead of Real32.

The documentation lists what Mathematica types correspond to what C (or C++) types. It's under "Details".