Mex function unresolved external

Question

I am trying to build a mex function in MATLAB. The function depends on a C++ library. However, I get unresolved externals in MATLAB no matter what I do. I have created three trivial files to demonstrate the problem:

my_test123.h

_declspec(dllexport) void my_test();

my_test.cpp

extern "C" {
#include "my_test123.h"
}
void my_test() {
}

I compile and link the two files above using the command:

cl /LD /Femy_test.dll my_test.cpp

This generates two files, my_test.lib and my_test.dll .

The third file is a trivial mexfunction:

my_mex.cpp

#include "mex.h"

extern "C" {
void my_test(); 
}

/* The gateway function */
void mexFunction(int nlhs, mxArray *plhs[],
int nrhs, const mxArray *prhs[])
{
    my_test();
}

In MATLAB, I use the command below:

mex  -v my_mex.cpp my_test.lib

I also tried:

mex  -v my_mex.cpp -lmy_test.lib

All files are in the same directory and the mex command is finding the .lib file (if I try a random name instead of my_test.lib , I get a file not found error).

The error I get is:

Error using mex Creating library my_mex.lib and object my_mex.exp my_mex.obj : error LNK2019: unresolved external symbol my_test referenced in function mexFunction my_mex.mexw64 : fatal error LNK1120: 1 unresolved externals

I have also tried making every file a C file (removing the externs and changing the mexfunciton extension to .c) and compiling in C. But I get the same exact error.

I am using Visual Studio 2013 and 64 bit version of MATLAB 2014b.

Any help is much appreciated.

Answer 1

After many hours working on this and help from the MathWorks support line, I discovered the following:

You need to take several factors into account:

• Is your MATLAB 32 bit or 64 bit?
• Is the extension of your mexfunction .c or .cpp?
• How are you using the extern "C"?
• Is the .dll a 64 bit .dll or a 32 bit .dll?

Let's assume the .dll is a 32 bit .dll and MATLAB is 32 bits

mex function has an extension .cpp, .dll is a C++ .dll You don't need to add any extern "C" neither in my_test.cpp or in my_mex.cpp.

mex function has an extension .c, .dll is a C++ .dll You need to add "extern "C"* in my_test.cpp.

mex function has an extension .cpp, .dll is a C .dll You don't need to add extern "C" to my_test.cpp but you need one in my_mex.cpp.

mex function has an extension .c, .dll is a C .dll You don't need to add any extern "C" neither in my_test.cpp or in my_mex.cpp.

It looks like depending on the extension of the mex function file, MATLAB compiles it as a C or a C++ file. Knowing this, the extern usage should make sense.

All the above is still valid but for a 64 bit MATLAB but you need a 64 bit dll.