Problem with C++ DLL in Matlab: The function was not found in the library

Hi there,

I'm trying to load a C++ DLL in Matlab. The compiler I'm using is Visual studio and I'm using Matlab 2012b. Everytime I'm trying to call a function in the library using "calllib" I get the following error:

"Error using calllib Method was not found".

The CPP file I'm using is just a simple add function as follows:

#include "stdafx.h"

#include "MyProject.h"

int MyProejct(int a,int b) { return(a+b); }

The header file I'm using is defined as follows:

#ifdef MYPROJECT_DLL_EXPORTS

#define MYPROJECT_DLL_API extern "C" _declspec(dllexport)

#else

#define MYPROJECT_DLL_API extern "C" _declspec(dllimport) #endif

MYPROJECT_DLL_API int MyProject(int,int);

I also added “MYPROJECT_DLL_EXPORTS” to the list of precompiler defines in the project properties tab.

The loadlibrary is successful when i use the following command: loadlibrary('MyProject.dll','MyProject.h')

But as I use the calllib as follows I get the "method was not found" error:

calllib('MyProject','MyProject', 5,6).

Any help would be appreciated. I researched a lot but I can't find the issue.

Thanks.

Answer 1

[Geoff Hayes]

Golnaz - while the library MyProject.dll may load successfully, are you sure there are no warnings being raised when you do load it? Try running the following when you load the library

 [notfound,warnings] = loadlibrary('MyProject.dll','MyProject.h')

What does the warnings variable show? Is it an empty string or not?

I created a very simple dylib (for my Mac) using the code that you provided above, and missed some key lines. So while I could load the library successfully (using R2014a), I observed the following warning message as the output from the loadlibrary call

 Warning: The function 'MyProject' was not found in the library 
 > In loadlibrary at 406

Several changes later, I got it to work fine (see attached) and could run calllib as

 calllib('MyProject','MyProject',4,38)

 ans =
     42

What I did notice, from your code in the above question, is your definition for the function in the cpp file

 #include "stdafx.h"
 #include "MyProject.h"

 int MyProejct(int a,int b) { return(a+b); }

Note how the method/function name is misspelt - MyProejct rather than MyProject. Could this be the problem?

I used gcc to compile the library as

 gcc -g -O -c MyProject.cpp
 gcc -dynamiclib MyProject.o -o MyProject.dylib

Then, in MATLAB, from the directory where the cpp, h, and dylib existed, I ran

 hfile = fullfile(pwd,'MyProject.h');
 loadlibrary('MyProject',hfile);
 calllib('MyProject','MyProject',4,38);
 unload library MyProject;

[agg gpo]

Dear Geoff,

Thank you for your response.

I actually modified the cpp code (changed MyProejct to MyProject). I also changed the header file to be the same as what you provided. I still get the same error when I use calllib.

In the load library I get the same warning as you expected:

Warning: The function 'MyProject' was not found in the library > In loadlibrary at 403

I'm looking to hear back from you.

Golnaz

[Geoff Hayes]

Hi Golnaz - when I attached the MyProject.h file I wondered if it was a good idea or not. Since it worked for gcc on a Mac there was no guarantee that it would work elsewhere.

I changed the header to the following (see attached as well)

 #ifndef MYMETHOD_H
 #define MYMETHOD_H

 #ifdef __cplusplus
 extern "C" {  // only need to export C interface if
               // used by C++ source code
 #endif

 __declspec( dllimport )  int MyProject(int a,int b);

 #ifdef __cplusplus
 }
 #endif

 #endif

I created the MyProject.dll with this file and the original MyProject.cpp in MS Visual C++ 2005 Express. Then did the following in MATLAB 2014a (as before)

 hfile = fullfile(pwd,'MyProject.h');
 loadlibrary('MyProject',hfile);
 calllib('MyProject','MyProject',4,38);
 unloadlibrary MyProject;

The library load was successful (no warnings) and the calllib returned the correct result of 42.

The above changes create an h-file that is very similar to yours. Looking more closely at what you pasted, I think that all you were missing was an underscore. In the updated header, there are two underscores before declspec as __declspec, whereas in your code, there is just the one underscore, _declspec.

You can try the above, or with your code again (with the corrected function name in the cpp file) and just add the extra underscore.

[agg gpo]

Thank you so much Geoff, it solved the problem. I'm getting the same error (method was not found) for the other functions, so now I'm sure that I've made a mistake in creating the header file.

Thank you so much again, it was a great help.

[Geoff Hayes]

Glad it worked out!

[Royi Avital]

What about Linux and macOS?