Buffer allocated in DLL crashes Matlab

Hi,

I am trying to get MATLAB to work with a DLL I wrote. The DLL allocates a buffer which remains static until a command is called to the DLL to release it.

I've been trying to work out a way to get the pointer to this buffer back to MATLAB and after a bit of research I think I have found a way. The trouble is if the buffer is allocated to be larger than ~200 or so bytes, MATLAB will crash and exit without any warning and I can't figure out why.

To simplify testing I made a much simpler stripped down DLL with just the buffer allocation stuff left over. Could someone have a look and see what I am doing wrong as I can't for the life of me work it out.

(p.s. I know it shouldn't be anything to do with I am using classes/C++ in my DLL as I have that working fine in another one, essentially all of the C++ is surrounded by C wrappers so MATLAB only ever calls C functions.)

I am using 64 bit MATLAB 2013a and 2014b, both do the same thing. The PC is Windows 7 x64 with 32GB RAM and Intel i7-5930 processor. I set the java heap size to the largest possible as well (I think it was ~16GB)

DLL TestingLib.cpp

// TestingDLL.cpp : Defines the exported functions for the DLL application.
//
#include "TestingLib.h"

#include <stdlib.h>

class TestingLib {
private:
  PINT16 _buffer;
  LONG _bufferLen;
public:
  PINT16 getBuffer(){
    return _buffer;
  }
  LONG getLength(){
    return _bufferLen;
  }
public:
  TestingLib(LONG len) {
    if (this) {
      _buffer = (PINT16)malloc(len);
      if (_buffer) {
        for (SHORT i = 0; i < len; i++){
          _buffer[i] = i;
        }
        _bufferLen = len;
      }
      else {
        _bufferLen = -1; //FAIL!
      }
    }
  }
  ~TestingLib(){
    if (_buffer) {
      free(_buffer);
    }
  }
};
typedef TestingLib *PTestingLib;

SYMBOL_DEF PVOID APIENTRY MakeBuffer(LONG length){
  PTestingLib _handle = new TestingLib(length);
  return (PVOID)_handle;
}

SYMBOL_DEF LONG APIENTRY GetBuffer(PVOID handle, PINT16 *buffer) {
  if (handle) {
    PTestingLib _handle = (PTestingLib)handle;
    *buffer = _handle->getBuffer();
    if (!*buffer) return -1;
    return _handle->getLength();
  }
  return -1;
}

SYMBOL_DEF VOID APIENTRY ClearBuffer(PVOID handle) {
  if (handle) {
    delete (PTestingLib)handle;
  }
}

DLL TestingLib.h (shared with MATLAB)

#pragma once
#include <windows.h>

#ifdef TESTINGLIB_EXPORTS
#define SYMBOL_DECLSPEC __declspec(dllexport)
#define SYMBOL_DEF      __declspec(dllexport)
#else
#define SYMBOL_DECLSPEC __declspec(dllimport)
#define SYMBOL_DEF      __declspec(dllimport)
#endif

#ifndef EXTERN_C
#ifdef __cplusplus
#define EXTERN_C extern "C"
#else
#define EXTERN_C
#endif
#endif

EXTERN_C SYMBOL_DEF PVOID APIENTRY MakeBuffer(LONG length);
EXTERN_C SYMBOL_DEF LONG APIENTRY GetBuffer(PVOID handle, PINT16 *buffer);
EXTERN_C SYMBOL_DEF VOID APIENTRY ClearBuffer(PVOID handle);

MATLAB Script:

loadlibrary('TestingLib')
profile on
length = 2047;
handle = calllib('TestingLib','MakeBuffer',length);
bufferPtr = lib.pointer('int16Ptr');
calllib('TestingLib','GetBuffer',handle,bufferPtr);
setdatatype(bufferPtr,'int16Ptr',1,length);
bufferPtr.Value
profile viewer
calllib('TestingLib','ClearBuffer',handle);
bufferPtr = [];
handle = [];

----------------------

In the MATLAB script, when I set the length to <200 it works fine, but if I set it larger than that, MATLAB will just crash. I've attached the compiled library in case you want to try yourself.

Answer 1

[Thomas Carpenter]

Never mind, just noticed a glaringly obvious mistake:

_buffer = (PINT16)malloc(len);

Should be:

_buffer = (PINT16)malloc(sizeof(INT16)*len);

Otherwise the following for loop starts writing into space it hasn't allocated. Duh.

It's funny how going to the effort of asking a question suddenly helps you spot the problem.

Thanks.

[Thomas Carpenter]

Actually, while that did help, it hasn't entirely solved the problem. If is try to use a very large array (which is the eventual aim) of length any higher than ~30000, MATLAB will still just crash without warning.

I wonder if that is pure chance that it coincides with the largest value a signed 16bit integer can store.

EDIT: Yes, it crashes at 32768, but doesn't with a length of 32767.

[Thomas Carpenter]

Ah, this line is wrong too:

    for (SHORT i = 0; i < len; i++)

It will be stuck in an infinite loop if len>32767. It should be:

    for (LONG i = 0; i < len; i++)

I think that solves it :)

[Philip Borghesani]

When writing a library for use with loadlibrary it is usually best to write a simple test program in the same language. Debugging a with a small exe is much faster and simpler then debugging with MATLAB. You can also be sure that when you have problems in MATLAB the problem is in how the function is called or parameters are passed and not worry about if the function actualy works.