C++ DLL crashes when calling from managed code

Question

there's a WinForms-application written in C# using .NET Framework 3.5. This application uses a C++ Dll which is imported using the following declaration:

[DllImport(DllName)]
public static unsafe extern int LoadDBData(String dsn, String userid, String password);

This method imports data from a given ODBC-DSN using a SQL Server database. The call crashes when there is too much data in database. The provider of this extern dll said this happens because the dll is unable to grab more heap size and my application should provide more heap memory.

How could I solve this problem? As far as I know the only possibility to exclude a component from automatic garbage collection is the unsafe keyword which I already used. Any idea would be appreciated.

Thanks in advance

Martin

Answer 1

You can't increase the heap size in .NET. You could create an EXE in c/c++ that your .NET app calls using Process.Start. Your c/c++ EXE would just call the DLL function and return the result (or if you have more than one function it could take a command line parameter). If you don't want a separate EXE you could try using RunDll32 instead.

Answer 2

This seems like a problem with the vendor's library, rather than your code.

Managed and unmanaged memory should be considered to be completely separate. Managed memory is typically memory allocated on a garbage-collected heap, while unmanaged memory is anything else: the ANSI C memory pool allocated through malloc(3), custom memory pools, and garbage-allocated heaps outside the control of the CLI implementation...

Note that the above quote is from the Mono documentation , but I believe (if I'm not mistaken) the same is true for .NET in general. If the data is being loaded in the DLL's internal data structures, then it should allocate its own memory. If you're providing a buffer which will get filled up with the data, then it will only get filled up with as much data as you've allocated for the buffer (and pinned before marshalling). So where is the data being loaded?

Answer 3

I doubt this is specific to .NET, managed memory, garbage collection etc. It's a native DLL so it uses regular, unmanaged memory. Of course, the .NET runtime will also use it's share of memory but a native application using the DLL would do the same.

If you're running in a 32 bit process, the total heap size for .NET and unmanaged code can be limited to 1.5 GB. It's difficult to tell without additional information, but you might have hit that limit.

So one option would be to ask your vendor, whether they have a 64 bit version of the library and switch to a 64 process. In a 64 bit process, memory is almost unlimited (according to today's standard).