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PInvoke: Allocate memory in C++ and free it in C#

We are using PInvoke to interop between C# and C++.
I have an interop struct as follows, with an identical layout C++ struct on the other side.
[StructLayout(LayoutKind.Sequential)]
public struct MeshDataStruct : IDisposable
{
public MeshDataStruct(double[] vertices, int[] triangles , int[] surfaces)
{
_vertex_count = vertices.Length / 3;
_vertices = Marshal.AllocHGlobal(_vertex_count*3*sizeof (double));
Marshal.Copy(vertices, 0, _vertices, _vertex_count);
}
// .. extract data methods to double[] etc.
private IntPtr _vertices;
private int _vertex_count;
public void Dispose()
{
if (_vertices != IntPtr.Zero)
{
Marshal.FreeHGlobal(_vertices);
_vertices = IntPtr.Zero;
}
}
}
Now I would like to add a second ctor
public MeshDataStruct(bool filled_in_by_native_codee)
{
_vertex_count = 0;
_vertices = IntPtr.Zero;
}
and then write a method in C++ that allows C++ to fill in the data. This would allow us to use the same structure for input as well as output data...
However, as far as I understand it, AllocHGlobal is available in C# and C++/Cli, but not pure C++.
So my question is: How can I allocate memory in C++ such that I can safely free it on the C# side with a call to Marshal.FreeHGlobal(...)?

This traditionally always ended up poorly, the Microsoft CRT created its own heap with HeapCreate() to service malloc/new calls in a C or C++ program. Can't deallocate such memory in C#, you don't have the heap handle.
That has changed however, starting with the CRT included with VS2012 (msvcr120.dll and up). It now uses the default process heap, the one returned by GetProcessHeap(). Also the one used by Marshal.Alloc/FreeHGlobal(). So you now have a shot at it, provided the native code doesn't use the debug allocator (crtdbg.h). Be careful throwing away that debug option.
The pinvoke marshaller was not changed, nor can it. If it has to release memory, like an array or string returned as a function return value, then it will call CoTaskMemFree(). It is not clear from your question which could apply. In case of doubt and if you have the choice in your native code then you can't go wrong with CoTaskMemAlloc(), paired to Marshal.FreeCoTaskMem() in your C# code.

From the documentation:
AllocHGlobal is one of two memory allocation methods in the Marshal
class. (Marshal.AllocCoTaskMem is the other.) This method exposes the
Win32 LocalAlloc function from Kernel32.dll.
When AllocHGlobal calls LocalAlloc, it passes a LMEM_FIXED flag, which
causes the allocated memory to be locked in place. Also, the allocated
memory is not zero-filled.
So, you can call LocalAlloc from your unmanaged code to allocate memory, and Marshal.FreeHGlobal from your managed code to deallocate it. Likewise, LocalFree can be be used in unmanaged code to deallocate memory allocated with Marshal.AllocHGlobal.
As the documentation also intimates, you could do the same with CoTaskMemAlloc/CoTaskMemFree and Marshal.AllocCoTaskMem/FreeCoTaskMem.
Having said that, you are setting yourself up for a fall doing it this way. It is far cleaner to keep the allocation and deallocation in the same modules. Mixing an matching in this way is very likely to lead to great confusion over who is responsible for deallocating the memory.

P/Invoke with arrays of double - marshalling data between C# and C++

I've read the various MSDN pages on C++ Interop with P/Invoke here and here but I am still confused.
I have some large arrays of doubles that I need to get into native code, and some resulting arrays that need to get back. I do not know the sizes of the output arrays in advance. For simplicity, I will use only a single array in the example. The platform is x64; I read that marshalling internals are quite different between 32- and 64-bit environments so this might be important.
C#
[DllImport("NativeLib.dll")]
public static extern void ComputeSomething(double[] inputs, int inlen,
[Out] out IntPtr outputs, [Out] out int outlen);
[DllImport("NativeLib.dll")]
public static extern void FreeArray(IntPtr outputs);
public void Compute(double[] inputs, out double[] outputs)
{
IntPtr output_ptr;
int outlen;
ComputeSomething(inputs, inputs.Length, out output_ptr, out outlen);
outputs = new double[outlen];
Marshal.Copy(output_ptr, outputs, 0, outlen);
FreeArray(output_ptr);
}
C++
extern "C"
{
void ComputeSomething(double* inputs, int input_length,
double** outputs, int* output_length)
{
//...
*output_length = ...;
*outputs = new double[output_length];
//...
}
void FreeArray(double* outputs)
{
delete[] outputs;
}
}
It works, that is, I can read out the doubles I wrote into the array on the C++ side. However, I wonder:
Is this really the right way to use P/Invoke?
Aren't my signatures needlessly complicated?
Can P/Invoke be used more efficiently to solve this problem?
I believe I read that marshalling for single dimensional arrays of built-in types can be avoided. Is there a way around Marshal.Copy?
Note that we have a working C++/Cli version, but there are some problems related to local statics in third-party library code that lead to crashes. Microsoft marked this issue as WONTFIX, which is why I am looking for alternatives.

It is okayish. The complete lack of a way to return an error code is pretty bad, that's going to hurt when the arrays are large and the program runs out of memory. The hard crash you get is pretty undiagnosable.
The need to copy the arrays and to explicitly release them doesn't win any prizes of course. You solve that by letting the caller pass a pointer to its own array and you just write the elements. You however need a protocol to let the caller figure out how large the array needs to be, that is going to require calling the method twice. The first call returns the required size, the second call gets the job done.
A boilerplate example would be:
[DllImport("foo.dll")]
private static int ReturnData(double[] data, ref int dataLength);
And a sample usage:
int len = 0;
double[] data = null;
int err = ReturnData(data, ref len);
if (err == ERROR_MORE_DATA) { // NOTE: expected
data = new double[len];
err = ReturnData(data, len);
}
No need to copy, no need to release memory, good thing. The native code can corrupt the GC heap if it doesn't pay attention to the passed len, not such a good thing. But of course easy to avoid.

If it were practical to separate the code that determines the output length from the code that populates the output then you could:
Export a function that returned the output length.
Call that from the C# code and then allocate the output buffer.
Call the unmanaged code again, this time asking it to populate the output buffer.
But I'm assuming that you have rejected this option because it is impractical. In which case your code is a perfectly reasonable way to solve your problem. In fact I would say that you've done a very good job.
The code will work just the same in x86 once you fix the calling convention mismatch. On the C++ side the calling convention is cdecl, but on the C# side it is stdcall. That doesn't matter on x64 since there is only one calling convention. But it would be a problem under x86.
Some comments:
You don't need to use [Out] as well as out. The latter implies the former.
You can avoid exporting the deallocator by allocating off a shared heap. For instance CoTaskMemAlloc on the C++ side, and then deallocate with Mashal.FreeCoTaskMem on the C# side.

If you knew the array size beforehand, you could write a C++/CLI DLL that takes the managed array as parameter, pins it, and calls the native C++ DLL on the pinned pointer it obtains.
But if it's output-only, I don't see any version without a copy. You can use a SAFEARRAY so P/Invoke does the copying instead of you, but that's all.

Receiving char* from C++ method, converting to managed string, then deallocating memory

I have a third party unmanaged C++ dll that I need to call from C#. The C++ function returns a char*. I've figured out how to convert that to a managed string in C#. But I don't know if I need to deallocate the memory. The following code works except that Marshal.FreeHGlobal(p) throws "The handle is invalid.". So do I need to deallocate the memory, and if so, how?
[DllImport("abc.dll", EntryPoint = "?GetVersion#ABC##QEAAPEADXZ", ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
private static extern unsafe char* GetVersionRaw();
public static unsafe string GetVersion()
{
char* x = Abc.GetVersionRaw(); // call the function in the unmanaged DLL
IntPtr p = new IntPtr(x);
string s = Marshal.PtrToStringAnsi(p);
Marshal.FreeHGlobal(p);
return s;
}

It is impossible to say in general how to free an arbitrary pointer returned from a C++ function. There are many ways the pointer could have been produced (new, malloc, VirtualAlloc when you're on Windows, etc). There is no way to determine by inspection which one the pointer came from, and using the wrong deallocation function is an error (which may crash your process). Also, even if you know which function to call, your code may be linked against a different version of the language runtime library - so you might be correctly calling free or delete, but an incompatible version.
Normal procedure is for libraries that perform allocation to provide a corresponding deallocation function that frees the allocated memory appropriately. If the library you're using doesn't have this, you'll just have to try what they've told you and follow up if it doesn't work.

Are there memory security levels in .NET interop?

I have a quite strange problem:
I am testing several function calls to a unmanaged C dll with NUnit. The odd thing is, the test fails when it runs normally, but when i run it with the debugger (even with no break point) it passes fine.
So, has the debugger a wider memory access as the plain NUnit application?
i have isolated the call which fails. its passing back a char pointer to a string, which the marshaller should convert to a C# string. the C side looks like this:
#define get_symbol(a) ((a).a_w.w_symbol->s_name)
EXTERN char *atom_get_symbol(t_atom *a);
...
char *atom_get_symbol(t_atom *a) {
return get_symbol(*a);
}
and the C# code:
[DllImport("csharp.dll", EntryPoint="atom_get_symbol")]
[return:MarshalAs(UnmanagedType.LPStr)]
private static extern string atom_get_symbol(IntPtr a);
the pointer which is returned from c is quite deep inside the code and part of a list. so do i just miss some security setting?
EDIT: here is the exception i get:
System.AccessViolationException : (translated to english:) there was an attempt to read or write protected memory. this might be an indication that other memory is corrupt.
at Microsoft.Win32.Win32Native.CoTaskMemFree(IntPtr ptr)
at ....atom_get_symbol(IntPtr a)
SOLUTION:
the problem was, that the marshaller wanted to free the memory which was part of a C struct. but it sould just make a copy of the string and leave the memory as is:
[DllImport("csharp.dll", EntryPoint="atom_get_symbol")]
private static extern IntPtr atom_get_symbol(IntPtr a);
and then in the code get a copy of the string with:
var string = Marshal.PtrToStringAnsi(atom_get_symbol(ptrToStruct));
great!

This will always cause a crash on Vista and up, how you avoided it at all isn't very clear. The stack trace tells the tale, the pinvoke marshaller is trying to release the string buffer that was allocated for the string. It always uses CoTaskMemFree() to do so, the only reasonable guess at an allocator that might have been used to allocate the memory for the string. But that rarely works out well, C or C++ code almost always uses the CRT's private heap. This doesn't crash on XP, it has a much more forgiving memory manager. Which produces undiagnosable memory leaks.
Notable is that the C declaration doesn't give much promise that you can pinvoke the function, it doesn't return a const char*. The only hope you have is to declare the return type as IntPtr instead of string so the pinvoke marshaller doesn't try to release the pointed-to memory. You'll need to use Marshal.PtrToStringAnsi() to convert the returned IntPtr to a string.
You'll need to test the heck out of it, call the function a billion times to ensure that you don't leak memory. If that test crashes with an OutOfMemoryException then you have a big problem. The only alternative then is to write a wrapper in the C++/CLI language and make sure that it uses the exact same version of the CRT as the native code so that they both use the same heap. Which is tricky and impossible if you don't have the source code. This function is just plain difficult to call from any language, including C. It should have been declared as int atom_get_symbol(t_atom* a, char* buf, size_t buflen) so it can be called with a buffer that's allocated by the client code.

Passing C# struct to C++/CLI for C++ wrapper

After posting a question yesterday I thought I had this cleared up but I'm still having problems, I have a C++/CLI wrapper for a C++ class, some functions of the C++ class take buffers for recv as parameters, the packet structures are defined as C++ structs and that is what is taken as a parameter.
In C# I have replicated these C++ structs using structlayout so that I have equivalent structs in C# which are laid out the same in memory as my C++ structs. In my C++/CLI code I attempted the following
UINT GetValues(value class^ JPVals) // value class, as C# structs are value types
{
IntPtr ptr;
Marshal::StructureToPtr(JPVals,ptr,false);
return m_pComms->GetValues(ptr,0); // m_pComms is a wrapped unmanaged class
//GetValues takes a pointer to a C++ struct
}
The error I get is cannot convert parameter 1 from 'System::IntPtr' to 'SJPVal *', why is it not possible to Marshall from value class to C++ struct pointer? And in this case what should I be passing in and how should I be marshalling it?

You didn't get the serialization process:
// !! Note the % !!
UINT GetValues(value class% JPVals) // value class, as C# structs are value types
{
// Allocate a buffer for serialization, pointer to NULL otherwise
IntPtr ptr = Marshal::AllocHGlobal(Marshal::SizeOf(JPVals));
try {
// Serialize the managed object to "static" memory (not managed by the GC)
Marshal::StructureToPtr(JPVals, ptr, false);
// Pass it to unmanaged code that will modify it.
auto ret = m_pComms->GetValues(reinterpret_cast<SJPVal*>(ptr.ToPointer()), 0);
// Copies the modifications back
Marshal::PtrToStructure(ptr, JPVals);
// Free resources
Marshal::FreeHGlobal(ptr);
return ret;
} catch (...) {
// Make sure we free the memory
Marshal.FreeHGlobal(ptr);
throw;
}
}
EDIT: shown how to copy back the value.
As you are using a C# struct you need to pass it by reference to make sure the changes are copied back. Alternatively, the code will work the same with a C# class. The first step (StructureToPtr) is probably useless, now, since you probably don't care about what was in there before your call to GetValues.
By the way your naming convention is a bit bad. You should NOT start variable names by a capital letter in C++.