I'm writing a C# library where the calling app will pass in a large amount of contiguous, unmanaged memory. This calling app can be either from .Net or Visual C++ (it will go through an intermediate C++/CLI library before calling my library if from C++). It would be useful to validate that there is sufficient memory, so I decided to call the _msize() function. Unfortunately, _msize always seems to give me the wrong size back.
I went back and modified my allocation routine in my sample app and then immediately call _msize. Here is my code snipet:
public unsafe class MyMemory
{
/// <returns></returns>
[DllImport("msvcrt.dll", SetLastError = true)]
public static extern int _msize(IntPtr handle);
public static IntPtr MyAlloc(int size)
{
IntPtr retVal = Marshal.AllocHGlobal(size);
...
int memSize = MyMemory._msize(retVal);
if (memSize < size)
{
...
}
return retVal;
}
When I pass in the size 199229440, I get back memSize of 199178885. I've seen similar results for different numbers. It is less than 0.01% off, which I would totally understand if it was over, but the fact is it is under, meaning _msize thinks the allocated memory is less than what was asked for. Anyone have any clue why this is? And any recommendations on what I should do instead would be appreciated as well.
P/Invoke the LocalSize function instead.
_msize is for determining the size of a block allocated with malloc (and its friends). AllocHGlobal is a wrapper around GlobalAlloc or LocalAlloc (depending on what reference you believe; but I think the two are equivalent), and you want the LocalSize function to determine the size of the block that actually returned. So far as I can tell, Marshal doesn't contain a wrapper for LocalSize, but you can call it using P/Invoke.
So it seems like it's only by sheer good luck that _msize is returning anything useful for you at all. Perhaps malloc uses GlobalAlloc (or LocalAlloc), either always or just when asked for large blocks, and requests a bit of extra space for bookkeeping; in which case _msize would be trying to compensate for that.
Related
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.
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.
I am using PInvoke to call a C++ function from my C# program. The code looks like this:
IntPtr data = Poll(this.vhtHand);
double[] arr = new double[NR_FINGERS /* = 5 */ * NR_JOINTS /* = 3*/];
Marshal.Copy(data, arr, 0, arr.Length);
With Poll()'s signature looking like this:
[DllImport("VirtualHandBridge.dll")]
static public extern IntPtr Poll(IntPtr hand);
The C-function Poll's signature:
extern "C" __declspec(dllexport) double* Poll(CyberHand::Hand* hand)
Unless I'm having a huge brain failure (admittedly, fairly common for me), this looks to me like it should be working.
However, the double values I am getting are completely incorrect, and I think this is because of incorrect memory usage. I have looked it up, and I think doubles in C# and C++ are identical in size, but maybe there is some other issue playing here. One thing that rubs me the wrong way is that Marshal.Copy is never told what type of data it should expect, but I read that it is supposed to be used this way.
Any clues, anyone? If needed, I can post the correct results and the returned results.
You are missing the CallingConvention property, it is Cdecl.
You really want to favor a better function signature, the one you have is extremely brittle due to the memory management problem, the required manual marshaling, the uncertainty of getting the right size array and the requirement to copy the data. Always favor the caller passing a buffer that your native code fills in:
extern "C" __declspec(dllexport)
int __stdcall Poll(CyberHand::Hand* hand, double* buffer, size_t bufferSize)
[DllImport("foo.dll")]
private static extern int Poll(IntPtr hand, double[] buffer, int bufferSize)
Use the int return value to report a status code. Like a negative value to report an error code, a positive value to return the number of elements actually copied into the buffer.
You shouldn't even need to marshal the data like that, as long as you declare the P/Invoke correctly.
If your CyberHand::Hand* is in reality a pointer to a double, then you should declare your P/Invoke as
[DllImport("VirtualHandBridge.dll")]
static public extern IntPtr Poll(double[] data);
And then just call it with your array of doubles.
If it isn't really an array of doubles, then you certainly can't do what you're doing.
Also, how does your 'C' function know how big the array will be? Is it a fixed size?
The IntPtr return value will be a problem. What is the double* pointing to? An array or a single item?
You could find that it's easier (if you can) to write a simpler more friendly 'C' wrapper for the function you're calling, and call the wrapper function itself. You can of course only do that if you can change the source code of the 'C' DLL. But without knowing exactly what your function does, I can't give you specific advice.
[EDIT]
Ok, your code should theoretically work if the memory being passed back isn't being messed around with (e.g. freed up). If it's not working, then I suspect something like that is happening. You'd definitely be better writing a wrapper 'C' function that fills in an array allocated by the C# and passed to the function, rather than passing back a pointer to some internal memory.
BTW: I don't like code which passes around pointers to blocks of memory without also passing the size of that block. Seems a bit prone to nasty things.
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.
Here's the native (Delphi 7) function:
function Foo(const PAnsiChar input) : PAnsiChar; stdcall; export;
var
s : string;
begin
s := SomeInternalMethod(input);
Result := PAnsiChar(s);
end;
I need to call this from C#, but the name of the dll is not known at compile time - so I must use LoadLibrary to get to it.
This is what my C# code looks like so far:
[DllImport("kernel32.dll")]
public extern static IntPtr LoadLibrary(String lpFileName);
[DllImport("kernel32.dll")]
public extern static IntPtr GetProcAddress(IntPtr handle, string funcName);
[UnmanagedFunctionPointer(CallingConvention.StdCall)]
private delegate string FooFunction(string input);
...
IntPtr dllHandle = LoadLibrary(dllName);
IntPtr fooProcAddr = GetProcAddress(dllHandle, "Foo");
FooFunction foo = (FooFunction)Marshal.GetDelegateForFunctionPointer(
fooProcAddr, typeof(FooFuncion)
);
string output = foo(myInputString);
Now, this actually works - at least, the delphi code receives the string correctly, and the C# code receives the output string.
However, I've noticed some weirdness when debugging the delphi code when it's called from the C# code - the debugger skips lines when it shouldn't..
And I'm concerned that I'm leaking memory - is anyone cleaning up those PChars?
Can anyone give me some feedback / advice on how this should be done?
The only reasonable thing that you can do is trash this function and rewrite it. There is no way this is ever going to work. s is a local string variable of the Foo() function, so the memory the string occupies will be freed when you leave Foo(). The pointer you return points to an invalid memory location, which by chance still contains the string data. If you use a memory manager that clears the memory when pointers to it are freed it won't even contain the data any more. If memory is reused it will contain something else, if the block containing that chunk of memory is released you will get an AV.
There are more questions here on StackOverflow how to return character sequence data from a DLL. Either use a string type that is compatible with the way the Windows API does business, a COM string, or pass a preallocated buffer to your function and fill that with data. In the latter case you can use the same way of using your function like with every similar API function.
For memory leak detection you can use the open source FastMM4 memory manager for Delphi.
"FastMM is a lightning fast
replacement memory manager for
Embarcadero Delphi Win32 applications
that scales well in multi-threaded
applications, is not prone to memory
fragmentation, and supports shared
memory without the use of external
.DLL files."
It is great for speed, leak checking and memory sharing between dll's.
Also very useful is the FastMM4 Options Interface which helps to configure FastMM4.