I'm writting a program in c# that uses a C++ library, and for some reason I need to allocate an unmanaged buffer to pass it to the lib. Is there a way to do this in c# ?
Basically I would just need to do a malloc in C#...
Thanks
Try something like this:
using System;
using System.Runtime.InteropServices;
class Example
{
static void Main()
{
IntPtr pointer = Marshal.AllocHGlobal(1024);
}
}
This uses the Marshal.AllocHGlobal method:
Allocates memory from the unmanaged memory of the process by using the specified number of bytes.
You can also use a byte array for this.
You do this by using an unsafe routine and the fixed statement:
static unsafe void PerformOperation()
{
byte[] buf = new byte[1024];
fixed (void* ptr = &buf[0])
{
SomeUnmanagedFunction(new IntPtr(ptr));
}
}
The issue - and this is an important one - is that SomeUnmanagedFunction is not allowed to touch that pointer after it has returned and code has exited the fixed block. So if you do something like this:
static void PerformFabulousTrick()
{
byte[] buf = new byte[1024];
fixed (void *ptr = &buf[0])
{
SetBuffer(ptr, buf.Length);
}
FillBuffer(); // puts data in buf - NOT - may crash hard
}
you are asking for nothing but trouble. In this case you probably want to use a GCHandle, which can pin a managed object in the heap. This can also be troublesome in that you NEED to unpin it in a timely manner or you risk fragmenting your heap.
In general, I would recommend making sure that you're P/Invoking correctly into the function so that the maybe marshaller can do this work for you. I like fixed better than GlobalAlloc since its scope is clear. I can't decide which I like least of GlobalAlloc and GCHandle. Both require you to do more work since the GC or language won't do it for you.
This is how we need to assign and free unmanaged memory by using specific number of bytes.
// Demonstrate how to call GlobalAlloc and
// GlobalFree using the Marshal class.
IntPtr hglobal = Marshal.AllocHGlobal(100);
Marshal.FreeHGlobal(hglobal)
Years later, in the meantime:
In .net6, 7, MS gifted us with the class "NativeMemory" with methods like "AlignedAlloc", which might be useful for vectorized applications: MS writes for this example:
public static void* AlignedAlloc (UIntPtr byteCount, UIntPtr alignment);
"This method is a thin wrapper over the C aligned_alloc API or a platform dependent aligned allocation API such as _aligned_malloc on Win32."
Of course, no garbage collection in that case. One has to release the memory oneself to avoid memory leaks, by using "AlignedFree".
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 am working on a C++ CLI wrapper of a C API. One function in the C API expected data in the form:
void setData(byte* dataPtr, int offset, int length);
void getData(byte* buffer, int offset, int length);
For the C++ CLI it was suggested that we use a System.Collections.BitArray (Yes the individual Bits have meaning). A BitArray can be constructed from an array of bytes and copied to one:
array<System::Byte>^ bytes = gcnew array<System::Byte>(40);
System::Collections::BitArray^ ba = gcnew System::Collections::BitArray(bytes);
int length = ((ba->Length - 1)/8) +1;
array<System::Byte>^ newBytes = gcnew array<System::Byte>(length);
ba->CopyTo(newBytes, 0);
pin_ptr<unsigned char> rawDataPtr = &buffer[0];
My concern is the last line. Is it valid to get a pointer from the array in this way? Is there a better alternative in C# for working with arbitrary bits? Remember the individual bits have meaning.
Is it valid to get a pointer from the array in this way?
Yes, that's valid. The pin_ptr<> helper class calls GCHandle.Alloc() under the hood, asking for GCHandleType.Pinned. So the pointer is stable and can be passed to unmanaged code without fear that the garbage collector is going to move the array and make the pointer invalid.
A very important detail is missing from the question however. The reason that pin_ptr<> exists instead of just letting you use GCHandle directly is exactly when the GCHandle.Free() method will be called. You don't do this explicitly, pin_ptr<> does it for you, it uses the standard C++ RAII pattern. In other words, the Free() method is automatically called, it happens when the variable goes out of scope. Which gets the C++ compiler to emit the destructor call, it in turns calls Free().
This will go very, very wrong when the C function stores the passed dataPtr and uses it later. Later being the problem, the array won't be pinned anymore and can now exist at an arbitrary address. Major data corruption, very hard to diagnose. The getData() function strongly suggests that is fact the case. This is not good.
You will need to fix this, using GCHandle::Alloc() yourself to pin the array permanently is very painful to garbage collector, a rock in the road that won't budge and has a long-lasting effect on the efficiency of the program. Instead you should copy the managed array to stable memory that you allocate with, say, malloc() or Marshal::AllocHGlobal(). That's unmanaged memory, it will never move. Marshal::Copy() is a simple way to copy it.
I want to get data from an IntPtr pointer into a byte array. I can use the following code to do it:
IntPtr intPtr = GetBuff();
byte[] b = new byte[length];
Marshal.Copy(intPtr, b, 0, length);
But the above code forces a copy operation from IntPtr into the byte array. It is not a good solution when the data in question is large.
Is there any way to cast an IntPtr to a byte array? For example, would the following work:
byte[] b = (byte[])intPtr
This would eliminate the need for the copy operation.
Also: how can we determine the length of data pointed to by IntPtr?
As others have mentioned, there is no way you can store the data in a managed byte[] without copying (with the current structure you've provided*). However, if you don't actually need it to be in a managed buffer, you can use unsafe operations to work directly with the unmanaged memory. It really depends what you need to do with it.
All byte[] and other reference types are managed by the CLR Garbage Collector, and this is what is responsible for allocation of memory and deallocation when it is no longer used. The memory pointed to by the return of GetBuffer is a block of unmanaged memory allocated by the C++ code and (memory layout / implementation details aside) is essentially completely separate to your GC managed memory. Therefore, if you want to use a GC managed CLR type (byte[]) to contain all the data currently held within your unmanaged memory pointed to by your IntPtr, it needs to be moved (copied) into memory that the GC knows about. This can be done by Marshal.Copy or by a custom method using unsafe code or pinvoke or what have you.
However, it depends what you want to do with it. You've mentioned it's video data. If you want to apply some transform or filter to the data, you can probably do it directly on the unmanaged buffer. If you want to save the buffer to disk, you can probably do it directly on the unmanaged buffer.
On the topic of length, there is no way to know the length of an unmanaged memory buffer unless the function that allocated the buffer also tells you what the length is. This can be done in lots of ways, as commenters have mentioned (first field of the structure, out paramtere on the method).
*Finally, if you have control of the C++ code it might be possible to modify it so that it is not responsible for allocating the buffer it writes the data to, and instead is provided with a pointer to a preallocated buffer. You could then create a managed byte[] in C#, preallocated to the size required by your C++ code, and use the GCHandle type to pin it and provide the pointer to your C++ code.
Try this:
byte* b = (byte*)intPtr;
Requires unsafe (in the function signature, block, or compiler flag /unsafe).
You can't have a managed array occupy unmanaged memory. You can either copy the unmanaged data one chunk at a time, and process each chunk, or create an UnmanagedArray class that takes an IntPtr and provides an indexer which will still use Marshal.Copy for accessing the data.
As #Vinod has pointed out, you can do this with unsafe code. This will allow you to access the memory directly, using C-like pointers. However, you will need to marshal the data into managed memory before you call any unsafe .NET method, so you're pretty much limited to your own C-like code. I don't think you should bother with this at all, just write the code in C++.
Check out this Code Project page for a solution to working with unmanaged arrays.
I have tons of C code that I need to use in C#. Example:
long foo(char** mystring);
void free_string(char* mystring);
foo() is using malloc() to allocate memory for mystring. I have tried several ways of calling this function from C#, but I am failing to free mystring. Can you please give me some guidelines on how to call foo() so that I can later free mystring?
For example, if char** is represented by StringBuilder[], then how can I use it to be freed in free_string()?
Allocating memory in native to be freed in managed puts quite an overhead on ensuring the caller knows exactly what they are doing, You might want to consider other techniques for allocating the memory in managed code.
One example might be to make a callback into the managed code to get a string buffer
extern "C" __declspec void GetString( char* buffer, int bufferSize );
Matching C# would be the following:
void GetString( StringBuilder buffer, int bufferSize );
If you allocate memory using LocalAlloc in kernel32.dll then you could free it using Marshal.FreeHGlobal(IntPtr). Though you can't free malloc'ed memory with it.
As another solution consider passing C# StringBuilder reference and fill it with C code.
Also take a look at MSDN article on memory models.
I am interfacing with an ActiveX component that gives me a memory address and the number of bytes.
How can I write a C# program that will access the bytes starting at a given memory address? Is there a way to do it natively, or am I going to have to interface to C++? Does the ActiveX component and my program share the same memory/address space?
You can use Marshal.Copy to copy the data from native memory into an managed array. This way you can then use the data in managed code without using unsafe code.
I highly suggest you use an IntPtr and Marshal.Copy. Here is some code to get you started.
memAddr is the memory address you are given, and bufSize is the size.
IntPtr bufPtr = new IntPtr(memAddr);
byte[] data = new byte[bufSize];
Marshal.Copy(bufPtr, data, 0, bufSize);
This doesn't require you to use unsafe code which requires the the /unsafe compiler option and is not verifiable by the CLR.
If you need an array of something other than bytes, just change the second line. Marshal.Copy has a bunch of overloads.
I think you are looking for the IntPtr type. This type (when used within an unsafe block) will allow you to use the memory handle from the ActiveX component.
C# can use pointers. Just use the 'unsafe' keyword in front of your class, block, method, or member variable (not local variables). If a class is marked unsafe all member variables are unsafe as well.
unsafe class Foo
{
}
unsafe int FooMethod
{
}
Then you can use pointers with * and & just like C.
I don't know about ActiveX Components in the same address space.