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.
Related
I'm pinning and unpinning with:
GCHandle pinArray(object a)
{
return GCHandle.Alloc(a, GCHandleType.Pinned);
}
void unpinArray(GCHandle h)
{
h.Free();
}
before and after an opencl method so array does not move while computing on it. Now I need to exchange backing array pointer with an aligned unmanaged array pointer to have faster read/write operations on it.
But I couldn't find anything like "change value of gchandle backing array pointer" info.
I need something like an "exchange" method:
GCHandle h=pinArray(array);
// how to?
IntPtr oldBackingArray=exchange(h,alignedMallocCSpace(10000000,4096));
// unmanaged operations
copyValues(h,oldBackingArray);
compute(array,...); // only passing with "array" for simplicity everywhere
array[3]=5;
l=array.toList();
compute(array,....);
Console.WriteLine(array[3]);
copyValues(oldBackingArray,h);
freeCSpace(exchange(h,oldBackingArray));
unpinArray(h);
does this need reflection to access and change that variable? There are also many C# methods using those arrays inside compute method so will it give more speed even on C# space too? So I'm tring to let C# to use alignedAlloc space for everything using "array" object until I unpin it.
You don't need to. The GCHandle relates to managed memory; unmanaged memory does not require any kind of GCHandle. Nor is it possible to talk to that memory as though it is a managed array. Instead, you need to accept that the unmanaged data is a pointer and only a pointer. You can abstract over the top of that to hide these details, but it doesn't change the reality. Fortunately, to the casual observer, talking to a SomeType[] is very similar to talking to a SomeType* - as long as you pass the lengths around yourself.
In the future, the upcoming Span<T> does a great job of unifying pointers and arrays, but that is only experimental at the moment.
I have structure:
public struct MyStruct
{
public int a;
public int b;
public byte[] mass;
}
I need:
Pass poiner to "mass" array to C++ unmanaged function.
And after it done all work it will return me pointer to "mass".
So I have the list of MyStruct. And I need to know what the MyStruct in the list contains returned "mass"(wich pointer to I have).
If I know pointer to "mass" can I reduce pointer to 8 bytes and take pointer to MyStruct?
HOW TO :
1.Get IntPtr to "mass" array?
2.Get IntPtr to MyStruct structure?
3.Get MyStruct from IntPtr?
But, do not using any copy procedure, like Marshal.Copy...
Or is there a better way to do what I need ? Can I use pointers like in C++ or IntPtr is enought, and how can I do that?
Assuming that the memory for the array is allocated by the managed code:
When you pass an array to an unmanaged function via P/Invoke, then by default the array is generally pinned in memory by the marshaller so that the memory used by the array does not have to be copied.
You should not need to use an IntPtr at all - you just need to declare the P/Invoke so that it is accepting an array parameter.
However, things are different if the unmanaged code is allocating memory to be returned to the managed code; then things get MUCH more difficult.
Assuming that's not the case, then if you can show us the "C" function declaration we might be able to come up with a P/Invoke declaration.
(I do have a feeling that your situation may be a bit more complicated though...)
Some useful links for you:
http://msdn.microsoft.com/en-us/library/z6cfh6e6%28v=vs.80%29.aspx
http://msdn.microsoft.com/en-us/library/zah6xy75.aspx
How can I pass a pointer to an array using p/invoke in C#?
And some lower level information about the optimizations that the marshaller makes when calling unmanaged code and passing arrays. Essentially, if it can it doesn't make a copy of the data at all:
http://msdn.microsoft.com/en-us/library/23acw07k%28v=vs.80%29.aspx
Take care as the structure may have some specific memory alignment.
You may be interested by this link
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'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".
How can read a value of a variable in memeory, as binary, which I have its pointer address for in C#?
This article shows how to use the Win32 ReadProcessMemory function to do it. Note that using the unsafe keyword is not sufficient, as it does not give you the ability to specify an explicit memory location.
How to write a Memory Scanner using C#
http://www.codeproject.com/KB/cs/sojaner_memory_scanner.aspx
To view the relevant code, you will need to download the project source, and examine the MemoryScanner.cs class.
You could also use the Marshal.Read* functions like Marshal.ReadInt32(). Cast the pointer value to an IntPtr and use helper methods in the Marshall class to get at the data.
Use unsafe
http://msdn.microsoft.com/en-us/library/28k1s2k6.aspx
Check out the unsafe keyword. This allows you to access the memory directly, like in C++.
http://msdn.microsoft.com/en-us/library/28k1s2k6.aspx