allocating “unmanaged” memory in c#

Question

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

Answer 1

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.

Answer 2

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.

Answer 3

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)