CODEWITHSUNDEEP
linux.net-coremarshallingkernel32virtualalloc
Hadar
Hadar

Reputation: 11

Marshal.AllocHGlobal instead of VirtualAlloc from kernel32.dll on dotnet core 6

I have some legacy unmanaged code (unsafe, intptr) which uses VirtualAlloc method from kernel32.dll many times in order to allocate unmanaged intptrs. This code is in a project which was upgraded from .net framework to .net 6 (core).

When I publish and run this code on linux ubuntu platform I get this exception:

An unhandled exception was thrown by the application.
   2022-11-03T10:55:31.08+0200 [APP/PROC/WEB/0] OUT System.AggregateException: One or more errors occurred. (Unable to load shared library 'kernel32.dll' or one of its dependencies. In order to help diagnose loading problems, consider setting the LD_DEBUG environment variable: libkernel32.dll: cannot open shared object file: No such file or directory)

I assume kernel32.dll cannot be used in .net core as a cross platform dll?

I tried to use Marshal.AllocHGlobal() instead, but I am getting this exception:

attempted to read write protected memory

because some intptr sizes in application are now corrupted

Marshal.AllocHGlobal() and VirtualAlloc() does the same? I cannot use Marshal.AllocHGlobal() instead of VirtualAlloc() without changing the sizes of variables in the application?

Upvotes: 1

Views: 340

Answers (1)

Hadar
Hadar

Reputation: 11

VirtualAlloc method can allocate memory with IntPtr.Zero:

VirtualAlloc(IntPtr.Zero, size, MEM_COMMIT | MEM_RESERVE | f, PAGE_READWRITE);

The problem was that Marshal.AllocHGlobal was allocating memory with garbage. From msdn: https://learn.microsoft.com/en-us/dotnet/api/system.runtime.interopservices.marshal.allochglobal?view=netframework-4.8

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.

Therefore writing '0' to array solved the problem:

 var zeroPtr = Marshal.AllocHGlobal(size);
 for (int i = 0; i < size; i++)
 {
      Marshal.WriteByte(zeroPtr, i, 0);
 }

Upvotes: 0

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