Import unmanaged DLL function

I need to import unmanaged DLL fuunction to C#.
C++ function signature is:
char *fnname(const DWORD Id, char *buffer32, DWORD *UId, BYTE *Cid, BYTE *Mask, WORD *ID)

How should C# import look?
I crrently have:
extern static String fnname(Int32 Id,[MarshalAs(UnmanagedType.LPStr)] ref StringBuilder buffer32, Int32 UId,  byte CId,  byte Mask,ref Int16 ID);
This is runable (no exception) but not working. Problem is that buffer which should be preallocated to 32 bytes and should contain empty string.  C++ code receives something wrong.

I have no source code, only LIB and H files.

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