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    <p>I haven't tested any of this, so it may need some changes, but here's what I see off hand.</p> <p>First off, <code>STRUCT</code> should be declared as follows:</p> <pre><code>struct STRUCT { public UInt32 length; } </code></pre> <p>Notice that we changed from <code>class</code> to <code>struct</code> so that the memory layout of the object is known and matches what the C code is expecting.</p> <p><strong>Update</strong></p> <p>After a bit of further consideration, there's an easier way of declaring and calling the method, I'll leave the original answer below for another way of doing it.</p> <p>Your P/Invoke signature should be:</p> <pre><code>[DllImport(LINUXLIB, CallingConvention=CallingConvention.Cdecl)] public static extern int simpletran(ref STRUCT req); </code></pre> <p>Notice that we changed <code>STRUCT</code> to <code>ref STRUCT</code> since <code>STRUCT</code> is a value type and the C code requires a pointer to that structure.</p> <p>And you'd call it like this:</p> <pre><code>STRUCT st = new STRUCT(); st.length = (UInt32)100; simpletran(ref st); </code></pre> <p><strong>Original</strong></p> <pre><code>[DllImport(LINUXLIB, CallingConvention=CallingConvention.Cdecl)] public static extern int simpletran(IntPtr req); </code></pre> <p>Notice that we changed <code>STRUCT</code> to <code>IntPtr</code> since <code>STRUCT</code> has to be a value type and the C code requires a pointer to that structure.</p> <p>And you'd call it like this:</p> <pre><code>STRUCT st = new STRUCT(); st.length = (UInt32)100; IntPtr ptr = Marshal.AllocHGlobal(sizeof(STRUCT)); Marshal.StructureToPtr(st, ptr, false); simpletran(ptr); Marshal.FreeHGlobal(ptr); </code></pre> <p>Adding the extra steps in between creating the instance and calling the method for allocating a chunk of unmanaged memory to store the value of the struct in (<a href="http://msdn.microsoft.com/en-us/library/s69bkh17.aspx" rel="nofollow">Marshal.AllocHGlobal</a>) and copying the value of <code>st</code> into that memory with <a href="http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.marshal.structuretoptr.aspx" rel="nofollow">Marshal.StructureToPtr(...)</a>. Be sure to free the allocated memory after the call with a call to <a href="http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.marshal.freehglobal.aspx" rel="nofollow">Marshal.FreeHGlobal</a></p>
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    1. PODo we have to marshall unsigned long?
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    1. PODo we have to marshall unsigned long?
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    1. COthe reason why i am using class is that class is passed by reference (since it is heap allocated) and struct is passed by value, of course we can use the ref.
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    2. COIt needs to be a struct as you can't control the memory layout of a class, only structs
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