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POSafe and Correct Struct Marshalling
 

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    <h2>Unmanaged and Managed Memory Regions</h2> <p>I am attempting to execute unmanaged code from a C-library. One of the methods takes a <code>void*</code> as a parameter but under the hood it's cast to a struct of type <code>nc_vlen_t</code></p> <h3>C struct for nc_vlen_t</h3> <pre><code>/** This is the type of arrays of vlens. */ typedef struct { size_t len; /**&lt; Length of VL data (in base type units) */ void *p; /**&lt; Pointer to VL data */ } nc_vlen_t; </code></pre> <p>Executing the method is correct and it works, I am concerned more about the pinning and safe handling of managed and unmanaged memory regions. I want to be as certain as possible that I am not going to cause memory leaks or a SEGFAULT. I wrote a struct that will be marshalled to and from the <code>nc_vlen_t</code> when I execute the C-library method calls.</p> <h3>C# struct</h3> <pre><code>[StructLayout(LayoutKind.Sequential)] public struct VlenStruct { public Int32 len; public IntPtr p; // Data } </code></pre> <p>The struct consists of a <code>size_t</code> that indicates the array length and a <code>void *</code> to the data. Inside the library it has attributes that allow it to cast the (void*) to the appropriate numeric types and I've had great success with that so far. </p> <p>What I want to understand is the best way to handle the memory regions. After reading some articles and other SO questions this is my best <em>guess</em> for how to handle it. I have a class that acts as an arbiter for creating and managing the structs and their memory. I rely on a destructor to free the handle which will unpin the array so that the GC can do it's job. </p> <h3>C# Vlen Helper</h3> <pre><code>public class Vlen { private GCHandle handle; private VlenStruct vlen_t; public Vlen() { isNull = true; } public Vlen(Array t) { isNull = false; handle = GCHandle.Alloc(t, GCHandleType.Pinned); // Pin the array vlen_t.len = t.Length; vlen_t.p = Marshal.UnsafeAddrOfPinnedArrayElement(t, 0); // Get the pointer for &amp;t[0] } ~Vlen() { if(!isNull) { handle.Free(); // Unpin the array } } public VlenStruct ToStruct() { VlenStruct retval = new VlenStruct(); retval.len = vlen_t.len; retval.p = vlen_t.p; return retval; } private bool isNull; } </code></pre> <h3>C Method Declaration</h3> <pre><code> //int cmethod(const int* typep, void *data) // cmethod copies the array contents of the vlen struct to a file // returns 0 after successful write // returns -1 on fail [DllImport("somelib.dll", CharSet = CharSet.Ansi, SetLastError = true, ExactSpelling = true, CallingConvention=CallingConvention.Cdecl)] public static extern Int32 cmethod(ref Int32 typep, ref VlenStruct data); </code></pre> <p>If I use this class to create the struct is there a possibility that the GC will clean the array before the C-library is called in this situation:</p> <h3>C# Use-case</h3> <pre><code>{ double[] buffer vlenBuffer = new double[] { 0, 12, 4}; Vlen data = new Vlen(vlenBuffer); // The instance now pins buffer VlenStruct s = data.ToStruct() Int32 type = VLEN_TYPE; cmethod(ref type, ref s); } </code></pre> <p>Is it possible for the <code>data</code> instance to be cleaned and thereby unpin <code>buffer</code> which could cause unpredictable behavior when executing the external library method?</p>
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    1. CO@HansPassant The C function copies the array contents to a file. It returns after the copy is complete.
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