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POCould accessing a variable directly (rather than with a pointer and dereferecer) give different values?
 

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  1. POCould accessing a variable directly (rather than with a pointer and dereferecer) give different values?
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    <p>I know it's an odd question and I risk showing my greenness, but I'm tearing my hair out here.</p> <p>I have a C++ example that works as expected. Here's a snippet of the bothersome bit:</p> <pre><code>BIRDFRAME frame; birdGetMostRecentFrame(GROUP_ID,&amp;frame); BIRDREADING *bird_data; bird_time=frame.dwTime; for(FBBloop=FBBstart; FBBloop&lt;FBBend; FBBloop++ ) { bird_data = &amp;frame.reading[FBBloop]; // Do stuff } </code></pre> <p>Note that <code>bird_data</code> is a pointer and is assigned the address of <code>frame.reading[FBBloop]</code>. My app is written in C# and so doesn't have any of this pointer malarkey. The corresponding bit looks like this:</p> <pre><code>FlockOfBirds.BIRDFRAME frame = new FlockOfBirds.BIRDFRAME(); FlockOfBirds.birdGetMostRecentFrame(1, ref frame); Console.WriteLine("T:{0}",frame.dwTime.ToString()); FlockOfBirds.BIRDREADING reading; for (int k = 1; k &lt;= sysconf.byNumDevices; k++) { reading = frame.readings[k]; Console.WriteLine(" [{0}][{1}][{2}]", reading.position.nX.ToString(), reading.position.nY.ToString(), reading.position.nZ.ToString()); } </code></pre> <p>The problem is that in the C# example, only <code>reading[1]</code> has meaningful data. In the C++ example both <code>bird_data[1]</code> and <code>bird_data[2]</code> have good data. Oddly, <code>reading[2-n]</code> don't all have zeroed values, but appear to be random and constant. Here's what the output from the C# app looks like:</p> <pre><code>T:17291325 [30708][-2584][-5220] [-19660][1048][-31310] T:17291334 [30464][-2588][-5600] [-19660][1048][-31310] T:17291346 [30228][-2600][-5952] [-19660][1048][-31310] T:17291354 [30120][-2520][-6264] [-19660][1048][-31310] T:17291363 [30072][-2388][-6600] [-19660][1048][-31310] </code></pre> <p>Notice that the top triplet of each entry is slightly different from those of the entries pre- and succeeding it. In the C++ app, the bottom line behaves similarly, but here it stays the same throughout.</p> <p>Could this be to do with the lack of pointing and dereferencing? Or am I barking up entirely the wrong tree? Have I done something else that's obviously stupid? Most importantly: how do I make it work?</p> <hr> <p><strong>Update: Structs &amp; externs</strong></p> <p>C++</p> <pre><code>// Bird reading structure typedef struct tagBIRDREADING { BIRDPOSITION position; // position of receiver BIRDANGLES angles; // orientation of receiver, as angles BIRDMATRIX matrix; // orientation of receiver, as matrix BIRDQUATERNION quaternion; // orientation of receiver, as quaternion WORD wButtons; // button states } BIRDREADING; // Bird frame structure // // NOTE: In stand-alone mode, the bird reading is stored in reading[0], and // all other array elements are unused. In master/slave mode, the "reading" // array is indexed by bird number - for example, bird #1 is at reading[1], // bird #2 is at reading[2], etc., and reading[0] is unused. typedef struct tagBIRDFRAME { DWORD dwTime; // time at which readings were taken, in msecs BIRDREADING reading[BIRD_MAX_DEVICE_NUM + 1]; // reading from each bird } BIRDFRAME; BOOL DLLEXPORT birdGetMostRecentFrame(int nGroupID, BIRDFRAME *pframe); </code></pre> <p>C#:</p> <pre><code>[StructLayout(LayoutKind.Sequential, Pack = 0)] public struct BIRDREADING { public BIRDPOSITION position; public BIRDANGLES angles; public BIRDMATRIX matrix; public BIRDQUATERNION quaternion; public ushort wButtons; } [StructLayout(LayoutKind.Sequential, Pack = 0)] public struct BIRDFRAME { public uint dwTime; [MarshalAs(UnmanagedType.ByValArray, SizeConst = 127)] public BIRDREADING[] readings; } [DllImport(@"Bird.dll", CallingConvention = CallingConvention.Cdecl)] public static extern bool birdGetMostRecentFrame(int nGroupID, ref BIRDFRAME frame); </code></pre> <hr> <p>** Update 2: More structs: **</p> <p>C++</p> <pre><code>#pragma pack(1) // pack the following structures on one-byte boundaries // Bird position structure typedef struct tagBIRDPOSITION { short nX; // x-coordinate short nY; // y-coordinate short nZ; // z-coordinate } BIRDPOSITION; // Bird angles structure typedef struct tagBIRDANGLES { short nAzimuth; // azimuth angle short nElevation; // elevation angle short nRoll; // roll angle } BIRDANGLES; // Bird matrix structure typedef struct tagBIRDMATRIX { short n[3][3]; // array of matrix elements } BIRDMATRIX; // Bird quaternion structure typedef struct tagBIRDQUATERNION { short nQ0; // q0 short nQ1; // q1 short nQ2; // q2 short nQ3; // q3 } BIRDQUATERNION; #pragma pack() // resume normal packing of structures </code></pre> <p>C#</p> <pre><code>[StructLayout(LayoutKind.Sequential, Pack = 1)] public struct BIRDPOSITION { public short nX; public short nY; public short nZ; } [StructLayout(LayoutKind.Sequential, Pack = 1)] public struct BIRDANGLES { public short nAzimuth; // azimuth angle public short nElevation; // elevation angle public short nRoll; // roll angle } [StructLayout(LayoutKind.Sequential, Pack = 1)] public unsafe struct BIRDMATRIX { public fixed short n[9]; } [StructLayout(LayoutKind.Sequential, Pack = 1)] public struct BIRDQUATERNION { public short nQ0; // q0 public short nQ1; // q1 public short nQ2; // q2 public short nQ3; // q3 } </code></pre>
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    1. COI can't say I understand what all your objects might be doing, or why this wouldn't cause a failure but most arrays are zero based, is this definitely supposed to be starting with `int k = 1;`?
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      1. POCould accessing a variable directly (rather than with a pointer and dereferecer) give different values?
      1. USJamie Treworgy
    2. COYeah, it's weird, but definitely starting from 1. 0 is reserved for a special case where there's only one value to query.
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      1. POCould accessing a variable directly (rather than with a pointer and dereferecer) give different values?
      1. USTom Wright
    3. CO"0 is reserved for a special case where there's only one value" - So if you have 2 items, the array will have 3 elements in it with the first one at index 0 being unused? If so, it seems like there should be some thought given to that. Especially since index 0 already handles the 'special case' of a one item array quite nicely.
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      1. USMichael Burr
 

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