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POMPI BCast (broadcast) of a std::vector of structs
 

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  1. POMPI BCast (broadcast) of a std::vector of structs
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    <p>I've got a question regarding passing of a std::vector of structs via MPI.</p> <p>First off, details. I'm using OpenMPI 1.4.3 (MPI-2 compliant) with gcc. Note that I can't use boost MPI or OOMPI -- I'm bound to using this version.</p> <p>I've got a struct to aggregate some data:</p> <pre><code> struct Delta { Delta() : dX(0.0), dY(0.0), dZ(0.0) {}; Delta(double dx, double dy, double dz) : dX(dx), dY(dy), dZ(dz) {}; Delta(const Delta&amp; rhs) : dX(rhs.dX), dY(rhs.dY), dZ(rhs.dZ) {}; double dX; double dY; double dZ; }; typedef std::vector&lt;Delta&gt; DeltaLine; </code></pre> <p>and I have a DeltaLine that I'd like to broadcast, via MPI, to all the nodes.</p> <p>Can I do the following safely and portably? This works for me in my test case. I just want to make sure it's legal and kosher across different platforms and according to the C++ and MPI standards.</p> <p>Thanks! Madeleine.</p> <pre><code> //Create an MPI struct for the Delta class const int nItems=3; int blocklengths[nItems] = {1, 1, 1}; MPI_Datatype types[nItems] = {MPI_DOUBLE, MPI_DOUBLE, MPI_DOUBLE}; MPI_Datatype MPI_DeltaType; MPI_Aint offsets[nItems]; offsets[0] = offsetof(Delta, dX); offsets[1] = offsetof(Delta, dY); offsets[2] = offsetof(Delta, dZ); MPI_Type_create_struct(nItems, blocklengths, offsets, types, &amp;MPI_DeltaType); MPI_Type_commit(&amp;MPI_DeltaType); //This is the vector to be filled, and its size DeltaLine deltaLine; unsigned deltaLineSize; //If this is the master proc, get the DeltaLine and its size if(amMaster()) { deltaLine = getMasterDeltaLine(); deltaLineSize = deltaLine.size(); } //Send out the correct size MPI_Bcast(&amp;deltaLineSize, 1, MPI_UNSIGNED, COMM_PROC, MPI_COMM_WORLD); //Size the delta line vector, and broadcast its contents deltaLine.reserve(deltaLineSize); MPI_Bcast(&amp;deltaLine.front(), deltaLineSize, MPI_DeltaType, COMM_PROC, MPI_COMM_WORLD); //Free up the type MPI_Type_free(&amp;MPI_DeltaType); </code></pre>
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    1. COJust for completeness sake: In order to avoid with padding issues mentioned by Hristo, and the size vs capacity issue I mentioned, on could also follow an approach that represents the data as a flat, 1-dimensional array (plain double* or contained in a std::valarray, e.g.) of 3*N size and transmit a "stride" (= 3) along with the number N of tuples in a first BCast.
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