How to send(MPI_Send) nested structure having pointer fields in MPI using C

Question

I have a structure :

    struct vertex 
    {  
     double a; double b;
    }      

    struct polygon
    {
     int numofVertex;
     vertex *v;
    }

How to send this nested structure in MPI using MPI_Send? The problem is that the structure contains pointer field "v" due to which MPI_Send crashes. I have tried MPI_Datatype to define new data type, it doesnot work. I read that serialization is the only solution but C doesnot provide such implementation. Any suggestions how to get around with this problem?

You're right that C doesn't provide an implementation to serialize arbitrary structures. What's stopping you from writing one for these structures? — Carl Norum
– Carl Norum, Commented May 12, 2011 at 0:21
@Carl I think it is difficult to do so, and in case there is some existing implementation, I dont want to reinvent the wheel. @CarlNorum — justin waugh
– justin waugh, Commented May 12, 2011 at 0:23
I think you're going to have to write it... it shouldn't be that hard. Please come back and ask a specific question if you have a problem with your implementation. — Carl Norum
– Carl Norum, Commented May 12, 2011 at 0:25

ThiefMaster · Accepted Answer · 2011-05-12 06:14:46Z

7

You'll have to send it in two messages:

// 'p' is a pointer to your polygon
vertex *tmp = p->v;
p->v = NULL;
MPI_Send(p, sizeof(struct polygon), MPI_BYTES, dest, 1, ...);
MPI_Send(tmp, sizeof(struct vertex), MPI_BYTES, dest, 2, ...);
p->v = tmp;

On the receiving end, you simply receive the struct in two steps:

polygon p;
MPI_Recv(&p, sizeof(struct polygon), MPI_BYTES, src, 1, ...);
p.vertex = malloc(sizeof(struct vertex));
MPI_Recv(p.vertex, sizeof(struct vertex), MPI_BYTES, src, 2, ...);

Obviously this is not very nice, so it'll be easier if you keep your struct pointer-less so you can send it at once.

answered May 12, 2011 at 6:14

ThiefMaster

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