Using MPI_Reduce in c - c

Question

I am trying to write a program in c using the MPI library.
In my program I am solving TSP (but not with any special algorithm...).
My input parameters are int nCites, int xCoord[] and int yCoord[].
I am bundling them into Coordinates and using MPI_Bcast to make them available to all the threads.
My problem is this: after I've finished calculating the weight of all the paths in each thread, I want to reduce them into one single result, the best one. I've tried using MPI_Reduce, but something, and this is where I get confused, causes a segmentation fault (only in one of the threads, usually root).
This is the main code and structs:
typedef struct Coordinates_t {
int* x;
int* y;
int n;
} Coordinates;
typedef struct PathAndLength_t {
int* path;
int pathSize;
int length;
} PathAndLength;
void comparePaths(void* a, void* b, int* len, MPI_Datatype* dataType) {
...
}
int tsp_main(int nCites, int xCoord[], int yCoord[], int P[]){
int numOfProcs, rank;
if (MPI_Comm_size(MPI_COMM_WORLD, &numOfProcs))
throw "Error: MPI_Comm_size failed";
if (MPI_Comm_rank(MPI_COMM_WORLD, &rank))
throw "Error: MPI_Comm_rank failed";
Coordinates crds;
crds.x = xCoord;
crds.y = yCoord;
crds.n = nCites;
MPI_Datatype data;
createInDataType(&crds, &data);
if (MPI_Bcast(&crds, 1, data, 0, MPI_COMM_WORLD))
throw "Error: MPI_Comm_size failed";
...
PathAndLength* pal = (PathAndLength*)malloc(sizeof(PathAndLength));
pal->path = (int*)malloc(sizeof(int)*crds.n);
pal->length = min_length;
for (int i = 0; i < crds.n; ++i) {
(pal->path)[i] = min_path[i];
}
pal->pathSize = crds.n;
MPI_Datatype outDatatype;
MPI_Op op;
createOutDataType(pal, &outDatatype);
MPI_Op_create(&comparePaths, 1, &op);
PathAndLength* result = (PathAndLength*)malloc(sizeof(PathAndLength));
result->path = (int*)malloc(sizeof(int)*crds.n);
MPI_Reduce(pal, result, crds.n, outDatatype, op, 0, MPI_COMM_WORLD);
...
return result->length;
}
And these are the createOutDataType and createInDataType I use in my code:
void createInDataType(Coordinates* indata, MPI_Datatype* message_type_ptr) {
// Build a derived datatype
int block_lengths[3];
MPI_Aint displacements[3];
MPI_Aint addresses[4];
MPI_Datatype typelist[3];
// First specify the types
typelist[0] = typelist[1] = typelist[2] = MPI_INT;
// Specify the number of elements of each type
block_lengths[0] = block_lengths[1] = indata->n;
block_lengths[2] = 1;
// Calculate the displacements of the members relative to indata
MPI_Address(indata, &addresses[0]);
MPI_Address(indata->x, &addresses[1]);
MPI_Address(indata->y, &addresses[2]);
MPI_Address(&indata->n, &addresses[3]);
displacements[0] = addresses[1] - addresses[0];
displacements[1] = addresses[2] - addresses[0];
displacements[2] = addresses[3] - addresses[0];
// Create the derived type
MPI_Type_struct(3, block_lengths, displacements, typelist, message_type_ptr);
// Commit it so that it can be used
MPI_Type_commit(message_type_ptr);
}
void createOutDataType(PathAndLength* outdata, MPI_Datatype* message_type_ptr) {
// Build a derived datatype
int block_lengths[2];
MPI_Aint displacements[2];
MPI_Aint addresses[3];
MPI_Datatype typelist[2];
// First specify the types
typelist[0] = MPI_INT;
typelist[1] = MPI_INT;
// Specify the number of elements of each type
block_lengths[0] = outdata->pathSize;
block_lengths[1] = 1;
// Calculate the displacements of the members relative to outdata
MPI_Address(outdata, &addresses[0]);
MPI_Address(outdata->path, &addresses[1]);
MPI_Address(&outdata->length, &addresses[2]);
displacements[0] = addresses[1] - addresses[0];
displacements[1] = addresses[2] - addresses[0];
// Create the derived type
MPI_Type_struct(2, block_lengths, displacements, typelist, message_type_ptr);
// Commit it so that it can be used
MPI_Type_commit(message_type_ptr);
}
Sorry for including so much code, but I couldn't decide what, if any, was irrelevant...
Thank you.

Answer 1

PathAndLength* result = (PathAndLength*)malloc(sizeof(PathAndLength));
result->path = (int*)malloc(sizeof(int)*crds.n);
MPI_Reduce(pal, result, crds.n, outDatatype, op, 0, MPI_COMM_WORLD);
you are receiving crds.n*outDatatypes into result buffer of size sizeof(PathAndLength). You seem to have design flaw.