mpi - Trouble with MPI_Allgather -

i trying implement matrix matrix multiplication in mpi using rowwise , columnwise broadcast mpi_allgather. although commented out code part works well, other one(below it) not work(i.e. gives signal -5 , mpi process killed). please? thanks

#include <stdlib.h> #include <string.h> #include "mpi.h"  #define n 4  // nxn origininal(big) matrices size multiply   int sqrtroot(int a) {     switch(a)     {         case 1: return 1 ;          case 4: return 2;             case 9: return 3;          case 16: return 4;          default: return -1;     } }  int main( argc, argv ) int  argc; char **argv; {       int myid, p;     double t1,t2;     double *rowwisea, *colwiseb; // 1d array of size n*n/sqrt(p) hold 2d data of                                   // processors own datatogether data of rowwise neighbours     double* myresult; // result of size of nxn(n = n/p) stored 1d      int mydatastartindex;      int i,j,n;       mpi_comm myrowpartners, mycolpartners;      mpi_init( &argc, &argv );     mpi_comm_size( mpi_comm_world, &p );     mpi_comm_rank( mpi_comm_world, &myid );      mpi_comm_split(mpi_comm_world,myid/sqrtroot(p), myid,&myrowpartners);     mpi_comm_split(mpi_comm_world,myid%sqrtroot(p), myid,&mycolpartners);     int dataperprocess = n*n/(p);      rowwisea = (double*) malloc(n*n/sqrtroot(p) * sizeof(double));     colwiseb = (double*) malloc(n*n/sqrtroot(p) * sizeof(double));     myresult = (double*) malloc(n*n/p * sizeof(double));      mydatastartindex = myid * dataperprocess;      n = sqrtroot(n*n/p); // 1 deimension of local square matrix of nxn      // initialize dat     for(i = 0; < n; i++){         for(j = 0; j < n; j++)  {         rowwisea [mydatastartindex + i*n + j] = (i + j) * (myid + 1);         colwiseb [mydatastartindex + i*n + j] = (i - j) * (myid + 1);         }     }         // ------------ 1 works ------------------     //double* sum = (double*)malloc(4*sizeof(double));     //     //double mydata = myid*1.0f;      //mpi_allgather(     //              &mydata ,                //              1,                   //              mpi_double,          //              sum,             //              1,                  //              mpi_double,          //              myrowpartners       //          );     //       //--------------------------------------------------              //--------this 1 not work----------------------     mpi_allgather(                     &rowwisea[mydatastartindex],                                 dataperprocess,                                      mpi_double,                          rowwisea ,                           dataperprocess,                                 mpi_double,                          myrowpartners                   );     //-----------------------------------------------------       return 0; }  

i found bug. should use new rank inside myrowpartners instead of myid in mpi_allgather , in matrix indexes. though..