distributed algorithm in C - c

Question

I am a beginner in C. I have to create a distributed architecture with the library MPI. The following code is:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <time.h>
#include <mpi.h>
int main(int argc, char **argv)
{
int N, w = 1, L = 2, M = 50; // with N number of threads
int T= 2;
int myid;
int buff;
float mit[N][T]; // I initialize a 2d array
for(int i = 0; i < N; ++i){
mit[i][0]= M / (float) N;
for (int j = 1; j < T; ++j){
mit[i][j] = 0;
}
}
float tab[T]; // 1d array
MPI_Status stat;
/*********************************************
start
*********************************************/
MPI_Init(&argc,&argv); // Initialisation
MPI_Comm_size(MPI_COMM_WORLD, &N);
MPI_Comm_rank(MPI_COMM_WORLD, &myid);
for(int j = 0; j < T; j++) {
for(int i = 0; i < N; i++) { // I iterate for each slave
if (myid !=0) {
float y = ((float) rand()) / (float) RAND_MAX;
mit[i][j + 1] = mit[i][j]*(1 + w * L * y);
buff=mit[i][j+1];
MPI_Send(&buff, 128, MPI_INT, 0, 0, MPI_COMM_WORLD); // I send the variable buff to the master
buff=0;
}
if( myid == 0 ) { // Master
for(int i = 1; i < N; i++){
MPI_Recv(&buff, 128, MPI_INT, i, 0, MPI_COMM_WORLD, &stat);
tab[j] += buff; // I need to receive all the variables buff sent by the salves, sum them and stock into the tab at the index j
}
printf("\n%.20f\n",tab[j]); // I print the result of the sum at index j
}
}
}
MPI_Finalize();
return 0;
}
}
I use the command in the terminal: mpicc .c -o my_file to compile the program
Then mpirun -np 101 my_file_c to start the program with 101 threads
But the problem is I have the following error int the terminal:
It seems that [at least] one of the processes that was started with
> mpirun did not invoke MPI_INIT before quitting (it is possible that
> more than one process did not invoke MPI_INIT -- mpirun was only
> notified of the first one, which was on node n0).
>
> mpirun can *only* be used with MPI programs (i.e., programs that
> invoke MPI_INIT and MPI_FINALIZE). You can use the "lamexec" program
> to run non-MPI programs over the lambooted nodes.
It seems that I have a problem with the master but i don't know why...
Any idea ???
Thank you :)

Answer 1

This behavior is very likely the result of a memory corruption.
You cannot
int buff=mit[i][j+1];
MPI_Send(&buff, 128, MPI_INT, ...);
depending on what you want to achieve, you can try instead
int buff=mit[i][j+1];
MPI_Send(&buff, 1, MPI_INT, ...);
// ...
MPI_Recv(&buff, 1, MPI_INT, ...);
or
int *buff=&mit[i][j+1];
MPI_Send(buff, 128, MPI_INT, ...);
// fix MPI_Recv()