Packing Arrays using MPI_Pack - parallel-processing

Question

I am trying to pack an array and send it from one process to another. I am doing the operation on only 2 processes. I have written the following code.
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include "mpi.h"
int main( int argc, char *argv[] )
{
MPI_Init(&argc, &argv);
int myrank, size; //size will take care of number of processes
MPI_Comm_rank(MPI_COMM_WORLD, &myrank) ;
MPI_Comm_size(MPI_COMM_WORLD, &size);
//declaring the matrix
double mat[4]={1, 2, 3, 4};
int r=4;
//Now we will send from one matrix to another using MPI_Pack
// For that we will need buffers which will have same number of rows as number of columns
double snd_buf[r];
double recv_buf[r];
double buf[r];
//total size of the data that is beign sent
int outs=r*8;
int position=0;
MPI_Status status[r];
//packing and sending the data
if(myrank==0)
{
for(int i=0;i<r;i++)
{
MPI_Pack(&mat[i], 1 , MPI_DOUBLE,snd_buf,outs,&position,MPI_COMM_WORLD);
}
MPI_Send (&snd_buf, r , MPI_PACKED, 1 /*dest*/ , 100 /*tag*/ , MPI_COMM_WORLD);
}
//receiving the data
if(myrank==1)
{
MPI_Recv(recv_buf, r, MPI_PACKED, 0 /*src*/ , 100 /*tag*/, MPI_COMM_WORLD,&status[0]);
position=0;
for(int i=0;i<r;i++)
{
MPI_Unpack(recv_buf,outs,&position,&buf[i], 1, MPI_DOUBLE, MPI_COMM_WORLD);
}
}
//checking whether the packing in snd_buff is taking place correctly or not
if(myrank==1)
{
for(int i=0;i<r;i++)
{
printf("%lf ",buf[i]);
}
printf("\n");
}
MPI_Finalize();
return 0;
}
I am expecting the output--> 1 2 3 4 but I am only getting 0 0 0 0 in my output.
I was suspecting whether it is a problem of the snd_buffer, but the snd_buffer seems to be fine as it is having all elements 1 2 3 4 correctly.
I have also tried to send and receive like this
//packing and sending the data
if(myrank==0)
{
{
MPI_Pack(&mat[0], 4 , MPI_DOUBLE,snd_buf,outs,&position,MPI_COMM_WORLD);
}
MPI_Send (snd_buf, r , MPI_PACKED, 1 /*dest*/ , 100 /*tag*/ , MPI_COMM_WORLD);
}
//receiving the data
if(myrank==1)
{
MPI_Recv(recv_buf, r, MPI_PACKED, 0 /*src*/ , 100 /*tag*/, MPI_COMM_WORLD,&status[0]);
position=0;
{
MPI_Unpack(recv_buf,outs,&position,&buf[0], 4, MPI_DOUBLE, MPI_COMM_WORLD);
}
Still, this was of no help and the output was only 0s.
I am not able to get why I am facing this error. Any help will be appreciated. Thank you.

Answer 1

Answering my own question. The mistake that I have done was pointed out by Gilles in the comments.
This is the solution to the problem that I have faced
send/recv outs MPI_PACKED (instead of r).
PS--> Consider declaring send_buf and recv_bufas char[] in order to avoid this kind of confusion. char[] won't solve the issue, but make the code more readable (and more obvious sending/receiving r MPI_PACKED is not the right thing to do)