Send dynamic array inside a struct over MPI - c

Question

In my C program I have a structure like the one below
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <mpi.h>
typedef struct Message
{
int elemNr;
char *elem;
} Msg;
I think I made all the steps to create custom data type in MPI
int main(int argc, char **argv) {
int size, rank;
int i;
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &size);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
if (size < 2) {
fprintf(stderr,"Requires at least two processes.\n");
exit(-1);
}
// just for simplicity
const int n = 5;
// create a new type for struct message
MPI_Datatype myType;
Msg msgSnd;
int block_length[2] = {1, n};
MPI_Aint elemNrAddr, elemAddr;
MPI_Aint displacement[2] = {0, 0};
MPI_Get_address(&msgSnd.elemNr, &elemNrAddr);
MPI_Get_address(&msgSnd.elem, &elemAddr);
// just displacement[1] because displacement[0] starts from 0
displacement[1] = elemAddr - elemNrAddr;
MPI_Datatype types[2] = {MPI_INT, MPI_CHAR};
MPI_Type_create_struct(2, block_length, displacement, types, &myType);
MPI_Type_commit(&myType);
// populate the message
msgSnd.elemNr = n;
msgSnd.elem = malloc(sizeof(char) * msgSnd.elemNr);
srand(time(NULL));
for (i = 0; i < msgSnd.elemNr; i++)
msgSnd.elem[i] = rand() % 26 + 'a';
if (rank != 0) {
printf("I'm sending\n");
MPI_Send(&msgSnd, 1, myType, 0, 0, MPI_COMM_WORLD);
printf("I sent\n");
} else {
MPI_Status status;
Msg msgRcv;
printf("I'm receiving\n");
MPI_Recv(&msgRcv, 1, myType, 1, 0, MPI_COMM_WORLD, &status);
printf("I received\n");
for (i = 0; i < msgRcv.elemNr; i++)
printf("element %d: %c\n", i, msgRcv.elem[i]);
if (msgRcv.elem != NULL)
free(msgRcv.elem);
}
if (msgSnd.elem != NULL)
free(msgSnd.elem);
MPI_Type_free(&myType);
MPI_Finalize();
return 0;
}
I ran the above code, but unfortunately when process 0 receives the message, the elem pointer points to null and the program ends with segmentation fault.
Can you help me to find the problem? Or, how can I send a dynamic array inside a struct on MPI?