Portability of sending values stored in char array using MPI - c

Question

If I have a char array that represents, for example, integer value(s), and I used it to send these values via MPI with the appropriate MPI datatype for send and receive operations as follows:
int main(int argc, char* argv[]){
int my_rank; /* rank of process */
MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);
unsigned char buff[100];
if (my_rank == 0){
int n = 99;
int i;
for(i = 0; i < sizeof(n); i++){
buff[i] = (n >> (8 * i)) & 0xFF;
}
MPI_Send(&buff, 1, MPI_INT, 1, 0, MPI_COMM_WORLD);
}
else{
MPI_Recv(&buff, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, NULL);
int n = *(int *)buff;
printf("%d\n", n);
}
/* shut down MPI */
MPI_Finalize();
return 0;
}
Is this portable across machines of different architecture/endiness?
I suspect the only part that is not portable is the conversion from integer value to char array:
int i;
for(i = 0; i < sizeof(n); i++){
buff[i] = (n >> (8 * i)) & 0xFF;
}
But, anyways if not, is there a way to make the above program fully portable with the existence of the char array to store value(s)?

Answer 1

The way you serialize integers into a buffer is not portable. But if you are sending the integer, why not send integers directly, just take endianness into account. Suppose you want to sent 32bits integers:
int n = 99;
int sent = htonl(n);
MPI_Send(&send, 1, MPI_INT, 1, 0, MPI_COMM_WORLD);
For receiving:
int n;
int recv;
MPI_Recv(&recv, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, NULL);
n = ntonl(recv);