What should be the value of message in MPI send-receive program? - c

Question

I have developed a given simple MPI program such that process 0 sends message to process 1 and receives message from process p-1. Following is the code :
In the skeleton given to me ,
char *message;
message= (char*)malloc(msg_size);
is confusing me. To check the correctness of program, I am trying to look value of message that been sent or received. So should it be hexadecimal value?
int main(int argc, char **argv)
{
double startwtime, endwtime;
float elapsed_time, bandwidth;
int my_id, next_id; /* process id-s */
int p; /* number of processes */
char* message; /* storage for the message */
int i, k, max_msgs, msg_size, v;
MPI_Status status; /* return status for receive */
MPI_Init( &argc, &argv );
MPI_Comm_rank( MPI_COMM_WORLD, &my_id );
MPI_Comm_size( MPI_COMM_WORLD, &p );
if (argc < 3)
{
fprintf (stderr, "need msg count and msg size as params\n");
goto EXIT;
}
if ((sscanf (argv[1], "%d", &max_msgs) < 1) ||
(sscanf (argv[2], "%d", &msg_size) < 1))
{
fprintf (stderr, "need msg count and msg size as params\n");
goto EXIT;
}
**message = (char*)malloc (msg_size);**
if (argc > 3) v=1; else v=0; /*are we in verbose mode*/
/* don't start timer until everybody is ok */
MPI_Barrier(MPI_COMM_WORLD);
int t=0;
if( my_id == 0 ) {
startwtime = MPI_Wtime();
// do max_msgs times:
// send message of size msg_size chars to process 1
// receive message of size msg_size chars from process p-1
while(t<max_msgs) {
MPI_Send((char *) message, msg_size, MPI_CHAR, 1 , 0, MPI_COMM_WORLD);
MPI_Recv((char *) message, msg_size, MPI_CHAR, p-1, 0, MPI_COMM_WORLD, &status);
t++;
}
MPI_Barrier(MPI_COMM_WORLD);
endwtime = MPI_Wtime();
elapsed_time = endwtime-startwtime;
bandwidth = 2.0 * max_msgs * msg_size / (elapsed_time);
printf("Number, size of messages: %3d , %3d \n", max_msgs, msg_size);
fflush(stdout);
printf("Wallclock time = %f seconds\n", elapsed_time );
fflush(stdout);
printf("Bandwidth = %f bytes per second\n", bandwidth);
fflush(stdout);
} else if( my_id == p-1 ) {
// do max_msgs times:
// receive message of size msg_size from process to the left
// send message of size msg_size to process to the right (p-1 sends to 0)
while(t<max_msgs) {
MPI_Send((char *) message, msg_size, MPI_CHAR, 0, 0, MPI_COMM_WORLD);
MPI_Recv((char *) message, msg_size, MPI_CHAR, my_id-1, 0, MPI_COMM_WORLD, &status);
t++;
}
} else {
while(t<max_msgs) {
MPI_Send((char *) message, msg_size, MPI_CHAR, my_id+1, 0, MPI_COMM_WORLD);
MPI_Recv((char *) message, msg_size, MPI_CHAR, my_id-1, 0, MPI_COMM_WORLD, &status);
t++;
}
}
MPI_Barrier(MPI_COMM_WORLD);
EXIT:
MPI_Finalize();
return 0;
}

Answer 1

I am not completely sure if this is what you mean, but I will try.
For what I understand, you want to know what is the message being sent. Well, for the code you provide, memory is assign to the message but any real "readable" message is specify. In this line.
message = (char*)malloc (msg_size);
malloc reserves the memory for the messages, so anyone can write it, however, it doesn't provide any initial value. Sometimes, the memory contains other information previously stored and freed. Then, the message being sent is that "garbage" that is before. This is probably what you call hexadecimal (I hope I understand this right).
The type of value in this case is char (defined as MPI_CHAR in the MPI_Send and MPI_Recv functions). Here you can find more data types for MPI.
I will suggest to assign a value to the message with the with my_id and next_id. So you know who is sending to whom.