add MPI_Probe

slides/pages/recap.md

@@ -280,3 +280,61 @@ token -> 0 -> 1 -> 2 -> 3 -> 0 -> token
 
 Add all missing function calls in `exercises/MPI_examples/round_trip/round_trip.c`
 
+
+---
+title: MPI IX
+---
+
+### Message Reception and Status
+
+The receive buffer must be able to fit the entire message:
+
+- send count <= receive count (<span class="text-green-500">OK</span>) 
+- send count > receive count (<span class="text-rose-500">Error: message truncation</span>)
+
+The MPI status object holds information about the received message.
+
+**MPI_Status status**:
+
+```c
+status.MPI_SOURCE // message source rank
+status.MPI_TAG    // message tag
+status.MPI_ERROR  // receive status code
+```
+
+### Message Size Inquiry
+
+**MPI_Get_count (MPI_Status \*status, MPI_Datatype datatype, int \*count)**
+
+- Calculates how many **datatype** elements can be formed from the data in the message referenced by status
+- Can be used with the status from **MPI_Recv** or **MPI_Probe** 
+
+
+---
+title: MPI Probe
+---
+
+### MPI Probe
+
+Blocks until a matching message appears without actually receiving the message:
+
+**MPI_Probe (int source, int tag, MPI_Comm comm, MPI_Status \*status)**
+
+One must still call **MPI_Recv** to receive the message and copy the data into the buffer.
+
+When probing tells you that there is a message, you can use **MPI_Get_count** to determine its size, allocate a large enough receive buffer, and do a regular receive to have the data copied.
+
+```c
+if (rank == receiver) {
+    MPI_Status status;
+    MPI_Probe(sender, 0, comm, &status);
+    int count;
+    MPI_Get_count(&status,MPI_FLOAT,&count);
+    float recv_buffer[count];
+    MPI_Recv(recv_buffer, count, MPI_FLOAT, sender, 0, comm, MPI_STATUS_IGNORE);
+} else if (rank == sender) {
+    float buffer[buffer_size];
+    MPI_Send(buffer, buffer_size, MPI_FLOAT, receiver, 0, comm);
+}
+```
+