The MPI_PROBE, MPI_IPROBE, MPI_MPROBE, and MPI_IMPROBE procedures allow incoming messages to be checked for, without actually receiving them. The user can then decide how to receive them, based on the information returned by the probe (basically, the information returned by status). In particular, the user may allocate memory for the receive buffer, according to the length of the probed message.
The MPI_CANCEL procedure allows pending communication operations to be cancelled. This is required for cleanup. Posting a send or a receive ties up user resources (send or receive buffers), and a cancel may be needed to free these resources gracefully.
Cancelling a send request by calling MPI_CANCEL is deprecated. Cancelling a send-recv request by calling MPI_CANCEL is not allowed.