c - When is MPI_wait necessary with non-blocking calls? -

i'm little bit confused when should call mpi_wait (or other variants such as: mpi_waitall, mpii_waitsome, etc). consider following situations: (note: pseudo code)

case (1)

mpi_isend (send_buffer, send_req);     // local work mpi_probe (recv_msg); mpi_irecv (recv_buffer, recv_req); // wait msgs finish mpi_wait (recv_req);   // <--- needed? mpi_wait (send_req);   // <--- how this? 

so confusion stems mpi_probe in case. since blocking call, wouldn't mean blocks caller until message received? if case, think mpi_waits unnecessary here.

how following case?

case (2)

mpi_isend (send_buffer, send_req);     // local work mpi_probe (recv_msg); mpi_recv (recv_buffer); // wait msgs finish mpi_wait (send_req);   // <--- necessary? 

similar first case mpi_irecv replaced blocking version. in case, message received time mpi_wait called means mpi_isend must have been finished ...

also separate question, mean when mpi_probe blocking? block until of message received process or block until "meta-data" (such msg size, sender rank, etc) received? in other words mpi_probe + mpi_irecv better mpi_probe + mpi_recv ?

only when wants receives data asynchronously, need use "mpi_wait" function or mpi_waitall if initiated various asynchronous read. problem mpi_wait blocking call. if want have non-blocking should use mpi_test check completion of function.

in case, mpi_probe block until message received. call mpi_probe not necessary in case.