Point-to-Point Routines

Point-to-point routines include the basic send and receive routines in both blocking and nonblocking forms and in four modes.

A blocking send blocks until its message buffer can be written with a new message. A blocking receive blocks until the received message is in the receive buffer.

Nonblocking sends and receives differ from blocking sends and receives in that they return immediately and their completion must be waited or tested for. It is expected that eventually nonblocking send and receive calls will allow the overlap of communication and computation.

MPI's four modes for point-to-point communication are:

• Standard, in which the completion of a send implies that the message either is buffered internally or has been received. Users are free to overwrite the buffer that they passed in with any of the blocking send or receive routines, after the routine returns.

• Buffered, in which the user guarantees a certain amount of buffering space.

• Synchronous, in which rendezvous semantics occur between sender and receiver; that is, a send blocks until the corresponding receive has occurred.

• Ready, in which a send can be started only if the matching receive is already posted. The ready mode for sends is a way for the programmer to notify the system that the receive has been posted, so that the underlying system can use a faster protocol if it is available.