The Building Blocks: Send and Receive Operations

• Since interactions are accomplished by sending and receiving messages, the basic operations in the message-passing programming paradigm are send and receive.
• In their simplest form, the prototypes of these operations are defined as follows:

  
  • sendbuf points to a buffer that stores the data to be sent,
  • recvbuf points to a buffer that stores the data to be received,
  • nelems is the number of data units to be sent and received,
  • dest is the identifier of the process that receives the data,
  • source is the identifier of the process that sends the data.

• Process $P_0$ sends a message to process $P_1$ which receives and prints the message.
• The important thing to note is that process $P_0$ changes the value of a to 0 immediately following the send.
• The semantics of the send operation require that the value received by process $P_1$ must be 100 (not 0).
• That is, the value of $a$ at the time of the send operation must be the value that is received by process $P_1$.
• It may seem that it is quite straightforward to ensure the semantics of the send and receive operations.
• However, based on how the send and receive operations are implemented this may not be the case.

• Most message passing platforms have additional hardware support for sending and receiving messages.
• They may support DMA (direct memory access) and asynchronous message transfer using network interface hardware.
• Network interfaces allow the transfer of messages from buffer memory to desired location without CPU intervention.
• Similarly, DMA allows copying of data from one memory location to another (e.g., communication buffers) without CPU support (once they have been programmed).
• As a result, if the send operation programs the communication hardware and returns before the communication operation has been accomplished, process $P_1$ might receive the value 0 in a instead of 100!