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:

  send(void *sendbuf, int nelems, int dest) 
  receive(void *recvbuf, int nelems, int source)
  

  • 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.

  1         P0                               P1 
  2 
  3         a = 100;                         receive(&a, 1, 0) 
  4         send(&a, 1, 1);                  printf("%d\n", a); 
  5         a=0;
  

• 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 as opposed to 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!