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)
  

    
  $\bullet$ sendbuf points to a buffer that stores the data to be sent,
  $\bullet$ recvbuf points to a buffer that stores the data to be received,
  $\bullet$ nelems is the number of data units to be sent and received,.
  $\bullet$ dest is the identifier of the process that receives the data,.
  $\bullet$ 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;

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