Producer-Consumer Example Using Shared Memory and Message Passing

Producer-Consumer Example Using Shared Memory

·         This is a classic example, in which one process is producing data and another process is consuming the data. ( In this example in the order in which it is produced, although that could vary. )

·         The data is passed via an intermediary buffer, which may be either unbounded or bounded. With a bounded buffer the producer may have to wait until there is space available in the buffer, but with an unbounded buffer the producer will never need to wait. The consumer may need to wait in either case until there is data available.

·         This example uses shared memory and a circular queue. Note in the code below that only the producer changes "in", and only the consumer changes "out", and that they can never be accessing the same array location at the same time.

First the following data is set up in the shared memory area:

#define BUFFER_SIZE 10

typedef struct {
     . . .
} item;

item buffer[ BUFFER_SIZE ];
int in = 0;
int out = 0;

Then the producer process. Note that the buffer is full when "in" is one less than "out" in a circular sense:

item nextProduced;

while( true ) {

/* Produce an item and store it in nextProduced */
nextProduced = makeNewItem( . . . );

/* Wait for space to become available */
while( ( ( in + 1 ) % BUFFER_SIZE ) == out )
      ; /* Do nothing */

/* And then store the item and repeat the loop. */
buffer[ in ] = nextProduced;
in = ( in + 1 ) % BUFFER_SIZE;

}

Then the consumer process. Note that the buffer is empty when "in" is equal to "out":

item nextConsumed;

while( true ) {

/* Wait for an item to become available */
while( in == out )
      ; /* Do nothing */

/* Get the next available item */
nextConsumed = buffer[ out ];
out = ( out + 1 ) % BUFFER_SIZE;

/* Consume the item in nextConsumed
     ( Do something with it ) */

}

Producer-Consumer Example Using Message passing