Process Coordination & Semaphores
Background: consider the following Producer and Consumer programs, which share access to the buffer array:
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;
}
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;
}
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 ) */
}
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 ) */
}
These notes by Miriam Briskman are licensed under CC BY-NC 4.0 and based on sources.