Consider the following two programs in a shared address space. Remember that semaphores are signals (like the flags and lights that railroads use) item buffer [5]; semaphore full (0); semaphore empty (5); semaphore mutex; while (1) { // produce next item to store wait (empty); wait (mutex); // add item to item_buffer signal (mutex); signal (full); while (1) { wait (full); wait (mutex); // remove item from item_buffer; signal (mutex); signal (empty); 11 consume the item } Why must "wait(mutex)" be the last wait performed? • Why does the program on the left wait on the "empty" semaphore, but signal the "full" semaphore? • Why was the "mutex" semaphore added? (What is its purpose?)

Database System Concepts
7th Edition
ISBN:9780078022159
Author:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Publisher:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Chapter1: Introduction
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Consider the following two programs in a shared address space. Remember that semaphores are signals
(like the flags and lights that railroads use)
item buffer [5];
semaphore full (0);
semaphore empty (5);
semaphore mutex;
while (1)
{
// produce next item to store
wait (empty);
wait (mutex);
// add item to item_buffer
signal (mutex);
signal (full);
while (1)
{
wait (full);
wait (mutex);
// remove item from item_buffer;
signal (mutex);
signal (empty);
11 consume the item
}
Why must "wait(mutex)" be the last wait performed?
• Why does the program on the left wait on the "empty" semaphore, but signal the "full"
semaphore?
• Why was the "mutex" semaphore added? (What is its purpose?)
Transcribed Image Text:Consider the following two programs in a shared address space. Remember that semaphores are signals (like the flags and lights that railroads use) item buffer [5]; semaphore full (0); semaphore empty (5); semaphore mutex; while (1) { // produce next item to store wait (empty); wait (mutex); // add item to item_buffer signal (mutex); signal (full); while (1) { wait (full); wait (mutex); // remove item from item_buffer; signal (mutex); signal (empty); 11 consume the item } Why must "wait(mutex)" be the last wait performed? • Why does the program on the left wait on the "empty" semaphore, but signal the "full" semaphore? • Why was the "mutex" semaphore added? (What is its purpose?)
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