Cigarette Smoker's Problem


There are four processes in this problem: three smoker processes and an agent process. Each of the smoker processes will make a cigarette and smoke it. To make a cigarette requires tobacco, paper, and matches. Each smoker process has one of the three items. I.e., one process has tobacco, another has paper, and a third has matches. The agent has an infinite supply of all three. The agent places two of the three items on the table, and the smoker that has the third item makes the cigarette. Synchronize the processes.

Please note this is the formulation of the problem used in recent OS texbooks, but not the problem as originally described by Parnas in CACM March 1975.


This seems like a fairly easy solution. The three smoker processes will make a cigarette and smoke it. If they can't make a cigarette, then they will go to sleep. The agent process will place two items on the table, and wake up the appropriate smoker, and then go to sleep. All semaphores except lock are initialized to 0. lock is initialized to 1, and is a mutex variable.

Here's the code for the agent process.

 1   do forever {
 2     P( lock );
 3     randNum = rand( 1, 3 ); // Pick a random number from 1-3
 4     if ( randNum == 1 ) {
 5        // Put tobacco on table
 6        // Put paper on table
 7        V( smoker_match );  // Wake up smoker with match
 8      } else if ( randNum == 2 ) {
 9        // Put tobacco on table
10        // Put match on table
11        V( smoker_paper );  // Wake up smoker with paper
12      } else {
13        // Put match on table
14        // Put paper on table
15        V( smoker_tobacco ); } // Wake up smoker with tobacco
16      V( lock );
17      P( agent );  //  Agent sleeps
18     }  // end forever loop

I will give code to one of the smokers. The others are analogous.

 1   do forever {
 2      P( smoker_tobacco );  // Sleep right away
 3      P( lock );
 4      // Pick up match
 5      // Pick up paper
 6      V( agent );
 7      V( lock );
 8      // Smoke (but don't inhale).
 9   }

The smoker immediately sleeps. When the agent puts the two items on the table, then the agent will wake up the appropriate smoker. The smoker will then grab the items, and wake the agent. While the smoker is smoking, the agent can place two items on the table, and wake a different smoker (if the items placed aren't the same). The agent sleeps immediately after placing the items out. This is something like the producer-consumer problem except the producer can only produce 1 item (although a choice of 3 kinds of items) at a time.