Operating Systems

The Cigarette-Smokers Problem.
contributed by Ramya

Consider a system with three smoker processes and one agent process. Each smoker continuously rolls a cigarette and then smokes it. But to roll and smoke a cigarette, the smoker needs three ingredients: tobacco, paper, and matches. One of the smoker processes has paper, another has tobacco, and the third has matches. The agent has an infinite supply of all three materials. The agent places two of the ingredients on the table. The smoker who has the remaining ingredient then makes and smokes a cigarette, signaling the agent on completion. The agent then puts out another two of the three ingredients, and the cycle repeats. Write a program to synchronize the agent and the smokers.

There are four processes in the system. Three represent the smokers, and one represents the supplier.
/* The Cigarette-smokers problem uses semaphores to solve. */
typedef int semaphore;
semaphore items=1; /*used for mutual exclusive access to the table on which the two ingredients are placed */
semaphore more=0; 
semaphore temp=0; /* used to queue the waiting smokers */
int count =0; /*indicates the number is of waiting smokers */
boolean flags[0..2]=initially all false;
/*a flag true indicates if the corresponding item is on the table */
/* the three items needed for smoking are named as 0,1,2 */
/* process i has item i but needs the other two items, i.e (i-1) mod2 and (i+1)mod 2*/
for 0<=i<=2;
Smoker process i;
{
repeat
wait(items); /*enter critical section */
if (flag[i-1 % 2] and flag[+1 % 2])
{
flag[i-1 % 2]=false; 
flag[i+1 % 2]=false;
SMOKE;
While(count >0)do
{
count --;
signal(temp);
}
signal(more);
}else{ /*both items needed for the smoking are not available */
count++;
signal(items);
wait(temp); /*wait for the next round */
}
until false;
}
Supplier process:
{
repeat
put any two items on the table and set the 
corresponding flags to true;
signal(items);
wait(more);
until false;
}

A full C program dealing with smokers problem is as follows:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <conio.h>
enum Ingredients /* Enum representing the ingredients */
{
None,
Paper,
Tobacco,
Matches
};
/* Structure representing a Smoker & Agent process */
typedef struct smoker
{ 
char SmokerID[25];
int Item;
}SMOKER;
typedef struct agent
{
char AgentID[25];
int Item1;
int Item2;
}AGENT;
char* GetIngredientName(int Item)
{
if(Item == Paper)
return "Paper";
else if(Item == Tobacco)
return "Tobacco";
else if(Item == Matches)
return "Matches";
}
void GetAgentIngredients(AGENT* agent)
{
/* Simulate random generation of ingredients*/
agent->Item1=random(3)+1;
while(1)
{
agent->Item2=random(3)+1;
if(agent->Item1 != agent->Item2)
break;
}
printf("\nAgent Provides Ingredients- %s,%s\n\n:,
GetIngredientName(agent->Item1),GetIngredientName(agent->Item2));
}
void GiveIngredientToSmoker(AGENT*agent,SMOKER* smoker)
{
int index=0;
while(smoker[index].Item !=NULL)
{
if((smoker[index].Item !=agent->Item1)&&(smoker[index].Item != 
agent->Item2));
{
printf("\nSmoker - \%s\"-is smoking his cigarette\n\n", smoker[index].SmokerID);
agent->Item1=None;
agent->Item2=None;
break;
}
index++;
}
}
void main()
{
/*Create the processes required -1 Agent, 3 Smokers */
AGENT agent;
SMOKER smoker[4] = {{SmokerWithPaper",Paper},
{"SmokerWithTobacco",Tobacco},
{"SmokerWithMatches",Matches},{"\0",None}};
int userChoice=0;
strcpy(agent.AgentID,"Agent");
agent.Item1=None;
agent>item2=None;
while(1)
{
GetAgentIngredients(&agent);
GiveIngredientToSmoker(&agent,smoker);
printf("Press ESC to exit or any key to continue\n\n");
UserChoice=getch();
If(UserChoice ==27)
break;
}
}/*Program Ends*/

 VAR

    tobacco_paper, tobacco_matches, paper_matches, done_smoking: SEMAPHORE;

PROCEDURE vendor
BEGIN
    WHILE (making_money) DO
        BEGIN
            CASE (random(1, 3)) OF
                1: 
                    signal(tobacco_paper);
                2: 
                    signal(tobacco_matches);
                3: 
                    signal(paper_matches);
            END;
            wait(done_smoking);
        END;
END;
 
PROCEDURE smoker1   { This smoker has matches }
BEGIN
    WHILE (not_dead) DO
        BEGIN
            wait(tobacco_paper);
            smoke;
            signal(done_smoking);
        END;
END;
 
PROCEDURE smoker2   { This smoker has paper }
BEGIN
    WHILE (addicted_to_nicotine) DO
        BEGIN
            wait(tobacco_matches);
            smoke;
            signal(done_smoking);
        END;
END;

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The Cigarette-Smokers Problem.

The Cigarette-smokers problem: Consider a system with three smoker processes and one agent process. Each smoker continuously rolls a cigarette and then smokes it. But to roll and cigarette and then smokes it. But to roll and smoke a cigarette, the smoker needs three ingredients: tobacco, paper and matches. One of the smoker processes has an infinite supply of all the three materials. The agent places two of the ingredients on the table. The smoker who has the remaining ingredient then makes the smokes a cigarette, signaling the agent on completion. The agent then puts out another two of the three ingredients, and the cycle repeats. Write a program to synchronize the agent and the smokers.