Computer Science 354
Operating Systems
Dickinson College
Spring Semester 2000
Grant Braught
<![CDATA[
/*
* SleepBarber.c
* Example program that uses semaphores
* to solve sleeping barber problem.
*
* Compile with:
* cc SleepBarber.c -o SleepBarber -lpthread -lm
*
* Grant Braught
* Dickinson College
* (c) 2000
*/
#define _REENTRANT
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <pthread.h>
#include <semaphore.h>
// The maximum number of customer threads.
#define MAX_CUSTOMERS 25
// Function prototypes...
void *customer(void *num);
void *barber(void *);
void randwait(int secs);
// Define the semaphores.
// waitingRoom Limits the # of customers allowed
// to enter the waiting room at one time.
sem_t waitingRoom;
// barberChair ensures mutually exclusive access to
// the barber chair.
sem_t barberChair;
// barberPillow is used to allow the barber to sleep
// until a customer arrives.
sem_t barberPillow;
// seatBelt is used to make the customer to wait until
// the barber is done cutting his/her hair.
sem_t seatBelt;
// Flag to stop the barber thread when all customers
// have been serviced.
int allDone = 0;
int main(int argc, char *argv[]) {
pthread_t btid;
pthread_t tid[MAX_CUSTOMERS];
long RandSeed;
int i, numCustomers, numChairs;
int Number[MAX_CUSTOMERS];
// Check to make sure there are the right number of
// command line arguments.
if (argc != 4) {
printf("Use: SleepBarber <Num Customers> <Num Chairs> <rand seed>\n");
exit(-1);
}
// Get the command line arguments and convert them
// into integers.
numCustomers = atoi(argv[1]);
numChairs = atoi(argv[2]);
RandSeed = atol(argv[3]);
// Make sure the number of threads is less than the number of
// customers we can support.
if (numCustomers > MAX_CUSTOMERS) {
printf("The maximum number of Customers is %d.\n", MAX_CUSTOMERS);
exit(-1);
}
printf("\nSleepBarber.c\n\n");
printf("A solution to the sleeping barber problem using semaphores.\n");
// Initialize the random number generator with a new seed.
srand48(RandSeed);
// Initialize the numbers array.
for (i=0; i<MAX_CUSTOMERS; i++) {
Number[i] = i;
}
// Initialize the semaphores with initial values...
sem_init(&waitingRoom, 0, numChairs);
sem_init(&barberChair, 0, 1);
sem_init(&barberPillow, 0, 0);
sem_init(&seatBelt, 0, 0);
// Create the barber.
pthread_create(&btid, NULL, barber, NULL);
// Create the customers.
for (i=0; i<numCustomers; i++) {
pthread_create(&tid[i], NULL, customer, (void *)&Number[i]);
}
// Join each of the threads to wait for them to finish.
for (i=0; i<numCustomers; i++) {
pthread_join(tid[i],NULL);
}
// When all of the customers are finished, kill the
// barber thread.
allDone = 1;
sem_post(&barberPillow); // Wake the barber so he will exit.
pthread_join(btid,NULL);
}
void *customer(void *number) {
int num = *(int *)number;
// Leave for the shop and take some random amount of
// time to arrive.
printf("Customer %d leaving for barber shop.\n", num);
randwait(5);
printf("Customer %d arrived at barber shop.\n", num);
// Wait for space to open up in the waiting room...
sem_wait(&waitingRoom);
printf("Customer %d entering waiting room.\n", num);
// Wait for the barber chair to become free.
sem_wait(&barberChair);
// The chair is free so give up your spot in the
// waiting room.
sem_post(&waitingRoom);
// Wake up the barber...
printf("Customer %d waking the barber.\n", num);
sem_post(&barberPillow);
// Wait for the barber to finish cutting your hair.
sem_wait(&seatBelt);
// Give up the chair.
sem_post(&barberChair);
printf("Customer %d leaving barber shop.\n", num);
}
void *barber(void *junk) {
// While there are still customers to be serviced...
// Our barber is omnicient and can tell if there are
// customers still on the way to his shop.
while (!allDone) {
// Sleep until someone arrives and wakes you..
printf("The barber is sleeping\n");
sem_wait(&barberPillow);
// Skip this stuff at the end...
if (!allDone) {
// Take a random amount of time to cut the
// customer's hair.
printf("The barber is cutting hair\n");
randwait(3);
printf("The barber has finished cutting hair.\n");
// Release the customer when done cutting...
sem_post(&seatBelt);
}
else {
printf("The barber is going home for the day.\n");
}
}
}
void randwait(int secs) {
int len;
// Generate a random number...
len = (int) ((drand48() * secs) + 1);
sleep(len);
}
]]>