Sleeping barber using semaphore
I am having a problem in the synchronisation of threads. I m trying to implement sleeping barber problem in a different approach. When i run the code the threads run in a completely vague manner. When i add join to the threads the processes run one by one which is not required. I know the solutions to the sleeping barber problem but can anybody tell me what is the problem with this code.?
Thank You
#include<stdio.h>
#include<pthread.h>
#include<time.h>
#include <unistd.h>
#include<semaphore.h>
#define n 1
#define n1 50
int status[n];
int b=0;
sem_t mutex;
sem_t barber;
void *barb()
{
printf("Barber Sleeping\n");
b=1;
sem_wait(&barber);
printf("Barber woke up\n");
sleep(1);
b=0;
}
void *handler(void *ptr)
{
int x;
x = *((int *) ptr);
int flag=0;
int i=0;
int cnt=0;
for(i=0;i<n;i++)
{
if(status[i]==1)
{
flag=1;
cnt++;
}
}
printf("Count=%d",cnt);
if(cnt==n)
{
printf("%d Customer Returned\n",x);
return 0;
}
if(flag==1)
{
status[x]=1;
printf("%d Customer waiting\n",x);
sem_wait(&mutex);
}
if(b==1)
{
printf("Customer %d woke up barber",x);
sem_post(&barber);
}
printf("Cutting Hair of customer %d\n",x);
status[x]=1;
sleep(5);
flag=0;
for(i=0;i<n;i++)
{
if(status[i]==1)
{
flag=1;
}
}
status[x]=0;
printf("Finished cutting Hair of customer %d\n",x);
if(flag==1)
sem_post(&mutex);
}
int main()
{
sem_init(&mutex, 0, n);
sem_init(&barber, 0, 1);
pthread_t thread[n1];
pthread_t barber;
int i=0;
for(i=0;i<n;i++)
{
status[i]=0;
}
pthread_create(&barber,NULL,barb,NULL);
pthread_join(barber,NULL);
for(i=0;i<n1;i++)
{
sleep(1);
printf("Customer entered %d\n",i);
pthread_create(&thread[i], NULL, handler, (void *)&i);
//pthread_join(thread[i],NULL);
}
for(i=0;i<n1;i++)
{
//pthread_join(thread[i],NULL);
}
//sem_post(&barber);
}
Answer
I have the code to help you
#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);
}