Question

hi there i’m implementing a multi-thread program which is called the sleeping barber problem. The issue is there is one barber shop with N chairs and if there are no any customers barber will fall asleep, whenever a customer comes into the shop barber will wake up and start to cut off customer’s hair. i use condition variables rather than semaphores. the thing is i couldnt have the output which i expect.

    #include <stdlib.h>
    #include <stdio.h>
    #include <pthread.h>
    #include <time.h>

    #define TRUE 1

    pthread_mutex_t mutex;
    pthread_cond_t fill;

    int counter;

    int* buffer;
    int size;

    void* producer(void*);
    void* consumer(void*);

    void initialize_variables();
    void insert(void);
    void remove_item(void);

    int main(int argc, char *argv[]) {

        int random_num;
        int num=0;
        pthread_t barber;

        srand( time(NULL) );

        if(argc!=2){
            printf("Not enough parameters\n");
            exit(0);
        }

        size=atoi(argv[1]);

        initialize_variables();

        pthread_create(&barber,NULL,consumer,NULL);

        while(TRUE){

            pthread_t customer;
            pthread_create(&customer,NULL,producer,NULL);

        }

        printf("Exit the program\n");
        return 0;
    }

    void initialize_variables(){

        int i;

        if(! (buffer=(int*)malloc(size*sizeof(int))) ){
            printf("Error while allocating memory for buffer\n");
            exit(1);
        }

        for(i=0; i<size; i++)
            buffer[i]=0;

        pthread_mutex_init(&mutex,NULL);

        pthread_cond_init(&empty, NULL);
        pthread_cond_init(&fill, NULL);

        counter = 0;
    }

  void* producer(void* arg){

    pthread_mutex_lock(&mutex);

        while(counter==size)
            pthread_cond_wait(&empty,&mutex);

        printf("One Customer has arrived and sit on the  #%d chair\n", counter);
        insert();
        sleep(1);
        pthread_cond_signal(&fill);

    pthread_mutex_unlock(&mutex);

    pthread_exit(NULL);
}

void* consumer(void* arg){

    while(TRUE){

        pthread_mutex_lock(&mutex);

            while(counter==0){
                printf("Barber is sleeping now\n");
                pthread_cond_wait(&fill,&mutex);
            }

            remove_item();
            sleep(1);
            pthread_cond_broadcast(&empty);

        pthread_mutex_unlock(&mutex);

    }
}

void insert(void){

    buffer[counter]=1;
    counter++;
}

void remove_item(void){

    buffer[counter]=0;
    printf("The barber is started to cut the hair of the customer at chair #%d\n", counter);
    counter--;
}

Moreover, when i start to program output should print "Barber is sleeping now" first, however main thread starts run producer thread first and then when counter is equal to zero it prints "Barber is sleeping now" can you help me why the sequence starts wrongly ?. and an another issue is main thread fills up whole the buffer and starts to count down (which refers that barber is going to start his work after shop is full. here is an example of output which will make clear for you. and assume that buffer size is 10.

One Customer has arrived and sit on the  #0 chair
One Customer has arrived and sit on the  #1 chair
One Customer has arrived and sit on the  #2 chair
One Customer has arrived and sit on the  #3 chair
One Customer has arrived and sit on the  #4 chair
One Customer has arrived and sit on the  #5 chair
One Customer has arrived and sit on the  #6 chair
One Customer has arrived and sit on the  #7 chair
One Customer has arrived and sit on the  #8 chair
One Customer has arrived and sit on the  #9 chair
The barber is started to cut the hair of the customer at chair #10
The barber is started to cut the hair of the customer at chair #9
The barber is started to cut the hair of the customer at chair #8
The barber is started to cut the hair of the customer at chair #7
The barber is started to cut the hair of the customer at chair #6
The barber is started to cut the hair of the customer at chair #5
The barber is started to cut the hair of the customer at chair #4
The barber is started to cut the hair of the customer at chair #3
The barber is started to cut the hair of the customer at chair #2
The barber is started to cut the hair of the customer at chair #1
Barber is sleeping now
One Customer has arrived and sit on the  #0 chair

i will be appreciated for every reply and thanks anyway

EDIT: first code which post was not working sorry for that and i updated with the true one.

Answer 1

You can’t assume nothing regarding the order of which threads will perform.
This is unexpected behaviour and it’s not deterministic.
If you need assurance that one thread starts before the other. Use a mutex or a semaphore.