Semaphores and concurrent programming

Question

For a homework assignment i need to program the following scenario. This is going to be done using semaphores using BACI (which is C--)

There are 2 unisex restrooms

Answer 1

Since you want to know how to code your algorithm for 1 restroom, I have done so in C. It will be a fairly simple task to convert it into C--, as all the semaphore constructs appear quite similar.

From what I could make of your answer,

C: sem_wait()  C--: wait()
   sem_post()       signal()
   sem_t            semaphore()
   sem_init()       initialsem() 

Bear in mind, as stated, I have worked out the problem for 1-restroom only. Since this is homework, I expect you to expand it into the 2-restrooms form yourself.

Working one's way from the Readers-writers problem to our "Unisex Restroom" problem, we make use of the following global variables:

int mcount,wcount; // count of number of men/women in restroom
sem_t x,y,z;       // semaphores for updating mcount & wcount values safely
sem_t wsem,msem;   // semaphores to block other genders' entry  
sem_t cap;         // capacity of the restroom

Incorporating these semaphores & counters into the man thread function,

void *man(void *param)
{           
    sem_wait(&z);                
        sem_wait(&msem);        
            sem_wait(&x);
                mcount++;
                if(mcount==1)   
                { sem_wait(&wsem); } // first man in, make women wait
            sem_post(&x);
        sem_post(&msem);
    sem_post(&z);

    sem_wait(&cap);  //wait here, if over capacity

    printf("\t\tman in!\n");
    delay();
    printf("\t\t\tman out!\n");

    sem_post(&cap);  //one man has left, increase capacity

    sem_wait(&x);
        mcount--;
        if(mcount==0)
        {sem_post(&wsem);}  // no man left, signal women 
    sem_post(&x);
}

Similarly, the woman thread function, substitutes mcount with wcount, msem with wsem, and x with y. Only z remains as is in the man function, so that both man & woman threads queue up on the same common semaphore. (Due to this, the code invariably has FIFO-like behaviour, which ensures fairness/non-starvation)

The complete code is as follows: (To compile, use gcc filename -lpthread)

#include 
#include 
#include 
#include 

int mcount,wcount;
sem_t x,y,z,wsem,msem,cap;

void delay(void)
{
    int i;
    int delaytime;
    delaytime = random();
    for (i = 0; i

While converting into the 2-restrooms form, make note of which semaphores & counter variables you would need to duplicate to satisfy all the conditions. Happy semaphoring!