Thread: Reader & Writer problem

Code:

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

#define NWRITER 2		/* total number of producers - 1 */
#define NREADER 5		/* total number of consumers - 1 */
#define NORW 4		/* number of items writen/read */

typedef struct {
    sem_t mutex;    	  /* enforce mutual exclusion to shared data */
    sem_t db;
} rwbuff_t;

/* instance shared of sbuf_t */
rwbuff_t shared;

/* number of processes reading or wanting to */
int rc = 0;
/* number of processes writing or wanting to */
int wc = 0;

/* function prototypes */
void *Writer(void *arg);
void *Read(void *arg);

/* increments within the mutex */
int twrite, tread;

int main()
{
   /* writer/read threads */
    pthread_t tWrite, tRead;

    /* creates semaphores */
    sem_init(&shared.mutex, 0, 1);
    sem_init(&shared.db, 0, 1);
    
    /* loop control */
    int index;
     
    /* creates write thread */
    for (index = 1; index < NWRITER; index++)
    {
       /* Create 1 write */
       pthread_create(&tWrite, NULL, Writer, (void*)index);
    }

    /* creates 4 read threads */
    for (index = 1; index < NREADER; index++)
    {
       pthread_create(&tRead, NULL, Read, (void*)index);
    }

    /* temp variables to compute the consumes and produces per hour */
    int writePH, readPH, ratio;

    /* keeps track of seconds */
    int seconds = 1;

    while(1)
    {
       sleep(1);

       readPH = ((tread * 3600) / seconds);
       printf("\nOperations after %d seconds: \n\n", seconds);
       printf("Number of read operations completed per hour : %d\n", readPH);
       
      
       writePH = ((twrite * 3600) / seconds);  
       ratio = writePH / readPH;
       printf("Number of write operations completed per hour: %d\n", writePH);
       printf("Ratio of reads per hour over writes per hour: %d\n\n", ratio);
       seconds++;
    }
    /* destroy semaphore */
    sem_destroy(&shared.mutex); 
    sem_destroy(&shared.db);
    pthread_exit(NULL);
}

void *Read(void *arg)
{
   int index, i;

   index = (int)arg;
    
    printf("Read thread - %d - in line to read...\n\n", index);

    for (i = 0; i < NORW; i++) 
    {

        sem_wait(&shared.mutex);
        rc++;
        if (rc == 1)           
        {
           sem_wait(&shared.db);
           sem_post(&shared.mutex);
           printf("Reader %d in criticial section\n",i);
          // read_data_base();
           sem_wait(&shared.mutex);
           rc--;
        }
        if(rc == 0)
        {
              sem_post(&shared.db);
              sem_post(&shared.mutex);
              printf("Reader %d in non-critical section\n", i);
        }
        sleep(1);
    }
}

void *Writer(void *arg)
{
    int data, index, i;

    index = (int)arg;

    printf("Write thread %d - in line to write...\n\n", index);

    for (i = 0; i < NORW; i++) {


        printf("Writer tries to enter the critical section \n");
        sem_wait(&shared.db);
      //  write_data_base();
        printf("Writer is in critical region\n");
        sem_post(&shared.db);
        printf("Writer is in non-critical region\n");
    }
    sleep(1);
}

It runs, but something is definitely amiss!

What does that even mean? I know there are a couple of people here that love to run random code just because people like you don't give them any idea as to what's really going on, but I'm not one of them.

So, what exactly is the problem with your program and the way it runs that is 'amiss'?


Quzah.

I know this is going to sound obvious, but you could actually try reading or writing something. I assume you know where you're supposed to be reading from/writing to?


Quzah.

Originally Posted by pantherman34

here is up'd code. i am getting the equal amount of reads as to writes....since i have 4 read threads and only 1 write thread....shouldnt i be getting 4x more reads than writes?!

Where would they be getting the extra data to read from? Your consumers shouldn't be consuming more than your producer is producing, right?

EDIT: Although, you're not actually reading or writing (still) are you. Just for fun, I ran the code for 50 seconds and did not get a single read event....