reader/writer problem using semaphores

This is a discussion on reader/writer problem using semaphores within the C Programming forums, part of the General Programming Boards category; I am working on writting 2 programs, the first was using the algorithm provided to me which basically blocks the ...

  1. #1
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    reader/writer problem using semaphores

    I am working on writting 2 programs, the first was using the algorithm provided to me which basically blocks the writter out as long as there are readers. That wasn't much of a problem. I then wanted to change it to allow the writter to gain access without changing the delay of the current algorithm so I creating a semaphore q basically like a que to allow the writter in and it seems to work however I'm a little confused on what I did and was hoping someone could help me- I believe I created q to allow the writter in after all readers are done reading but every now and then it will let the writter in while there is still a reader and I thought by gaining exclusive access it shouldn't. Anyways if you don't mind taking a look at my code and any suggestions would be helpful. keep in mind the only thing I added from my original was sem q
    Code:
    #include <pthread.h>
    #include <semaphore.h>
    #include <stdio.h>
    #include <unistd.h>
    
    #define MAXTHREAD 4  /* define # readers */
    
    
    void access_database();		/* prototypes */
    void non_access_database();	
    
    void* reader(void*);
    void* writer(void*);
    
    sem_t mutex;	/* controls access to rc */
    sem_t db;	/* controls access to db */
    sem_t q;		/* establish que */
    int rc = 0;		/* number of processes reading or wanting to */
    int main()
    {
        pthread_t readers[MAXTHREAD],writerTh;
        int index;
        int ids[MAXTHREAD];	e readers and initialize mutex, q and db-set them to 1 */
        sem_init(&mutex,0,1);
        sem_init(&db,0,1);
       sem_init (&q,0,1);
        for(index = 0; index < MAXTHREAD; index ++)	
        {
    	ids[index]=index+1;								/* create readers and error check */
    	if(pthread_create(&readers[index],0,reader,&ids[index])!=0){
    	perror("Cannot create reader!");
        exit(1);							
            }
        }
        if(pthread_create(&writerTh,0,writer,0)!=0){
    	perror("Cannot create writer");		/* create writers and error check */
    	exit(1);
        }
    	
        pthread_join(writerTh,0);	
        sem_destroy(&mutex);
        sem_destroy(&db);
    	sem_destroy (&q);
        return 0;
    }
    
    void* reader(void*arg)			/* readers function to read */
    {
        int index = *(int*)arg;
        while(1){
    	sem_wait(&q);
    	sem_wait(&mutex);
    	rc++;
    	if(rc == 1) sem_wait(&db);
    	sem_post(&mutex);
    	
    	access_database();
    	sem_post (&q);
        printf("Thread %d reading\n",index);
    	sleep(index);
    	sem_wait(&mutex);
    	rc--;
    	if(rc == 0) sem_post(&db);
    	sem_post(&mutex);	
    	non_access_database();
    	    }
        return 0;
    }
    ;
    void* writer(void*arg)		/* writer's function to write */
    {
        while(1){
    	non_access_database();
    	sem_wait (&q);
    	sem_wait(&db);
    	
    	access_database();	
    	sem_post (&q);
    	printf("Writer is now writing...Number of readers: %d\n",rc);
    	sleep(1);
    	sem_post(&db);
        }
        return 0;
    }
    
    void access_database()
    {
    
    }
    
    
    void non_access_database()
    {

  2. #2
    tjb
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    I haven't been able to find the issue with this code. It works fine for me (though under Cygwin).

    Here's a version that uses one semaphore (I think it's a little bit easier to analyze):

    Code:
    #include <pthread.h>
    #include <sched.h>
    #include <semaphore.h>
    #include <stdio.h>
    #include <unistd.h>
    
    #define MAXTHREAD 10  /* define # readers */
    
    
    void access_database();		/* prototypes */
    void non_access_database();	
    
    void* reader(void*);
    void* writer(void*);
    
    sem_t q;		/* establish que */
    int rc = 0;		/* number of processes reading or wanting to */
    int wc = 0;
    int write_request = 0;
    
    int main()
    {
        pthread_t readers[MAXTHREAD],writerTh;
        int index;
        int ids[MAXTHREAD];	/* readers and initialize mutex, q and db-set them to 1 */
        sem_init (&q,0,1);
        for(index = 0; index < MAXTHREAD; index ++)	
        {
            ids[index]=index+1;								/* create readers and error check */
            if(pthread_create(&readers[index],0,reader,&ids[index])!=0){
                perror("Cannot create reader!");
                exit(1);							
            }
        }
        if(pthread_create(&writerTh,0,writer,0)!=0){
            perror("Cannot create writer");		/* create writers and error check */
            exit(1);
        }
    
        pthread_join(writerTh,0);	
        sem_destroy (&q);
        return 0;
    }
    
    void* reader(void*arg)			/* readers function to read */
    {
        int index = *(int*)arg;
        int can_read;
        while(1){
            can_read = 1;
    
            sem_wait(&q);
            if(wc == 0 && write_request == 0)  rc++;
            else                               can_read = 0;
            sem_post(&q);
    
            if(can_read) {
                access_database();
                printf("Thread %d reading\n", index);
                sleep(index);
    
                sem_wait(&q);
                rc--;
                sem_post(&q);
            }
    
            sched_yield();
        }
        return 0;
    }
    ;
    void* writer(void*arg)		/* writer's function to write */
    {
        int can_write;
        while(1){
            can_write = 1;
            non_access_database();
    
            sem_wait (&q);
            if(rc == 0)   wc++;
            else          { can_write = 0; write_request = 1; }
            sem_post(&q);
    
            if(can_write) {
                access_database();	
                printf("Writer is now writing...Number of readers: %d\n",rc);
    
                sleep(3);
    
                sem_wait(&q);
                wc--;
                write_request = 0;
                sem_post(&q);
            }
    
            sched_yield();
        }
        return 0;
    }
    
    void access_database()
    {
    
    }
    
    
    void non_access_database()
    {
    
    }

  3. #3
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    Question

    I understand my code works. That isn't the problem. I guess I was just wondering why with my code does the reader still have access when the writter is writing sometimes it will say # of readers 1. I thought once the writter got access the reader was out. As far as writting using 3 semaphores that is the way the algorithm was given to us. I agree your is alot easier to understand. Before creating &q I just had &db and &mutex but it starved the writter so I was wondering when I created &q the way its written is it creating a que of some sort to enable the writter in because the writter gains access.

  4. #4
    tjb
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    Quote Originally Posted by jalex39 View Post
    I understand my code works. That isn't the problem. I guess I was just wondering why with my code does the reader still have access when the writter is writing sometimes it will say # of readers 1. I thought once the writter got access the reader was out. As far as writting using 3 semaphores that is the way the algorithm was given to us. I agree your is alot easier to understand. Before creating &q I just had &db and &mutex but it starved the writter so I was wondering when I created &q the way its written is it creating a que of some sort to enable the writter in because the writter gains access.
    That's what I meant by "it works". I was unable to get the program to print out anything other than zero for the number of reads in progress from the write thread (under Cygwin though).

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