Zibaldone

Codice ERRATO dei lettori scrittori che dovrebbe non soffrire di problemi di starvation... Esercizio: trovare gli errori.

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

#define NREADER 3
#define NWRITER 3
semaphore mutex;
semaphore ok2read;
semaphore ok2write;

int nr=0;
int nw=0;
int wr=0;
int ww=0;
int w_last;

// (nw == 0 && nr == 0) || (nr > 0 && nw == 0) || nw == 1
// (nw == 0 && nr >= 0) || nw == 1

// this "baton" function has been left for reference only.
// code to implement "passing the baton" has been copied at the end of
// each ({start}|{end})({read}|{write}) function and reduced.
void baton(void) {
        if (nw == 0 && nr == 0 && ww > 0)
                semaphore_V(ok2write);
        else if (nw == 0 && wr > 0 )
                semaphore_V(ok2read);
        else
                semaphore_V(mutex);
}

void startread(void) {
// <await nw == 0 -> nr++>
        semaphore_P(mutex);
        if (nw > 0 && ww == 0) {
                wr++;
                semaphore_V(mutex);
                semaphore_P(ok2read);
                wr--;
        }
        nr++;
        w_last=0;
        printf("NR %d - NW %d - WR %d - WW %d \n", nr, nw, wr, ww);
        if (wr > 0 )
                semaphore_V(ok2read);
        else
                semaphore_V(mutex);
}

void endread(void) {
// <nr-->
        semaphore_P(mutex);
        nr--;
        printf("NR %d - NW %d - WR %d - WW %d \n", nr, nw, wr, ww);
        if (nr == 0 && ww > 0)
                semaphore_V(ok2write);
        else
                semaphore_V(mutex);
}

void startwrite(void) {
// <await nw == 0 && nr == 0 -> nw++>
        semaphore_P(mutex);
        if (nw > 0 || nr > 0) {
                ww++;
                semaphore_V(mutex);
                semaphore_P(ok2write);
                ww--;
        }
        nw++;
        w_last=1;
        printf("NR %d - NW %d - WR %d - WW %d \n", nr, nw, wr, ww);
        semaphore_V(mutex);
}

void endwrite(void) {
// <nw-->
        semaphore_P(mutex);
        nw--;
        printf("NR %d - NW %d - WR %d - WW %d \n", nr, nw, wr, ww);
        if (wr > 0 && (ww == 0 || w_last))
                semaphore_V(ok2read);
        else if (ww > 0 && (wr == 0 || !w_last))
                semaphore_V(ok2write);
        else
                semaphore_V(mutex);
}

void *reader(void *arg) {
        int i = (uintptr_t)arg;
        while (1) {
                //other code
                usleep(random() % 200000);
                startread();
                //read
                usleep(random() % 200000);
                endread();
        }
}

void *writer(void *arg) {
        int i = (uintptr_t)arg;
        while (1) {
                //other code
                usleep(random() % 200000);
                startwrite();
                //write
                usleep(random() % 200000);
                endwrite();
        }
}


//printf("philo thinking: %d\n",i);
/*while*/

int main(int argc, char *argv[]) {
        int i;
        pthread_t t;
        srandom(time(NULL));
        mutex=semaphore_create(1);
        ok2read=semaphore_create(0);
        ok2write=semaphore_create(0);
        for (i=0; i<NREADER; i++)
                pthread_create(&t, NULL, reader, (void *)(uintptr_t) i);
        for (i=0; i<NWRITER; i++)
                pthread_create(&t, NULL, writer, (void *)(uintptr_t) i);
        while(1)
                pause();
}

Propongo la seguente soluzione. Mi sembra che funzioni ma devo ancora capire perchè e documentarla bene. Ho provato prima a cercare il bug ma non ci sono riuscito. Ho quindi modificato il codice secondo ciò che mi sembrava sensato in base alle condizioni del problema. Ne possiamo parlare in aula oggi. --FedericoB (talk) 12:48, 11 November 2016 (CET) Ho notato che l'errore non si manifesta più eliminando il controllo che ci siano scrittori in attesa quando un lettore vuole iniziare a scrivere. Non capisco come questo possa essere collegato al bug. --FedericoB (talk) 15:10, 11 November 2016 (CET)

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

#define NUMBER_OF_READERS 3
#define NUMBER_OF_WRITERS 3
semaphore mutex;
semaphore ok2read;
semaphore ok2write;

int numberOfReaders=0;
int numberOfWriters=0;
int waitingReaders=0;
int waitingWriters=0;

// (nw == 0 && nr == 0) || (nr > 0 && nw == 0) || nw == 1
// (nw == 0 && nr >= 0) || nw == 1

void startread(void) {
// <await nw == 0 -> nr++>
    semaphore_P(mutex);
    if (numberOfWriters > 0) {
        waitingReaders++;
        semaphore_V(mutex);
        semaphore_P(ok2read);
        waitingReaders--;
    }
    numberOfReaders++;
    printf("NR %d - NW %d - WR %d - WW %d \n", numberOfReaders, numberOfWriters, waitingReaders, waitingWriters);
    if (waitingReaders > 0 )
        semaphore_V(ok2read);
    else
        semaphore_V(mutex);
}

void endread(void) {
// <nr-->
    semaphore_P(mutex);
    numberOfReaders--;
    printf("NR %d - NW %d - WR %d - WW %d \n", numberOfReaders, numberOfWriters, waitingReaders, waitingWriters);
    if (numberOfReaders == 0 && waitingWriters > 0)
        semaphore_V(ok2write);
    else
        semaphore_V(mutex);
}

void startwrite(void) {
// <await nw == 0 && nr == 0 -> nw++>
    semaphore_P(mutex);
    if (numberOfWriters > 0 || numberOfReaders > 0) {
        waitingWriters++;
        semaphore_V(mutex);
        semaphore_P(ok2write);
        waitingWriters--;
    }
    numberOfWriters++;
    printf("NR %d - NW %d - WR %d - WW %d \n", numberOfReaders, numberOfWriters, waitingReaders, waitingWriters);
    semaphore_V(mutex);
}

void endwrite(void) {
// <nw-->
    semaphore_P(mutex);
    numberOfWriters--;
    printf("NR %d - NW %d - WR %d - WW %d \n", numberOfReaders, numberOfWriters, waitingReaders, waitingWriters);
    if (waitingReaders > 0)
        semaphore_V(ok2read);
    else if (waitingWriters > 0)
        semaphore_V(ok2write);
    else
        semaphore_V(mutex);
}

void *reader(void *arg) {
    int i = (uintptr_t)arg;
    while (1) {
        //other code
        usleep(random() % 200000);
        startread();
        //read
        usleep(random() % 200000);
        endread();
    }
}

void *writer(void *arg) {
    int i = (uintptr_t)arg;
    while (1) {
        //other code
        usleep(random() % 200000);
        startwrite();
        //write
        usleep(random() % 200000);
        endwrite();
    }
}


//printf("philo thinking: %d\n",i);
/*while*/

int main(int argc, char *argv[]) {
    int i;
    pthread_t t;
    srandom(time(NULL));
    mutex=semaphore_create(1);
    ok2read=semaphore_create(0);
    ok2write=semaphore_create(0);
    for (i=0; i<NUMBER_OF_READERS; i++)
        pthread_create(&t, NULL, reader, (void *)(uintptr_t) i);
    for (i=0; i<NUMBER_OF_WRITERS; i++)
        pthread_create(&t, NULL, writer, (void *)(uintptr_t) i);
    while(1)
        pause();
}