In order to solve the Readers-Writers problem and avoid readers starvation, we can alternate the preference between readers and writers: A boolean variable (readersTurn) indicates whose turn it is. When writers finish writing, they set the next turn to readers, and when readers finish reading, they set the next turn to writers. Modify the readers_writers.c file as follows:

#include

#include

#include //compile and link with -pthread

#include

const int NUM_READERS_WRITERS = 10;

pthread_cond_t cond; //condition

pthread_mutex_t mutex; //lock, a.k.a. mutex

int activeWritersNumber;

int activeReadersNumber;

int waitingWritersNumber = 0;

int waitingReadersNumber = 0;

bool readersTurn = false;

void *read( void *ptr ) {

int tid = (int) pthread_self();

int value;

while(1){

printf("\tReader %d wants to read! ", tid);

pthread_mutex_lock(&mutex);

while(activeWritersNumber > 0 || ( activeWritersNumber <= 0 && waitingWritersNumber > 0 && !readersTurn) ){

++waitingReadersNumber;

printf("\tReader %d is waiting... ", tid);

pthread_cond_wait(&cond, &mutex);

--waitingReadersNumber;

}

++activeReadersNumber;

pthread_mutex_unlock(&mutex);

printf(" \tReader %d is reading! ", tid);

printf("Number of active readers: %d, number of active writers: %d, number of waiting readers: %d, number of waiting writers: %d ", activeReadersNumber, activeWritersNumber, waitingReadersNumber, waitingWritersNumber );

usleep(rand() % 1000);

pthread_mutex_lock(&mutex);

--activeReadersNumber;

readersTurn = false;

printf("\tReader %d has finished reading. ", tid);

printf("Number of active readers: %d, number of active writers: %d, number of waiting readers: %d, number of waiting writers: %d ", activeReadersNumber, activeWritersNumber, waitingReadersNumber, waitingWritersNumber );

if(activeReadersNumber == 0){

pthread_cond_broadcast(&cond);

printf("\tReader %d was the last reader. Notifying one waiting writer. ", tid);

}

pthread_mutex_unlock(&mutex);

usleep(rand() % 1000);

}

}

void *write( void *ptr ) {

int tid = (int) pthread_self();

int value;

while(1){

printf("\t\t\tWriter %d wants to write! ", tid);

pthread_mutex_lock(&mutex);

while(activeReadersNumber > 0 || activeWritersNumber > 0 || (activeReadersNumber <= 0 && activeWritersNumber<=0 && waitingReadersNumber > 0 && readersTurn)){

++waitingWritersNumber;

printf("\t\t\tWriter %d is waiting... ", tid);

pthread_cond_wait(&cond, &mutex);

--waitingWritersNumber;

}

++activeWritersNumber;

pthread_mutex_unlock(&mutex);

printf("\t\t\tWriter %d is writing! ", tid);

printf("Number of active readers: %d, number of active writers: %d, number of waiting readers: %d, number of waiting writers: %d ", activeReadersNumber, activeWritersNumber, waitingReadersNumber, waitingWritersNumber );

usleep(rand() % 1000);

pthread_mutex_lock(&mutex);

--activeWritersNumber;

readersTurn = true;

printf("\t\t\tWriter %d has finished writing. Notifying waiting readers and writers. ",tid);

printf("Number of active readers: %d, number of active writers: %d, number of waiting readers: %d, number of waiting writers: %d ", activeReadersNumber, activeWritersNumber, waitingReadersNumber, waitingWritersNumber );

pthread_cond_broadcast(&cond);

pthread_mutex_unlock(&mutex);

usleep(rand() % 1000); }

}

int main() {

pthread_t readers[NUM_READERS_WRITERS];

pthread_t writers[NUM_READERS_WRITERS];

activeWritersNumber = 0;

activeReadersNumber = 0;

pthread_mutex_init(&mutex, NULL);

pthread_cond_init(&cond, NULL);

srand(time(NULL));

int i;

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

pthread_create( &readers[i], NULL, read, NULL);

pthread_create( &writers[i], NULL, write, NULL);

}

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

pthread_join( readers[i], NULL);

pthread_join( writers[i], NULL);

}

exit(0);

}

Compile readers_writers.c again:

student@Ubuntu:~/labs/lab3$ gcc -pthread readers_writers.c -o rw

Run the readers_writers program for a few seconds. Then press <Ctrl+ C> to stop the program:

student@Ubuntu:~ /labs/lab3 $ ./rw

Copy and paste the output below. Check the output. Has the problem been solved?