Synchronization : Reader Writer problem

1) First readers-writers problem (favors readers)

-No reader should be kept waiting unless a writer has already been granted permission to use the object.

-A reader that arrives after a waiting writer gets priority over the writer.

=> coding (solution):

int readcnt; /* Initially 0 */

sem_t mutex, w; /* Both initially 1 */

void reader(void)

{

while (1) {

P(&mutex);

readcnt++;

if (readcnt == 1) /* First in */

P(&w);

V(&mutex);

/* Reading happens here */

P(&mutex);

readcnt--;

if (readcnt == 0) /* Last out */

V(&w);

V(&mutex);

}

}

void writer(void)

{

while (1) {

P(&w);

/* Writing here */

V(&w);

}

}

2) Second readers-writers problem (favors writers)

-Once a writer is ready to write, it performs its write as soon as possible

-A reader that arrives after a writer must wait, even if the writer is also waiting.

=> coding...

How to implement second reader-writers problems (favors writer)? (reference solution one)