How to implement a solution to Readers-Writers problem using writers preference

Question

So I just now got into Multithreading and encountered the Readers-Writers problem.

In the following case:
-The driver creates 500 Reader threads and 500 Writer threads. After all threads have been started the driver waits for them to finish.
-Each Writer thread attempts to open a text file that contains a single number. If the file doesn't exist then the writer will create the file and store the number 1 in the file. If the file does exist the writer will read the value, add one to it and write the new value back to the file (replacing the original value).
-Each Reader will read the value from the file and display it. If the file doesn't exist it reports the error without crashing.

I have to modify the program so it employs a Writer-Preference solution to this instance of the Readers-Writers Problem. The basic logic of the provided code should remain intact.

Here are the following codes:

Driver - https://code.sololearn.com/cvbJnSbFXeEF
Reader - https://code.sololearn.com/c6i4eyhG071L
Writer - https://code.sololearn.com/c96698rqEgza

If anyone can help me, I would really appreciate it!

Answer 1

int readcount, writecount = 0;
semaphore rsem, wsem = 1; //
semaphore x,y,z = 1; //
void main(){
int p = fork();
if(p) reader; // assume multiple instances
else writer; // assume multiple instances
}
void reader(){
while(1){
wait(z);
wait(rsem);
wait(x);
readcount++;
if (readcount==1)
wait(wsem);
signal(x);
signal(rsem);
signal(z);
doReading();
wait(x);
readcount--;
if (readcount==0)
signal(wsem);
signal(x);
}
}
void writer(){
while(1){
wait(y);
writecount++;
if (writecount==1)
wait(rsem);
signal(y);
wait(wsem);
doWriting();
signal(wsem);
wait(y);
writecount--;
if (writecount==0)
signal(rsem);
signal(y);
}
}