Semaphore - Reader Writer Problem

semaphore.c

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

sem_t mutex,writeblock;
int data = 0,rcount = 0;

void *reader(void *arg)
{
  int f;
  f = ((int)arg);
  sem_wait(&mutex);
  rcount = rcount + 1;
  if(rcount==1)
   sem_wait(&writeblock);
  sem_post(&mutex);
  printf("Data read by the reader%d is %d\n",f,data);
  sleep(1);
  sem_wait(&mutex);
  rcount = rcount - 1;
  if(rcount==0)
   sem_post(&writeblock);
  sem_post(&mutex);
}

void *writer(void *arg)
{
  int f;
  f = ((int) arg);
  sem_wait(&writeblock);
  data++;
  printf("Data writen by the writer%d is %d\n",f,data);
  sleep(1);
  sem_post(&writeblock);
}

int main()
{
  int i,b; 
  pthread_t rtid[5],wtid[5];
  sem_init(&mutex,0,1);
  sem_init(&writeblock,0,1);
  for(i=0;i<=2;i++)
  {
    pthread_create(&wtid[i],NULL,writer,(void *)i);
    pthread_create(&rtid[i],NULL,reader,(void *)i);
  }
  for(i=0;i<=2;i++)
  {
    pthread_join(wtid[i],NULL);
    pthread_join(rtid[i],NULL);
  }
  return 0;
}

Why the loops are used?...please explain

There two pthread_create() in the for loop.
One is for reader and the other is for writer.

Loops are used to implement multiple reader and writer.

Is it reader's priority or writer's priority??

It is first writers priority. Once the writer has written something then only any reader can read.

…

On Fri, 27 Sep 2019, 1:49 pm Ferogle, ***@***.***> wrote: Is it reader's priority or writer's priority?? — You are receiving this because you commented. Reply to this email directly, view it on GitHub <https://gist.github.com/6209a575f00b122fe490?email_source=notifications&email_token=AK3DGUP7G52K2IEMZKCLJETQLW6Y3A5CNFSM4HK6C4BKYY3PNVWWK3TUL52HS4DFVNDWS43UINXW23LFNZ2KUY3PNVWWK3TUL5UWJTQAFZPPM#gistcomment-3038966>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AK3DGUOXBDQNQ4ZHXU2MSM3QLW6Y3ANCNFSM4HK6C4BA> .

Can you please explain with the lines of code in the writer function how it is the writers priority?

Can you please explain with the lines of code in the writer function how it is the writers priority?

sem_wait(&mutex); rcount = rcount + 1; if(rcount==1) sem_wait(&writeblock);

Inside reader you can see this piece of code, observe carefully:
rcount is 0 initially,
after statement : rcount = rcount+1;
value of rcount is 1,
If rcount is 1, It is calling sem_wait(&writeblock), this will make sure that the read block is on hold for long enough time.
Since now read block is on hold, the write block will be executed.

I hope I have answered your question.
Thank You!

I thought that would force write to wait. Like I thought it would force write to not be capable of executing if there were new writers after and that it would need to wait for the reader to finish.

…

On Fri, Apr 3, 2020 at 7:59 AM Mritunjay Choubey ***@***.***> wrote: ***@***.**** commented on this gist. ------------------------------ Can you please explain with the lines of code in the writer function how it is the writers priority? sem_wait(&mutex); rcount = rcount + 1; if(rcount==1) sem_wait(&writeblock); Inside reader you can see this piece of code, observe carefully: rcount is 0 initially, after statement : rcount = rcount+1; value of rcount is 1, If rcount is 1, It is calling sem_wait(&writeblock), this will make sure that the read block is on hold for long enough time. Since now read block is on hold, the write block will be executed. I hope I have answered your question. Thank You! — You are receiving this because you commented. Reply to this email directly, view it on GitHub <https://gist.github.com/6209a575f00b122fe490#gistcomment-3239055>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ALC3CNFFK4YB265V7SOHXLDRKXFQZANCNFSM4HK6C4BA> .

How can I solve this problem using shared_mutex?

I figured it out. Thank you.

…

On Fri, Apr 10, 2020 at 10:38 AM jprustom ***@***.***> wrote: ***@***.**** commented on this gist. ------------------------------ How can I solve this problem using shared_mutex? — You are receiving this because you commented. Reply to this email directly, view it on GitHub <https://gist.github.com/6209a575f00b122fe490#gistcomment-3248877>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ALC3CNFTZSE2JAJ6655OKQ3RL4VOZANCNFSM4HK6C4BA> .

it is binary semaphore