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@rajabishek
Created March 19, 2015 16:32
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Semaphore - Reader Writer Problem
#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;
}
@JaiParakh
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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.

@mjay4
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mjay4 commented Aug 26, 2019

Loops are used to implement multiple reader and writer.

@Ferogle
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Ferogle commented Sep 27, 2019

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

@mjay4
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mjay4 commented Sep 27, 2019 via email

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

@mjay4
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mjay4 commented Apr 3, 2020

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!

@angelitapuedes
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angelitapuedes commented Apr 3, 2020 via email

@jprustom
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How can I solve this problem using shared_mutex?

@angelitapuedes
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angelitapuedes commented Apr 10, 2020 via email

@rohith0510
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it is binary semaphore

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