Question

Can you add more comments explaining what this code does? i commented what I know so far

#include<stdio.h>

#include<pthread.h>

#include<semaphore.h>

#include<unistd.h>

sem_t mutex,writeblock;

int data = 0,rcount = 0;

int sleepLength = 2; // used to represent work

void *reader(void *arg)

{

int f;

f = ((int)arg);

sem_wait(&mutex); // decrement by 1 if

rcount = rcount + 1;

if(rcount==1)

sem_wait(&writeblock);

sem_post(&mutex);

printf("Data read by the reader%d is %d\n",f,data); //shows current reader and data

sleep(sleepLength); // 1 second of "work" is being done here

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++; //increases data by 1

printf("Data writen by the writer%d is %d\n",f,data); //shows current writer and data

sleep(sleepLength); // 1 second of "work" is being done here

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=1;i<=5;i++)//loop to create writers and readers

{

pthread_create(&wtid[i],NULL,writer,(void *)i);

pthread_create(&rtid[i],NULL,reader,(void *)i);

//printf("looping in for\n");

}

for(i=1;i<=5;i++)//loop for

{

pthread_join(wtid[i],NULL);

pthread_join(rtid[i],NULL);

}

return 0;//resetting

}

Answer 1

This is standard reader writer problem.for reference , I am attaching below image.

===========================================================================

#include<stdio.h>

#include<pthread.h>

#include<semaphore.h>

#include<unistd.h>

sem_t mutex,writeblock; // These will be used to check if there is mutual exclusion between blocks

int data = 0,rcount = 0; //Data will be written by writer and rcount will store reader’s count

int sleepLength = 2; // used to represent work

void *reader(void *arg)

{

int f;

f = ((int)arg);

sem_wait(&mutex); // it will put wait condition on reader

rcount = rcount + 1; //increment reader count

if(rcount==1) //allows only one reader at time

sem_wait(&writeblock); //if reader is 1.meaning reader is writing then no one can write so it puts wait

sem_post(&mutex); //send signal to reader ..now ready to read

printf("Data read by the reader%d is %d\n",f,data); //shows current reader and data

sleep(sleepLength); // 1 second of "work" is being done here

sem_wait(&mutex);// it will put wait condition on reader

rcount = rcount - 1; //and it will reduce count and make it free

if(rcount==0) //Now no one is reading,then writer can write

sem_post(&writeblock); //signal writer for writing

sem_post(&mutex);// After all this,again reader can write,so it sends signal

}

void *writer(void *arg)

{

int f;

f = ((int) arg);

sem_wait(&writeblock); // It will stop all writer

data++; //increases data by 1 which will be written

printf("Data writen by the writer%d is %d\n",f,data); //shows current writer and data

sleep(sleepLength); // 1 second of "work" is being done here

sem_post(&writeblock);// It will send signal and makes value 1,so that other writers can write

}

int main()

{

int i,b;

pthread_t rtid[5],wtid[5]; // 5 threads are created

sem_init(&mutex,0,1);// 0 means it is shared across threads and 1 means it is initial value

sem_init(&writeblock,0,1); means it is shared across threads and 1 means it is initial value

for(i=1;i<=5;i++)//loop to create writers and readers

{

pthread_create(&wtid[i],NULL,writer,(void *)i);

pthread_create(&rtid[i],NULL,reader,(void *)i);

//printf("looping in for\n");

}

//All thread will be joined after execution

for(i=1;i<=5;i++)//loop for

{

pthread_join(wtid[i],NULL);

pthread_join(rtid[i],NULL);

}

return 0;//resetting

}