semaphore

beginner_84 · 08-24-2004, 03:08 PM

This program creates a circular queue of integers of size 20 in shared memory. It then creates 4 child processes. The second, third, and the fourth process created enters 50 consecutive integers each starting from 100, 200, and 300 respectively (so the second process enters 100, 101, 102,…,149; the third process enters 200, 201, 202, ..249 and so on). The first process sums up the integers entered and prints the final sum after all the integers have been entered and summed up.
The parent process waits until all child processes finish, cleans up all the shared memory and semaphore created, and exits.

HOWEVER, the program is not behaving in the expected manner..... the problem arises when the queue becomes full..
CAN ANYONE HELP ME OUT?... its urgent !!

Code:

#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/sem.h>
#include <errno.h>

#define P(s) semop(s, &pop, 1)
#define V(s) semop(s, &vop, 1)

int queue_is_empty(int z)
{
  if(z==0)
    return 1;
  else return 0;
}
int queue_is_full(int z)
{
  if(z==20)
    return 1;
  else return 0;
}
int insert(int a[],int number,int *indexs,int *size)
{
  if(*indexs==20)
    *indexs=0;
  a[(*indexs)++]=number;
  (*size)++;
  printf("%d\n",number);
  getchar();
  return 1;
}
int removes(int a[],int *indexs,int *size)
{
  if(*indexs==0)
    *indexs=20;
  (*size)--;
  printf("%d\n",a[(*indexs)-1]);
  getchar();  
  return a[--(*indexs)];
}

int main()
{
  int shmid,status;
  int semid1,semid2,semid3,semid4,sum=0,count=50,count1; 
  struct sembuf pop,vop;
  int *sto1,*sto2,*sto3,*sto4;
  int semparent,i,j;
  pid_t pid[4];

  shmid = shmget(IPC_PRIVATE, 22*sizeof(int), 0777|IPC_CREAT); 
  semid1 = semget(IPC_PRIVATE, 1, 0777|IPC_CREAT);
  semid2 = semget(IPC_PRIVATE, 1, 0777|IPC_CREAT);
  semid3 = semget(IPC_PRIVATE, 1, 0777|IPC_CREAT);
  semid4 = semget(IPC_PRIVATE, 1, 0777|IPC_CREAT);
  
  semctl(semid1, 0, SETVAL, 0);
  semctl(semid2, 0, SETVAL, 1);
  semctl(semid3, 0, SETVAL, 0);
  semctl(semid4, 0, SETVAL, 0);
 
  pop.sem_num = vop.sem_num = 0;
  pop.sem_flg = vop.sem_flg = 0;
  pop.sem_op = -1 ; vop.sem_op = 1 ;
  
  if(fork() == 0)
    {
      sto1 = (int *) shmat(shmid, 0, 0);      
      P(semid1);
      
      sto1[20]=0;
      sto1[21]=0;
    
      while (count1) {
	if(queue_is_empty(sto1[20]))
	  {
	    V(semid2);
	    P(semid1);
	  }
      	else
	  { 
	    printf("Process1 :");
	    //getchar();
	    sum+=removes(sto1,&sto1[21],&sto1[20]);
	    count1--;
	  }
      }
      shmdt(sto1);
      printf("SUM :%d\n",sum);
      getchar();
      exit(0);
    }
  else if(fork() ==0)
    {
      printf("\nentering 2");
      sto2 = (int *) shmat(shmid, 0, 0);
      //P(semid3);
      //P(semid4);
      while(count)
	{
	  P(semid2);
     
	  if(queue_is_full(sto2[20]))
	    {	  
	      V(semid1);
	      P(semid2);
	    }
	  else
	    {
	      printf("Process2 :");
	 
	      insert(sto2,count+100,&sto2[21],&sto2[20]);
	      count--;
	 	
	    }
	  printf("2nd"); 
	  V(semid3);
      
	}
      shmdt(sto2);
      exit(0);
    }

  else if(fork() ==0)
    {
      sto3 = (int *) shmat(shmid, 0, 0);
 
      while(count)
	{
	  P(semid3);
	  // P(semid2);
	  //   P(semid4);
	  if(queue_is_full(sto3[20]))
	    {	  printf("\nFull");
	    V(semid1);
	    P(semid3);
	    }
	  else
	    {
	      printf("Process3 :");
	      //getchar();
	      insert(sto3,count+200,&sto3[21],&sto3[20]);
	      count--;
	    }
	  printf("nunua");
	  V(semid4);
      
	}
      shmdt(sto3);
      exit(0);
    }
  else if(fork() ==0)
    {
      sto2 = (int *) shmat(shmid, 0, 0);
  
      while(count)
	{
	  printf("\nhug me");
	  P(semid4);
	  if(queue_is_full(sto2[20]))
	    {	  
	      printf("hagga");
	      V(semid1);
	      P(semid4);
	    }
	  else
	    {
	      printf("Process4 :");
	      //getchar();
	      insert(sto2,count+300,&sto2[21],&sto2[20]);
	      count--;
	    }
	 
	  V(semid2);
      
	}
      shmdt(sto4);
      exit(0);
    }
  else
    {
      sleep(3);
      wait(&status);
      shmctl(shmid, IPC_RMID, 0);
      semctl(semid1, 0, IPC_RMID, 0);
      semctl(semid2, 0, IPC_RMID, 0);
      semctl(semid3, 0, IPC_RMID, 0);
      semctl(semid4, 0, IPC_RMID, 0);
      
    }
}

acid_kewpie · 08-24-2004, 05:14 PM

this question looks an awful lot like homework. lpease note that it is against LQ rules to request help for homework, and if this is indeed some sort of assignment / coursework i'd be grateful if you could remove the content yourself.

beginner_84 · 08-25-2004, 06:22 PM

this is not a homework but a program that i encountered myself during learning semaphores and shared memory....... i have written the whole code myself but i m puzzled to see that its not working.so thats y I'd b grateful if any one of u can plz have a look and point out the problem....

itsme86 · 08-25-2004, 06:33 PM

I'd recommend starting smaller. Maybe just 1 child. See if simplifying the problem leads you to a solution. If you can get it to work with 1 child, then try 2 children before jumping to 5.

beginner_84 · 08-25-2004, 06:52 PM

i have worked successfully with 1 and 2 child processes......... in this program, the queue takes input perfectly for first 20 entries but after that instead of emptying the queue, the program goes to child2 and then the problem starts....data are taken in random order from the children after that.

beginner_84 · 08-25-2004, 06:54 PM

one mistake that i have found was taking sto2 again in 4th process in place of sto4. That was a typo.But correcting that hasn't solved the problem .