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

#define array_size 1000 /* no of elements in shared memory */ 
//extern char *shmat();
void P(int *s);
void V(int *s) ;

int main(){
    int shmid, s, pid; /* shared memory, semaphore, proc id */ 
    char *shm; /*shared mem. addr returned by shmat()*/ 
    int *a, *addr, *sum; /* shared data variables*/
    int partial_sum; /* partial sum of each process */
    int i;
    /* initialize semaphore set */
    union semun {
	int val;
	struct semid_ds *buf;
	unsigned short int *array;
	struct seminfo *__buf;
    } su;

    int init_sem_value = 1;

    su.val = 1;
    s = semget(IPC_PRIVATE, 1, (0600 | IPC_CREAT));
    if (s == -1) { /* if unsuccessful*/
	perror("semget");
	exit( 1 );
    }
/*
    if (semctl(s, 0, SETVAL, init_sem_value) < 0) {
*/
    if (semctl(s, 0, SETVAL, su) < 0) {
	perror("semctl") ;
	exit ( 1 ) ;
    }
    /* create segment*/
    shmid = shmget(IPC_PRIVATE, (array_size*sizeof(int)+1),
		   (IPC_CREAT | 0600)) ;
    if (shmid == -1) {
	perror("shmget") ;
	exit (1) ;
    }
    /* map segment to process data space */ 
    shm = shmat(shmid, NULL, 0);
    /* returns address as a character*/
    if (shm == (char*)(-1)) {
	perror("shmat");
	exit(1);
    }

    addr = (int*)shm; /* starting address */ 
    sum = addr; /* accumulating sum */ 
    addr++;
    a = addr; /* array of numbers, a[] */
    *sum = 0;
    for (i = 0; i < array_size; i++) /* load array with numbers */
	*(a + i) = i+1;
    pid = fork() ; /* create child process */
    if (pid == 0) { /* child does this */
	partial_sum = 0;
	for (i = 0; i < array_size; i = i + 2)
	    partial_sum += *(a + i);
    }
    else { /* parent does this */
	partial_sum = 0;
	for (i = 1; i < array_size; i = i + 2)
	    partial_sum += *(a + i);
    }
    P(&s) ; /* for each process, add partial sum */ 
    *sum += partial_sum;
    V(&s) ;
    printf("\nprocess pid = %d, partial sum = %d\n", pid,
	   partial_sum);
	
    if (pid == 0) 
	exit(0); 
    else 
	wait(0); /* terminate child proc */ 
    printf("\nThe sum of 1 to %i is %d\n", array_size, *sum) ;
    /* remove semaphore */
    if (semctl(s, 0, IPC_RMID, 1) == -1) {
	perror("semctl") ;
	exit ( 1 ) ;
    }
    /* remove shared memory */ 
    if (shmctl(shmid, IPC_RMID, NULL) == -1) {
	perror("shmctl");
	exit ( 1 ) ;
    }
    /* end of main */
    return 0;
}

void P(int *s) /* P(s) routine*/
{
    struct sembuf sembuffer, *sops;
    sops = &sembuffer; 
    sops->sem_num = 0; 
    sops->sem_op = -1; 
    sops->sem_flg = 0;
    if (semop(*s, sops, 1) < 0) { 
	perror("semop") ;
	exit( 1 ) ;
    }
    return;
}

void V(int *s) /* V(s) routine */
{
    struct sembuf sembuffer, *sops;
    sops = &sembuffer;
    sops->sem_num = 0;
    sops->sem_op = 1;
    sops->sem_flg = 0;
    if (semop(*s, sops, 1) <0) {
	perror("semop");
	exit ( 1) ;
    }
    return;
}