//i%2==0 reader else writer
for(i=0; i<20; i++) {
    switch(fork()) {
        case 0:



    switch(i%2) {
        case 0://reader

            while(semctl(sem_mutex, 0, GETVAL) != 1); //wait(mutex)

            if(writers > 0 || readers == 0) {
                semctl(sem_mutex, 0, SETVAL, 1);          //signal(mutex)
                while(semctl(sem_resource, 0, GETVAL) != 1); //wait(mutex)
                while(semctl(sem_mutex, 0, GETVAL) != 1); //wait(mutex)
            }

            readers++;
            semctl(sem_mutex, 0, SETVAL, 1);          //signal(mutex)

            for(j = 0; j<20; j++) { //reading
                if(j%2!=0)
                    printf("%d %d ", getpid(), list[j]);
            }printf("n");

            while(semctl(sem_mutex, 0, GETVAL) != 1); //wait(mutex)
            readers--;
            if(readers==0)
                semctl(sem_resource, 0, SETVAL, 1);       //signal(resource)
            semctl(sem_mutex, 0, SETVAL, 1);          //signal(mutex)


            break;

        default://writer

            while(semctl(sem_mutex, 0, GETVAL) != 1); //wait(mutex)
            writers++;                                //writers <- +1
            semctl(sem_mutex, 0, SETVAL, 1);          //signal(mutex)
            while(semctl(sem_resource, 0, GETVAL) != 1); //wait(resource)

            list[i] = i; //writing

            while(semctl(sem_mutex, 0, GETVAL) != 1); //wait(mutex)
            writers--;
            semctl(sem_mutex, 0, SETVAL, 1);          //signal(mutex)
            semctl(sem_resource, 0, SETVAL, 1);       //signal(resource)

            break;

    }



shmdt(list);

    exit(0);
    break;

    default:
        break;

}