Shared memory to read an modify arrays - c

Question

I am starting to learn basics of OS and I have no experience at all at C and pointers. I am doing something wrong in my code but I don't know how to fix it.I am trying to populate an array first process initializes the array the second updates the values randomly. my issues are:
the first process writes to shared memory then exits I want it to actually read the array after it is updated
I think I am storing the array in a wrong way
I need a way to tell each process when the other one is done to keep the communication live until the first process decide it is over. below are my codes:
First Process
int main()
{
key_t key;
int shm_id;
int *shm_ptr;
if ((key = ftok("/tmp", 'y')) == -1)
{
perror("ftok() failed");
exit(EXIT_FAILURE);
}
shm_id = shmget(key, SIZE*sizeof(int), IPC_CREAT | IPC_EXCL | 0600 );
if (shm_id == -1)
{
perror("Failed to get memory");
exit(EXIT_FAILURE);
}
shm_ptr = (int *) shmat ( shm_id, NULL, 0);
if (shm_ptr == (int *) -1)
{
perror( "shmat failed" ) ;
exit( EXIT_FAILURE ) ;
}
printf("Array Initialized as : \n");
for(int i=0; i<SIZE-1; i++)
{
shm_ptr[i]=-1;
}
// for(int i = 0; i< 30; i++)
// {
// printf(" %d", shm_ptr[i]);
// }
shmdt(shm_ptr);
// shmctl(shm_id,IPC_RMID, NULL);
return 0;
}
Second Process
int main()
{
key_t key;
int sum = 0;
int shm_id;
int *shm_ptr;
key = ftok("/tmp", 'y');
shm_id = shmget( key, 0 , 0600 );
if (shm_id == -1)
{
perror("Failed to get memory");
exit(EXIT_FAILURE);
}
shm_ptr = (int*) shmat(shm_id,NULL,0);
printf("Client array \n");
srand(time(NULL));
for(int i = 0; i< 30; i++)
{
shm_ptr[i] = rand() % 20 + 1;
}
for(int i = 0; i< 30; i++)
{
printf(" %d", shm_ptr[i]);
}
shmdt(shm_ptr);
shmctl(shm_id,IPC_RMID, NULL);
return 0;
}

Answer 1

There is a lack of process synchronization in your program. Essentially, your second process may try to obtain a shared memory segment which doesn't exist at this point in time. Since, you are working across two executables, procuring PID is not an option hence, we cannot use signals. But, we can make it work with shared memory segments alone.
Create an additional shared memory segment such as data_ready. You want to use IPC_CREAT on both but, not IPC_EXCL. And have your first process set it to one once your array has been initialized. Since, we have used IPC_CREAT, neither of the process will overwrite over the data if the process doesn't exist and if it doesn't exist, the first process will anyway initialize it to 0. You can in fact, remove IPC_EXCL in your first process if you're employing this.
Have your first process set data_ready to 1 once it has initialized the array and block your second process using a command such as
```while(data_ready==0);````
This is rather a crude method for doing this. You can of course, do this much better with threads, semaphores, message queues etc. But, for the time being, this should get the job done.