Read-Write shared memory - c

Question

I have a program in which i have a parent process and i want to create 3 children processes.
I also have created 2 shared memories (IN, OUT) and 1 semaphore for each shared memory.
The idea is:
The parent process has 3 integers and has to write in shared memory (IN) one of these integers. Then one of the children processes reads it, does some calculations and writes something in shared memory (OUT) from where the parent reads the result. Then the parent process adds the next integer to the shared memory and so on..
This is part of my code where i try to implement the first part (write-read from shared memory IN):
// create and attach shared memory
memidIN = shmget(...);
memidOUT= shmget(...);
pointerIN = (...) shmat(...);
pointerOUT = (...) shmat(...);
// create and init semaphores
semIN = semget(...);
semOUT = semget(...);
semctl(semIN, ...); // initialize both to 1
semctl(semOUT, ...);
for (i = 0; i < children; ++i)
{
pid = fork();
if (pid)
{
// parent process code
down sem_IN
write in shmIN
up sem_IN
}
else if (pid == 0)
{
// Children processes code
down sem_IN
read from shmIN
up sem_IN
exit(0);
}
else
//ERROR
}
// dont die until childrens die
for( j = 0; j < children; j++)
wait(&status);
The question is, how can i be sure that the children processes read the correct value? I mean if the parent process writes 5 then one process should take it, do something, write something in OUT. Then the parent should write another value, lets say 10.
In my program 5 could be read by the children processes 2 or more times.
Should i use a mutex semaphore to make sure that the children processes read the correct value and to make sure that the parent process updates the value when one of the children has read it?
Any ideas?

Answer 1

So, you can arrange data in you IN shared memory by this way:
int Number;
bool Number_IsRead; // Initialized by 'true'.
When you need to pass a Number, you should do
Number_IsRead = false;
Child should check Number_IsRead before reading Number.
When Number has read, child should assign true to Number_IsRead.
If you have 3 children who need to read Number, you should create 3 flags (bool Number_IsRead[3]).
Also you must use semaphores when you changing Number or Number_IsRead.
P.S. In general it is not a good idea to use shared memory instead of message queues (msgget/msgctl/msgsnd/msgrcv/etc.).