the problem is this:
AirCanada has 100 seats available for flight888, and there are 10 travel agents that's selling these 100 seats. So I am going to use 10 threads to indicate the 10 travel agents. Whichever a thread book a seat, the seats_available is decrement by 1. Basically the seats_available is a global variable. The ratio of booking/cancelling is 3:1, so every 3 seats booking will result a seat of cancelling.

I have to avoid the any two or more threads updating the seats_available at the same time. So what should I do? how do I code it in c or c++ that will act in a way so every 3 seats booked and 1 seat will be canceled? do I need to use random seed for the 10 threads to decide which one is going to perform some kind of action.

The program will end when seats_available is 0.

and constructive idea is welcomed.

you should reference some thread synchronization issues ,actually it's very easy,you can use Mutex or semaphore,
btw,I am not very clear about what you said "so every 3 seats booking will result a seat of cancelling."
good luck

Indeed it's quite easy!! But do I need a random seed to handle when each thread to make a booking/cancelling.

what I meant by ""so every 3 seats booking will result a seat of cancelling." is that 75% of the peple will make a booking and 25% will make a cancelling.

Is your programe used for simulation ?
if so,the random seed is not enough,because it's average rate will be 1:1