The easy solution is to use a lock or someone else's thread safe queue as has already been suggested. If you're just trying to solve a problem, this is definitely the way to go.

If you're interested in really digging into concurrent programming for the purpose of learning, you may want to investigate writing your own lockless queue where exactly one thread can write and exactly one thread can read. It doesn't require a ton of code to do something simple and it's well worth the learning experience. The need to pass data continuously from thread A to thread B in this way is extremely common. An easy implementation is to have a "read pointer" and a "write pointer" that wraps in a circular buffer - just be careful of order of operations and make the read pointer and write pointer volatile. The "pointer" can be an index into the static-sized circular buffer.

Using a queue like this is great for passing data to be processed. It can also proxy a function call to another thread by using the "command" pattern. You can even pass exceptions this way.

The easy solution is to use a lock or someone else's thread safe queue as has already been suggested. If you're just trying to solve a problem, this is definitely the way to go.

If you're interested in really digging into concurrent programming for the purpose of learning, you may want to investigate writing your own lockless queue where exactly one thread can write and exactly one thread can read. It doesn't require a ton of code to do something simple and it's well worth the learning experience. The need to pass data continuously from thread A to thread B in this way is extremely common. An easy implementation is to have a "read pointer" and a "write pointer" that wraps in a circular buffer - just be careful of order of operations and make the read pointer and write pointer volatile. The "pointer" can be an index into the static-sized circular buffer.