Threads let a process do many different things at the same time

process: thread 1: I’m calculating ten million digits of π! so fun!
thread 2: I’m finding a REALLY BIG prime number!

threads in the same process share memory

thread 1: I’ll write some digits of to π O x 129420 in memory thread 2: uh oh! that’s where I was putting my prime numbers.

and they share code

calculate-pi
find-big-prime-number

but each thread has its own stack and they can be run by different CPUs at the same time
CPU 1: π thread
CPU 2: primes thread

sharing memory can cause problems (race conditions!)

at the same time:
memory: 23
thread 1: I’m going to add 1 to that number!
thread 2: I’m going to add 1 to that number!

RESULT: 24
WRONG. Should be 25!

why use threads instead of starting a new process?

a thread takes less time to create.

sharing data between threads is very easy. But it’s also easier to make mistakes with threads.

thread 1: you weren’t supposed to CHANGE that data!