Threads
Whats a thread?
Threads can allow a program to execute multiple sections of code at once
Uses for threads
- Faster code (execute on multiple cores at the same time)
- Non blocking IO (one thread waits for input while another does work)
Side note: processes
- A process can have multiple threads
- Heavier weight than threads
- A process is more like an entire program
- Learn more in OS
Using threads
How to use threads
- pthread in C (compile with -lpthread)
- std::thread in C++ (compile with -lpthread)
- Thread in Java
- *Python threading gets a bit weird
C++ example
#include <iostream>
#include <string>
#include <thread>
void thread_test(std::string id, unsigned long n) {
for (int i = 0; i < n; i++) {
std::cout << id << " " << i << '\n';
}
}
void demo1() {
std::thread t1{thread_test, "thread1", 10};
//Main thread continues executing here
// ...
// ...
// ...
//Main thread waits here until
t1.join();
}Demo
Making a simple program with threads!
Make a program to find the sum of an array
Tips: Split the array in half and have one thread work on each half
//Useful code bits
#include <thread>
//compile with this
g++ mycode.cpp -lpthread
//std::thread t(myfunction, arg1, arg2);
//t.join();
Live coding demo
Shared memory
- Threads can access global variables
- Efficient way for threads to communicate
- Can be dangerous (race conditions)
Fixing race conditions
Mutex locks
- When locked other threads will not run during that code segment
- Used to give only one thread access to data