Zig cookbook

Share data between two threads

When we want to mutate data shared in different threads, Mutex(Mutually exclusive flag) must be used to synchronize threads, otherwise the result maybe unexpected.

const std = @import("std");
const Thread = std.Thread;
const Mutex = Thread.Mutex;

const SharedData = struct {
    mutex: Mutex,
    value: i32,

    pub fn updateValue(self: *SharedData, increment: i32) void {
        // Use `tryLock` if you don't want to block
        self.mutex.lock();
        defer self.mutex.unlock();

        for (0..100) |_| {
            self.value += increment;
        }
    }
};

pub fn main() !void {
    var shared_data = SharedData{ .mutex = Mutex{}, .value = 0 };
    // This block is necessary to ensure that all threads are joined before proceeding.
    {
        const t1 = try Thread.spawn(.{}, SharedData.updateValue, .{ &shared_data, 1 });
        defer t1.join();
        const t2 = try Thread.spawn(.{}, SharedData.updateValue, .{ &shared_data, 2 });
        defer t2.join();
    }
    try std.testing.expectEqual(shared_data.value, 300);
}

If we remove Mutex protection, the result will most like be less than 300.