Threads callback相关技术咨询：代码疑问及使用场景问题

Hey Tomas, let’s unpack these questions clearly—they’re common points of confusion when working with concurrent code and function patterns!

1) What’s the relationship and use case motivation between callbacks and threads? #

First, let’s clarify the basics:

• A callback is a design pattern: you pass a function pointer (or a callable object) to another function, which then executes it at the right time. It’s all about decoupling logic—letting one piece of code trigger another without hardcoding the relationship.
• A thread is an execution context: a separate path of execution in your program, allowing concurrent work.

Their main connections and why you’d use them together:

• Thread entry points: Many threading APIs (like pthread_create in C, or std::thread in C++) let you pass a callback as the function the new thread will run. This lets you reuse the thread setup code for different tasks—you just swap the callback instead of writing a new thread function every time.
• Asynchronous notifications: When a background thread finishes a task (like fetching data or processing a file), it can call a callback to notify the main thread (or another part of your program) about the result. This avoids blocking the main thread while waiting for the task to finish.
• Event-driven threading: In systems where threads wait for events (like network sockets), callbacks are used to define what happens when an event triggers—so the thread can dynamically handle different actions based on which callback is registered.

2) Is line (1) just a function pointer (callback) call, or a thread call? #

Without seeing the exact code, here’s how to tell:

• If line (1) looks like this: my_callback_function(some_args); — that’s just a regular synchronous function call using a callback pointer. It runs in the current thread, right then and there, no new thread is created.
• If line (1) is part of creating a new thread, like pthread_create(&my_thread, NULL, my_callback_function, some_args); — then you’re using the callback as the entry point for the new thread. The thread will start executing that callback function, but the line itself is just the thread creation call, not the callback execution.

In short: A callback call is just executing a function pointer; a thread call (creation) uses the callback as the thread’s starting logic.

3) When should I use a callback inside a thread, vs. a regular function call? #

Use callbacks inside threads when:

• You need flexibility: If the thread’s task isn’t fixed. For example, a thread pool where each thread can run different tasks—you pass a callback for each task instead of writing a separate thread function for every possible job.
• You want to decouple components: If the thread’s core logic (like managing concurrency or waiting for events) is separate from the actual work it does. The thread doesn’t need to know what task it’s running, just that it should execute the callback when the time comes.
• Asynchronous result handling: When the thread needs to notify another part of the program when it’s done, without blocking. The callback lets you define what happens after the task finishes, without hardcoding that logic into the thread.

Use regular function calls inside threads when:

• The thread’s task is fixed: If the thread always does the same thing (e.g., a dedicated log-writing thread that only calls write_log_entry()), there’s no need for a callback—just call the function directly. It makes the code simpler and easier to read.
• Performance matters: Callbacks add a tiny overhead (indirect function call), so if you’re in a tight loop inside a thread where every cycle counts, a direct function call is better.
• Simplicity is key: If the thread logic is straightforward and doesn’t need to be reused for other tasks, a regular function call keeps the code more direct and less abstract.

内容的提问来源于stack exchange，提问作者Tomas_cz