how polling works in linux

Polling in Linux is a mechanism that allows programs to monitor multiple file descriptors (such as sockets, pipes, or files) to see if any of them are ready for I/O operations (like reading or writing). This is useful in situations where a program needs to handle multiple streams of data simultaneously without constantly checking each one in a busy loop.

What Is Polling?

Polling in LinuxPolling in Linux is like waiting in line for something to happen. Imagine you’re at a party with multiple activities, but you can only do one at a time. You keep looking at each activity to see if it’s ready for you (like waiting for a basketball court to be free or waiting for the music to start playing). Polling is similar but for computers—your program keeps checking if different tasks are ready for it to handle (like reading data, writing data, etc.).

Step 1: Why Do We Need Polling?

In a computer program, sometimes you have to wait for multiple things at once, like:

• Waiting for data to come in from the internet.
• Waiting for a file to finish downloading.
• Waiting for someone to type something.

Instead of checking every millisecond if something is ready, which would waste time and resources, you use polling. It’s like saying, “Hey, let me know when you’re ready.”

Step 2: What is a File Descriptor?

Every file, network connection, or input device (like your keyboard) has something called a file descriptor. Think of it like a ticket number. When you want to do something with a file or connection, your program uses this ticket number to know what it’s working with.

Step 3: How Polling Works Step-by-Step

Let’s say you’re writing a program, and you want it to watch over two things:

1. Waiting for the user to type something.
2. Waiting for data from the internet.

Here’s how polling works:

1. Set Up the Things to Watch (File Descriptors):
   Imagine you have two things to watch. You tell the computer, “Watch the keyboard and the internet connection for me.” These are your file descriptors.
2. Tell the Computer What Events to Watch:
   You need to tell the computer exactly what you’re waiting for. For the keyboard, you’re waiting for someone to press a key. For the internet connection, you’re waiting for data to arrive. You specify these “events” for each file descriptor.
3. Start Polling:
   You then call a special function (like poll()). This is like saying, “I’ll wait here. Let me know when something happens.”
4. Wait and Check:
   The program pauses and waits. It doesn’t waste energy constantly checking the keyboard and internet—it just sits and waits. When something happens (like someone presses a key or data comes in), the computer wakes up your program and says, “Hey, one of the things you were waiting for is ready!”
5. Handle the Event:
   When the program wakes up, it checks which file descriptor is ready. If it’s the keyboard, it reads what the user typed. If it’s the internet, it grabs the incoming data.
6. Repeat:
   After handling the event, the program goes back to waiting for the next thing to happen.

Step 4: Example in Real Life

Let’s say you’re sitting in front of two video games. One game is downloading, and the other game is waiting for you to press a button to start. You keep checking to see if the download is complete, and you also check to see if the game is ready to play.

Instead of constantly checking both games every second, you tell someone, “Please tell me when either the download is complete or the game is ready to play.” Now, you can sit back and relax. When one of those things happens, that person taps you on the shoulder and says, “The game is ready!” or “The download is done!” Then you go handle that task.

Polling works like that: the program doesn’t keep checking everything itself. It waits for the computer to tell it when something is ready.

Step 5: Code Example (Simplified)

Let’s say we want to wait for someone to type something and also wait for a file to be ready for download. Here’s a simplified code snippet:

Step 6: Explaining the Code

• fds[0].fd = 0;: This tells the program to watch the keyboard (file descriptor 0 represents the keyboard or “standard input” in Linux).
• POLLIN: This means “I want to know when there is something to read” (like when the user types something).
• poll(fds, 1, 5000);: This function waits up to 5 seconds for something to happen. If nothing happens within 5 seconds, it will stop waiting.

Polling is a way to handle multiple things at once without wasting time constantly checking everything. It waits for something to be ready, and when it is, the program jumps into action. It’s efficient, useful for multitasking, and important for things like servers, games, and apps that deal with lots of input and output.Hope that makes it clearer! Let me know if you have more questions!