How Can I Efficiently Check if a Go Buffered Channel is Full?

How to Check if a Buffered Channel is Full

In Go, buffered channels have a limited capacity to store data. When the channel is full, sending data to it will block the sender. However, if you want to drop data instead of blocking, how can you determine if the channel is full?

Solution using Select Statement

One way to check if a buffered channel is full is to use the select statement with a default case. If none of the cases can be executed, like sending data to a full channel, the default case will be executed.

package main

import "fmt"

func main() {
    ch := make(chan int, 1)

    // Fill it up
    ch <- 1

    select {
    case ch <- 2: // Put 2 in the channel unless it is full
    default:
        fmt.Println("Channel full. Discarding value")
    }
}

Output:

Channel full. Discarding value

This code creates a buffered channel of size 1 and sends a value to fill it. The select statement then attempts to send another value to the channel. If the channel is full, the default case is executed, printing a message and discarding the value.

Check without Sending

Alternatively, you can check the number of elements queued in a channel using len(ch). This allows you to determine if a channel is full without attempting to send data.

if len(ch) == cap(ch) {
    // Channel was full, but might not be by now
} else {
    // Channel wasn't full, but might be by now
}

However, note that the result of the comparison may become invalid by the time you enter the if block due to concurrent data flow.

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