When are Function Arguments Evaluated in Go Goroutines?

Evaluating Function Arguments in Goroutines

In Go, when passing arguments to a function invoked with the go keyword, these arguments are evaluated in the main goroutine. This means that any variables passed as arguments are evaluated immediately, rather than when the function is executed within the newly created goroutine.

Example: Input from Main Goroutine

The code in question from "The Go Programming Language" explains that input.Text() is evaluated in the main goroutine because it is a function argument passed to the go echo() goroutine. As a result, the input is read and processed in the main goroutine before the echo() goroutine is started.

func handleConn(c net.Conn) {
    input := bufio.NewScanner(c)
    for input.Scan() {
        go echo(c, input.Text(), 1*time.Second)
    }
}

In the above example, input.Text() calls the Scan() method on the input buffer, which reads and returns the next line of text from the connection. Since this is an argument to go echo(), it is evaluated before the goroutine is started, ensuring that the actual text input is available to the echo() function.

Evaluation Order and Synchronization

Understanding when function arguments are evaluated is crucial for synchronization and data consistency in goroutine-based programs. By evaluating arguments in the main goroutine, Go ensures that the values passed to goroutines are up-to-date and thread-safe.

Conclusion

The evaluation of function arguments in goroutines helps maintain consistency and prevent race conditions. By having all argument evaluations occur in the main goroutine, Go eliminates the risk of using outdated or inconsistent data in concurrent operations.

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