Gosched之外还有什么？

Question

Code snippet below,

package main

import (
        "fmt"
        "runtime"
)

func main() {
        runtime.GOMAXPROCS(1)
        var s string
        done := make(chan bool)
        go func() {
                fmt.Scanln(&s)
                fmt.Println(s)
                done <- true
        }()

        var i int
        for i = 0; i < 1e10; i++ {
        }
        fmt.Println(i)
        <-done
}

Run it, quickly type a few chars e.g. abcd before the for loop ends; finally hit Enter. To my knowledge there is none of yield points available within the for loop for Gosched to switch the main goroutine to the subroutine, why had it promptly printed abcd even before the for loop was finished?

abcd10000000000

abcd

Answer 1

It is random you cannot predict the output. Because in your case the go routine will run and then the for loop will execute. Now if go routine finished before the for loop completes its iterations then it will print the value inside the go routine first and send the value on done channel and then it will print the value of i. The done channel will wait until the value is sent on it by the go routine.

For example if you try to scan only as and then enter fast your value will print first inside the go routine then 10000000000 will print in the last so it depends on if go routine will execute before the for loop completes.

The runtime can allocate more threads than the value of GOMAXPROCS to service multiple outstanding I/O requests. GOMAXPROCS only affects how many goroutines can actually execute at once; arbitrarily more may be blocked in system calls.