What does '-race' flag do in go build

Question

I am confusing for following code, what's the difference between go run with go run -race, Does the -race will change the program behavior?

// test.go
package main

import "fmt"

func main() {
    c := make(chan string)

    go func() {
        for i := 0; i < 2; i++ {
            c <- "hello there"
        }
    }()

    for msg := range c {
        fmt.Println(msg)
    }
}

when go run test.go, result is :

hello there
hello there
fatal error: all goroutines are asleep - deadlock!

goroutine 1 [chan receive]:
main.main()
        /Users/donghui6/go/src/jd.com/iaas-sre/test/test.go:14 +0xf4
exit status 2

when go run -race test.go, program will hang as following:

hello there
hello there

so, who can tell me what happened when use -race flag

Answer 1

what happen[s] when use '-race' flag in go build

Then the program is built with the so called "race detector" enabled. Dataraces are a programming error and any program with a datarace is invalid and its behaviour is undefined. You must never write code with data races.

A data race is when two or more goroutines read and write to the same memory without proper synchronisation. Data races happen but are a major fault of the programmer.

The race detector detects unsynchronised read/writes to the same memory and reports them as a failure (which it is). Note that if the race detector detects a data race your code is buggy even if it runs "properly" without -race.

The race detector is not on always because detecting data races slows down execution drastically.