dreamy1992 2018-10-08 22:25
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len()线程在golang中安全吗?

I'm logging every second the length of a map; I don't care if I have the "exact" value / race conditions (off by one is ok). I'm interested to know if this could cause a panic and if I have to enclose len() with some .RLock()/Unlock() or not.

I'm asking because concurrent reads/writes in a map will cause a panic (Go detects that) but I don't know if reading the length counts as a "read". I have tried with a test program but cannot produce a crash but I'd rather have an exact answer, at least for the sake of it.

If it matters I'm interested in both len for arrays and for maps. Thanks!

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1条回答 默认 最新

  • 普通网友 2018-10-08 22:44
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    It is a race condition. The results are undefined. For example,

    racer.go:

    package main
    
    func main() {
        m := make(map[int]int)
        l := 0
        go func() {
            for {
                l = len(m)
            }
        }()
        for i := 0; i < 10000; i++ {
            m[i] = i
        }
    }
    

    Output:

    $ go run -race racer.go
    ==================
    WARNING: DATA RACE
    Read at 0x00c00008e000 by goroutine 5:
      main.main.func1()
          /home/peter/gopath/src/racer.go:8 +0x5f
    
    Previous write at 0x00c00008e000 by main goroutine:
      runtime.mapassign_fast64()
          /home/peter/go/src/runtime/map_fast64.go:92 +0x0
      main.main()
          /home/peter/gopath/src/racer.go:12 +0xba
    
    Goroutine 5 (running) created at:
      main.main()
          /home/peter/gopath/src/racer.go:6 +0x92
    ==================
    Found 1 data race(s)
    exit status 66
    $
    

    References:

    Wikipedia: Race condition

    The Go Blog: Introducing the Go Race Detector

    Go: Data Race Detector

    Benign Data Races: What Could Possibly Go Wrong?

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