Is len() thread safe in golang?

Issue

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!

Solution

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?

Answered By – peterSO

Answer Checked By – Dawn Plyler (GoLangFix Volunteer)

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