go type_error ai_generated true

error: expected result 3, got 3 (all goroutines see same loop variable)

ID: go/loop-variable-capture

Also available as: JSON · Markdown · 中文
80%Fix Rate
90%Confidence
0Evidence
2024-02-05First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
1.18 active — — —
1.19 active — — —
1.20 active — — —
1.21 active — — —
1.22 active — — —

Root Cause

Before Go 1.22, goroutines launched inside a for loop captured the same loop variable, so all goroutines observed the final value. This causes subtle output bugs and races.

generic

中文

在 Go 1.22 之前，for 循环内启动的 goroutine 捕获同一个循环变量，导致所有 goroutine 看到最终值。这会造成微妙的输出错误和竞态。

Workarounds

  1. 95% success 
    Shadow the loop variable: for _, v := range items { v := v; go func(){ use(v) }() }
  2. 92% success 
    Upgrade to Go 1.22+ where loop variables are per-iteration by default, or pass v as an argument: go func(v T){...}(v)

Dead Ends

Common approaches that don't work:

  1. 90% fail

    The variable is shared; sleeping changes nothing about which value is read.

  2. 95% fail

    Scheduling does not affect variable capture semantics.