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Licsber
V2EX  ›  Go 编程语言

Go 语言用切片实现队列遇到无法理解的问题

  •  
  •   Licsber ·
    licsber · 2018-12-07 21:10:28 +08:00 · 2722 次点击
    这是一个创建于 2163 天前的主题,其中的信息可能已经有所发展或是发生改变。
    package main
    
    import (
    	"fmt"
    	"log"
    	"net/http"
    	"os"
    	"time"
    )
    
    var Q []string
    
    func main() {
    	go func() {
    		time.Sleep(1 * time.Second)
    		for {
    			test()
    		}
    	}()
    	Q = append(Q, "233")
    	Q = append(Q, "233")
    	Q = append(Q, "233")
    
    	time.Sleep(3 * time.Second)
    	Q = append(Q, "233")
    	Q = append(Q, "233")
    	Q = append(Q, "233")
    	Q = append(Q, "233")
    	Q = append(Q, "233")
    
    	time.Sleep(3 * time.Second)
    	Q = append(Q, "233")
    	time.Sleep(3 * time.Second)
    	Q = append(Q, "233")
    
    	http.HandleFunc("/", handle)
    	err := http.ListenAndServe(":8080", nil)
    	if err != nil {
    		log.Println("http port has been used.")
    		os.Exit(-1)
    	}
    }
    func handle(w http.ResponseWriter, r *http.Request) {
    	defer r.Body.Close()
    	_ = r.ParseForm()
    	_, _ = fmt.Fprint(w, "hello")
    
    }
    func test() {
    	file := queueOut()
    	if file != "" {
    		log.Println(file)
    	}
    
    }
    
    func queueOut() string {
    	res := Q[0]
    	if len(Q) == 1 {
    		Q[0] = ""
    		return res
    	}
    	Q = Q[1:]
    	return res
    }
    
    

    我觉得我这样写没错啊. 但是运行的时候有几率出现

    panic: runtime error: invalid memory address or nil pointer dereference
    [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x1058fe5]
    
    goroutine 19 [running]:
    fmt.(*buffer).WriteString(...)
    	/opt/local/lib/go/src/fmt/print.go:82
    fmt.(*fmt).padString(0xc000114040, 0x0, 0x3)
    	/opt/local/lib/go/src/fmt/format.go:110 +0x9d
    fmt.(*fmt).fmtS(0xc000114040, 0x0, 0x3)
    	/opt/local/lib/go/src/fmt/format.go:328 +0x61
    fmt.(*pp).fmtString(0xc000114000, 0x0, 0x3, 0x76)
    	/opt/local/lib/go/src/fmt/print.go:437 +0x122
    fmt.(*pp).printArg(0xc000114000, 0x1247a20, 0xc000010050, 0x76)
    	/opt/local/lib/go/src/fmt/print.go:671 +0x878
    fmt.(*pp).doPrintln(0xc000114000, 0xc0000cffa8, 0x1, 0x1)
    	/opt/local/lib/go/src/fmt/print.go:1146 +0xb1
    fmt.Sprintln(0xc0000cffa8, 0x1, 0x1, 0x1247a20, 0x1)
    	/opt/local/lib/go/src/fmt/print.go:271 +0x52
    log.Println(0xc0000cffa8, 0x1, 0x1)
    	/opt/local/lib/go/src/log/log.go:301 +0x3f
    main.test()
    	/Users/licsber/go/src/test/main.go:52 +0xd7
    main.main.func1()
    	/Users/licsber/go/src/test/main.go:17 +0x2f
    created by main.main
    	/Users/licsber/go/src/test/main.go:14 +0x39
    
    Process finished with exit code 2
    
    

    小白表示无法理解 这个问题是几率出现. 每次运行不一定都有

    17 条回复    2018-12-07 22:24:09 +08:00
    mlkr
        1
    mlkr  
       2018-12-07 21:16:27 +08:00
    要加锁吧
    AlphaTr
        2
    AlphaTr  
       2018-12-07 21:18:59 +08:00
    slice 的线程安全问题,加锁?
    Licsber
        3
    Licsber  
    OP
       2018-12-07 21:24:11 +08:00
    @mlkr
    @AlphaTr 感谢 我去试试
    mlkr
        4
    mlkr  
       2018-12-07 21:26:07 +08:00   ❤️ 1
    package main

    import (
    "fmt"
    "log"
    "net/http"
    "os"
    "sync"
    "time"
    )

    var Q []string

    var lock sync.RWMutex

    func main() {
    go func() {
    time.Sleep(1 * time.Second)
    for {
    test()
    }
    }()
    Q = append(Q, "233")
    Q = append(Q, "233")
    Q = append(Q, "233")

    time.Sleep(3 * time.Second)
    Q = append(Q, "233")
    Q = append(Q, "233")
    Q = append(Q, "233")
    Q = append(Q, "233")
    Q = append(Q, "233")

    time.Sleep(3 * time.Second)
    Q = append(Q, "233")
    time.Sleep(3 * time.Second)
    Q = append(Q, "233")

    http.HandleFunc("/", handle)
    err := http.ListenAndServe(":8080", nil)
    if err != nil {
    log.Println("http port has been used.")
    os.Exit(-1)
    }
    }
    func handle(w http.ResponseWriter, r *http.Request) {
    defer r.Body.Close()
    _ = r.ParseForm()
    _, _ = fmt.Fprint(w, "hello")

    }
    func test() {
    file := queueOut()
    if file != "" {
    log.Println(file)
    }

    }

    func queueOut() string {
    // 加写锁
    lock.Lock()
    res := Q[0]
    if len(Q) == 1 {
    Q[0] = ""
    return res
    }
    Q = Q[1:]
    lock.Unlock()
    return res
    }
    littlewing
        5
    littlewing  
       2018-12-07 21:29:03 +08:00
    有 channel 不用非要用非线程安全的 slice

    另外 slice 的 append 操作是会重新分配 slice 结构的,而且你的 slice 初始 len 为 0,每次扩容时存储数据的 array 也会重新分配,问题应该就出现在这里
    Licsber
        6
    Licsber  
    OP
       2018-12-07 21:31:03 +08:00
    @mlkr 好像这样子之后, 会有数据没有被加到队列里去?
    Licsber
        7
    Licsber  
    OP
       2018-12-07 21:32:33 +08:00
    @littlewing channel 是啥.. 刚学 go 还没看到 我去看看 谢谢
    mritd
        8
    mritd  
       2018-12-07 21:37:36 +08:00 via iPhone
    大哥你这个 ... channel 啊
    heimeil
        9
    heimeil  
       2018-12-07 22:00:55 +08:00
    Q = Q[1:]瞬间执行多次,这个并发直接把 Q 给 cut 没了,再访问 Q 的任意下标自然就崩了。

    要非得用 slice 就别直接操作 slice,另外封装两个增删方法,加上锁。
    mlkr
        10
    mlkr  
       2018-12-07 22:12:18 +08:00
    @Licsber
    func queueOut() string {
    res := Q[0]
    if len(Q) == 1 {
    Q[0] = ""
    return res
    }
    // 加写锁
    lock.Lock()
    Q = Q[1:]
    lock.Unlock()
    return res
    }
    AngryPanda
        11
    AngryPanda  
       2018-12-07 22:14:49 +08:00 via Android   ❤️ 1
    channel 没学等于没学 go
    Licsber
        12
    Licsber  
    OP
       2018-12-07 22:20:20 +08:00
    @mritd 感谢 成功解决问题
    Licsber
        13
    Licsber  
    OP
       2018-12-07 22:21:39 +08:00
    @heimeil 对, 看了 channel 之后恍然大悟 谢谢
    Licsber
        14
    Licsber  
    OP
       2018-12-07 22:22:27 +08:00
    @AngryPanda 刚看到 go routine , 感谢
    现在接触的 go 给我一种我一直在处理错误(err)的错觉
    mritd
        15
    mritd  
       2018-12-07 22:22:29 +08:00
    @Licsber #12 channel 是 go 的神器啊,怎么能不用呢
    Licsber
        16
    Licsber  
    OP
       2018-12-07 22:23:03 +08:00
    @mlkr 感谢
    Licsber
        17
    Licsber  
    OP
       2018-12-07 22:24:09 +08:00
    @mritd 刚刚看到 go 的 channel , 感谢!
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