golang 的指针地址问题 - V2EX
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yellowmarlboro

golang 的指针地址问题

  •  
  •   yellowmarlboro Jan 28, 2021 2852 views
    This topic created in 1930 days ago, the information mentioned may be changed or developed.
     s0 := "something" s1 := "something" s2 := "something"[7:] fmt.Println(&s0, &s1, &s2) fmt.Printf("%d \n", (*reflect.StringHeader)(unsafe.Pointer(&s0)).Data) fmt.Printf("%d \n", (*reflect.StringHeader)(unsafe.Pointer(&s1)).Data) fmt.Printf("%d \n", (*reflect.StringHeader)(unsafe.Pointer(&s2)).Data) 

    结果是

    0xc00010a040 0xc00010a050 0xc00010a060 4974445 4974445 4974452 
    • 为什么 & 出来的总差 16 ? unsafe.Pointer 是一样的?(请忽略切片的)
    Supplement 1    Feb 4, 2021
    我又凑不要脸来加问题了

    我该怎么调用字符串 struct 中的指针指向的底层数组并且修改他的值(如果可以修改的话)?
  • Unsafe
  • fmt
  • pointer
  • printf
    /div>12 replies    2021-02-03 20:02:34 +08:00
    yellowmarlboro
        1
    yellowmarlboro  
    OP
       Jan 28, 2021
    已经了解!
    End
    iQXQZX
        2
    iQXQZX  
       Jan 28, 2021
    为什么为什么为什么?插眼
    iamzuoxinyu
        3
    iamzuoxinyu  
       Jan 28, 2021 via Android
    分清栈上变量和字符串常量。
    koujyungenn
        4
    koujyungenn  
       Jan 29, 2021
    @yellowmarlboro 谜语人?
    kakach
        5
    kakach  
       Jan 29, 2021
    蹲一个大佬的回答
    kiddingU
        6
    kiddingU  
       Jan 29, 2021
    你自己用法姿势不对,正确的应该是这样吧:unsafe.Pointer((*reflect.StringHeader)(unsafe.Pointer(&s0)))
    yuguorui96
        7
    yuguorui96  
       Jan 29, 2021   1
    没翻源码,花了几分钟看了下汇编。

    s0, s1, s2 之间的地址总差 16 是因为 String 在栈上的结构是:
    struct String {
    char *data;
    int length;
    };
    sizeof(一个指针)+sizeof(一个 int)正好是 16 字节( 64 位环境下)。

    又因为栈上变量都是编译器按序分配的,所以有了楼主看到的结果。

    至于为啥 unsafe.Pointer 显示的结果是一致的,我个人猜测是因为字符串常量池(这个得去翻 Golang 的编译器,一会儿早上有会就不深挖了),即相同的字符串不会在内存中存在两次,有多次引用就把上面 data 的指针指向实际存储数据的 backing store 。这是一个常见优化,Python 中也存在。
    Dganzh
        8
    Dganzh  
       Jan 29, 2021
    字符串实际是一个**stuct
    struct string {
    byte* str;
    intgo len;
    }

    大小就是 16
    unsafe.Sizeof(s0) == 16
    kiddingU
        9
    kiddingU  
       Jan 29, 2021
    import (
    "github.com/davecgh/go-spew/spew"
    "reflect"
    "unsafe"
    )

    func main() {
    s0 := "something"
    s1 := "something"
    spew.Dump(&s0)
    spew.Dump(&s1)
    spew.Dump((*reflect.StringHeader)(unsafe.Pointer(&s0)))
    spew.Dump((*reflect.StringHeader)(unsafe.Pointer(&s1)))
    }

    --------------
    (*string)(0xc0001042b0)((len=9) "something")
    (*string)(0xc0001042c0)((len=9) "something")
    (*reflect.StringHeader)(0xc0001042b0)({
    Data: (uintptr) 0x1101229,
    Len: (int) 9
    })
    (*reflect.StringHeader)(0xc0001042c0)({
    Data: (uintptr) 0x1101229,
    Len: (int) 9
    })
    kiddingU
        10
    kiddingU  
       Jan 29, 2021
    @yuguorui96 编译一下,可以看到,golang 对于相同的字符串确实是做了优化处理,只存一份的
    iQXQZX
        11
    iQXQZX  
       Jan 29, 2021
    @kiddingU 学到了
    yellowmarlboro
        12
    yellowmarlboro  
    OP
       Feb 3, 2021
    下棋的时候翻出答案就没来得及回具体解决,抱歉~
    @kiddingU @yuguorui96 @Dganzh 等大佬都回答的要比我好!
    多谢各位~
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