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< ol class = "chapter" > < li class = "chapter-item expanded affix " > < a href = "../index.html" > Go语言圣经< / a > < / li > < li class = "chapter-item expanded affix " > < a href = "../preface-zh.html" > 译者序< / a > < / li > < li class = "chapter-item expanded affix " > < a href = "../preface.html" > 前言< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1.html" > < strong aria-hidden = "true" > 1.< / strong > 入门< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-01.html" > < strong aria-hidden = "true" > 1.1.< / strong > Hello, World< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-02.html" > < strong aria-hidden = "true" > 1.2.< / strong > 命令行参数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-03.html" > < strong aria-hidden = "true" > 1.3.< / strong > 查找重复的行< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-04.html" > < strong aria-hidden = "true" > 1.4.< / strong > GIF动画< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-05.html" > < strong aria-hidden = "true" > 1.5.< / strong > 获取URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-06.html" > < strong aria-hidden = "true" > 1.6.< / strong > 并发获取多个URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-07.html" > < strong aria-hidden = "true" > 1.7.< / strong > Web服务< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-08.html" > < strong aria-hidden = "true" > 1.8.< / strong > 本章要点< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2.html" > < strong aria-hidden = "true" > 2.< / strong > 程序结构< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-01.html" > < strong aria-hidden = "true" > 2.1.< / strong > 命名< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-02.html" > < strong aria-hidden = "true" > 2.2.< / strong > 声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-03.html" > < strong aria-hidden = "true" > 2.3.< / strong > 变量< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-04.html" > < strong aria-hidden = "true" > 2.4.< / strong > 赋值< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-05.html" > < strong aria-hidden = "true" > 2.5.< / strong > 类型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-06.html" > < strong aria-hidden = "true" > 2.6.< / strong > 包和文件< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-07.html" > < strong aria-hidden = "true" > 2.7.< / strong > 作用域< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3.html" > < strong aria-hidden = "true" > 3.< / strong > 基础数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-01.html" > < strong aria-hidden = "true" > 3.1.< / strong > 整型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-02.html" > < strong aria-hidden = "true" > 3.2.< / strong > 浮点数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-03.html" > < strong aria-hidden = "true" > 3.3.< / strong > 复数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-04.html" > < strong aria-hidden = "true" > 3.4.< / strong > 布尔型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-05.html" > < strong aria-hidden = "true" > 3.5.< / strong > 字符串< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-06.html" > < strong aria-hidden = "true" > 3.6.< / strong > 常量< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4.html" > < strong aria-hidden = "true" > 4.< / strong > 复合数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-01.html" > < strong aria-hidden = "true" > 4.1.< / strong > 数组< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-02.html" > < strong aria-hidden = "true" > 4.2.< / strong > Slice< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-03.html" > < strong aria-hidden = "true" > 4.3.< / strong > Map< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-04.html" > < strong aria-hidden = "true" > 4.4.< / strong > 结构体< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-05.html" > < strong aria-hidden = "true" > 4.5.< / strong > JSON< /
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2024-06-12 07:20:07 +00:00
< li > < em > Go语言圣经读书笔记(挖坑中...): < a href = "https://github.com/chai2010/gopl-notes-zh" > https://github.com/chai2010/gopl-notes-zh< / a > < / em > < / li >
< li > < em > 凹语言(Go实现, 面向WASM设计): < a href = "https://github.com/wa-lang/wa" > https://github.com/wa-lang/wa< / a > < / em > < / li >
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2022-08-04 06:59:33 +00:00
< hr >
< h2 id = "132-unsafepointer" > < a class = "header" href = "#132-unsafepointer" > 13.2. unsafe.Pointer< / a > < / h2 >
< p > 大多数指针类型会写成< code > *T< / code > ， （ ： < code > void*< / code > 类型的指针），它可以包含任意类型变量的地址。当然，我们不可以直接通过< code > *p< / code > 来获取unsafe.Pointer指针指向的真实变量的值， ， ， < / p >
< p > 一个普通的< code > *T< / code > 类型指针可以被转化为unsafe.Pointer类型指针， ， < code > *T< / code > 类型相同。通过将< code > *float64< / code > 类型指针转化为< code > *uint64< / code > 类型指针，我们可以查看一个浮点数变量的位模式。< / p >
< pre > < code class = "language-Go" > package math
func Float64bits(f float64) uint64 { return *(*uint64)(unsafe.Pointer(& f)) }
fmt.Printf(" %#016x\n" , Float64bits(1.0)) // " 0x3ff0000000000000"
< / code > < / pre >
< p > 通过转为新类型指针，我们可以更新浮点数的位模式。通过位模式操作浮点数是可以的，但是更重要的意义是指针转换语法让我们可以在不破坏类型系统的前提下向内存写入任意的值。< / p >
< p > 一个unsafe.Pointer指针也可以被转化为uintptr类型， （ ： ， ） ， （ ， ， ） ， ， < / p >
< p > 许多将unsafe.Pointer指针转为原生数字， < code > *int16< / code > 类型指针， ： < / p >
< p > < u > < i > gopl.io/ch13/unsafeptr< / i > < / u > < / p >
< pre > < code class = "language-Go" > var x struct {
a bool
b int16
c []int
}
// 和 pb := & x.b 等价
pb := (*int16)(unsafe.Pointer(
uintptr(unsafe.Pointer(& x)) + unsafe.Offsetof(x.b)))
*pb = 42
fmt.Println(x.b) // " 42"
< / code > < / pre >
< p > 上面的写法尽管很繁琐， ， ， （ ： ） ： < / p >
< pre > < code class = "language-Go" > // NOTE: subtly incorrect!
tmp := uintptr(unsafe.Pointer(& x)) + unsafe.Offsetof(x.b)
pb := (*int16)(unsafe.Pointer(tmp))
*pb = 42
< / code > < / pre >
< p > 产生错误的原因很微妙。有时候垃圾回收器会移动一些变量以降低内存碎片等问题。这类垃圾回收器被称为移动GC。当一个变量被移动， ， ， ； ， ， ， ， < code > & x.b< / code > 地址。第三个向之前无效地址空间的赋值语句将彻底摧毁整个程序！< / p >
< p > 还有很多类似原因导致的错误。例如这条语句：< / p >
< pre > < code class = "language-Go" > pT := uintptr(unsafe.Pointer(new(T))) // 提示: 错误!
< / code > < / pre >
< p > 这里并没有指针引用< code > new< / code > 新创建的变量， ， ， < / p >
< p > 虽然目前的Go语言实现还没有使用移动GC（ ： ） ， ： ， ， < / p >
< p > 在编写本文时， ， （ < a href = "https://github.com/golang/go/issues/7192" > Issue7192< / a > ） ） ， ， ， < / p >
< p > 当调用一个库函数， （ ： ， ， ） ， ， < / p >
< pre > < code class = "language-Go" > package reflect
func (Value) Pointer() uintptr
func (Value) UnsafeAddr() uintptr
func (Value) InterfaceData() [2]uintptr // (index 1)
< / code > < / pre >
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