2015-12-09 07:57:17 +00:00
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2015-12-16 02:56:29 +00:00
< div class = "book" data-level = "13.2" data-chapter-title = "unsafe.Pointer" data-filepath = "ch13/ch13-02.md" data-basepath = ".." data-revision = "Wed Dec 16 2015 10:54:29 GMT+0800 (中国标准时间)" >
2015-12-09 07:57:17 +00:00
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前言
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< b > 0.1.< / b >
Go語言起源
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< b > 0.2.< / b >
Go語言項目
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< b > 0.3.< / b >
本書的組織
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< b > 0.4.< / b >
更多的信息
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< b > 0.5.< / b >
緻謝
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< b > 1.< / b >
入門
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< b > 1.1.< / b >
Hello, World
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< b > 1.2.< / b >
命令行參數
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< b > 1.3.< / b >
査找重復的行
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< b > 1.4.< / b >
GIF動畫
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< b > 1.5.< / b >
穫取URL
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< b > 1.6.< / b >
併髮穫取多個URL
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< b > 1.7.< / b >
Web服務
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< b > 1.8.< / b >
本章要點
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< b > 2.< / b >
程序結構
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< b > 2.1.< / b >
命名
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< b > 2.2.< / b >
聲明
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< b > 2.3.< / b >
變量
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< b > 2.4.< / b >
賦值
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< b > 2.5.< / b >
類型
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< b > 2.6.< / b >
包和文件
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< b > 2.7.< / b >
作用域
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< b > 3.< / b >
基礎數據類型
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< b > 3.1.< / b >
整型
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< b > 3.2.< / b >
浮點數
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< b > 3.3.< / b >
復數
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< b > 3.4.< / b >
佈爾型
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< b > 3.5.< / b >
字符串
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< b > 3.6.< / b >
常量
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< b > 4.< / b >
復閤數據類型
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< b > 4.1.< / b >
數組
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< b > 4.2.< / b >
切片
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< b > 4.3.< / b >
字典
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< b > 4.4.< / b >
結構體
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< b > 4.5.< / b >
JSON
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< b > 4.6.< / b >
文本和HTML模闆
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< b > 5.< / b >
函數
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< b > 5.1.< / b >
函數聲明
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< li class = "chapter " data-level = "5.2" data-path = "ch5/ch5-02.html" >
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< b > 5.2.< / b >
遞歸
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< li class = "chapter " data-level = "5.3" data-path = "ch5/ch5-03.html" >
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< b > 5.3.< / b >
多返迴值
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< li class = "chapter " data-level = "5.4" data-path = "ch5/ch5-04.html" >
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< b > 5.4.< / b >
錯誤
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< b > 5.5.< / b >
函數值
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< li class = "chapter " data-level = "5.6" data-path = "ch5/ch5-06.html" >
< a href = "../ch5/ch5-06.html" >
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< b > 5.6.< / b >
匿名函數
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< / li >
< li class = "chapter " data-level = "5.7" data-path = "ch5/ch5-07.html" >
< a href = "../ch5/ch5-07.html" >
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< b > 5.7.< / b >
可變參數
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< li class = "chapter " data-level = "5.8" data-path = "ch5/ch5-08.html" >
< a href = "../ch5/ch5-08.html" >
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< b > 5.8.< / b >
Deferred函數
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< li class = "chapter " data-level = "5.9" data-path = "ch5/ch5-09.html" >
< a href = "../ch5/ch5-09.html" >
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< b > 5.9.< / b >
Panic異常
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< li class = "chapter " data-level = "5.10" data-path = "ch5/ch5-10.html" >
< a href = "../ch5/ch5-10.html" >
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< b > 5.10.< / b >
Recover捕穫異常
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< li class = "chapter " data-level = "6" data-path = "ch6/ch6.html" >
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< b > 6.< / b >
方法
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< ul class = "articles" >
< li class = "chapter " data-level = "6.1" data-path = "ch6/ch6-01.html" >
< a href = "../ch6/ch6-01.html" >
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< b > 6.1.< / b >
方法聲明
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< li class = "chapter " data-level = "6.2" data-path = "ch6/ch6-02.html" >
< a href = "../ch6/ch6-02.html" >
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< b > 6.2.< / b >
基於指鍼對象的方法
< / a >
< / li >
< li class = "chapter " data-level = "6.3" data-path = "ch6/ch6-03.html" >
< a href = "../ch6/ch6-03.html" >
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< b > 6.3.< / b >
通過嵌入結構體來擴展類型
< / a >
< / li >
< li class = "chapter " data-level = "6.4" data-path = "ch6/ch6-04.html" >
< a href = "../ch6/ch6-04.html" >
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< b > 6.4.< / b >
方法值和方法錶達式
< / a >
< / li >
< li class = "chapter " data-level = "6.5" data-path = "ch6/ch6-05.html" >
< a href = "../ch6/ch6-05.html" >
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< b > 6.5.< / b >
示例: Bit數組
< / a >
< / li >
< li class = "chapter " data-level = "6.6" data-path = "ch6/ch6-06.html" >
< a href = "../ch6/ch6-06.html" >
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< b > 6.6.< / b >
封裝
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< / li >
< li class = "chapter " data-level = "7" data-path = "ch7/ch7.html" >
< a href = "../ch7/ch7.html" >
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< b > 7.< / b >
接口
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< ul class = "articles" >
< li class = "chapter " data-level = "7.1" data-path = "ch7/ch7-01.html" >
< a href = "../ch7/ch7-01.html" >
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< b > 7.1.< / b >
接口是閤約
< / a >
< / li >
< li class = "chapter " data-level = "7.2" data-path = "ch7/ch7-02.html" >
< a href = "../ch7/ch7-02.html" >
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< b > 7.2.< / b >
接口類型
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< li class = "chapter " data-level = "7.3" data-path = "ch7/ch7-03.html" >
< a href = "../ch7/ch7-03.html" >
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< b > 7.3.< / b >
實現接口的條件
< / a >
< / li >
< li class = "chapter " data-level = "7.4" data-path = "ch7/ch7-04.html" >
< a href = "../ch7/ch7-04.html" >
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< b > 7.4.< / b >
flag.Value接口
< / a >
< / li >
< li class = "chapter " data-level = "7.5" data-path = "ch7/ch7-05.html" >
< a href = "../ch7/ch7-05.html" >
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< b > 7.5.< / b >
接口值
< / a >
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< li class = "chapter " data-level = "7.6" data-path = "ch7/ch7-06.html" >
< a href = "../ch7/ch7-06.html" >
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< b > 7.6.< / b >
sort.Interface接口
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< li class = "chapter " data-level = "7.7" data-path = "ch7/ch7-07.html" >
< a href = "../ch7/ch7-07.html" >
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< b > 7.7.< / b >
http.Handler接口
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< li class = "chapter " data-level = "7.8" data-path = "ch7/ch7-08.html" >
< a href = "../ch7/ch7-08.html" >
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< b > 7.8.< / b >
error接口
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< li class = "chapter " data-level = "7.9" data-path = "ch7/ch7-09.html" >
< a href = "../ch7/ch7-09.html" >
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< b > 7.9.< / b >
示例: 錶達式求值
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< li class = "chapter " data-level = "7.10" data-path = "ch7/ch7-10.html" >
< a href = "../ch7/ch7-10.html" >
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< b > 7.10.< / b >
類型斷言
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< li class = "chapter " data-level = "7.11" data-path = "ch7/ch7-11.html" >
< a href = "../ch7/ch7-11.html" >
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< b > 7.11.< / b >
基於類型斷言識彆錯誤類型
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< / li >
< li class = "chapter " data-level = "7.12" data-path = "ch7/ch7-12.html" >
< a href = "../ch7/ch7-12.html" >
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< b > 7.12.< / b >
通過類型斷言査詢接口
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< li class = "chapter " data-level = "7.13" data-path = "ch7/ch7-13.html" >
< a href = "../ch7/ch7-13.html" >
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< b > 7.13.< / b >
類型分支
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< li class = "chapter " data-level = "7.14" data-path = "ch7/ch7-14.html" >
< a href = "../ch7/ch7-14.html" >
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< b > 7.14.< / b >
示例: 基於標記的XML解碼
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< li class = "chapter " data-level = "7.15" data-path = "ch7/ch7-15.html" >
< a href = "../ch7/ch7-15.html" >
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< b > 7.15.< / b >
補充幾點
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< / ul >
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< li class = "chapter " data-level = "8" data-path = "ch8/ch8.html" >
< a href = "../ch8/ch8.html" >
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< b > 8.< / b >
Goroutines和Channels
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< li class = "chapter " data-level = "8.1" data-path = "ch8/ch8-01.html" >
< a href = "../ch8/ch8-01.html" >
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< b > 8.1.< / b >
Goroutines
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< li class = "chapter " data-level = "8.2" data-path = "ch8/ch8-02.html" >
< a href = "../ch8/ch8-02.html" >
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< b > 8.2.< / b >
示例: 併髮的Clock服務
< / a >
< / li >
< li class = "chapter " data-level = "8.3" data-path = "ch8/ch8-03.html" >
< a href = "../ch8/ch8-03.html" >
< i class = "fa fa-check" > < / i >
< b > 8.3.< / b >
示例: 併髮的Echo服務
< / a >
< / li >
< li class = "chapter " data-level = "8.4" data-path = "ch8/ch8-04.html" >
< a href = "../ch8/ch8-04.html" >
< i class = "fa fa-check" > < / i >
< b > 8.4.< / b >
Channels
< / a >
< / li >
< li class = "chapter " data-level = "8.5" data-path = "ch8/ch8-05.html" >
< a href = "../ch8/ch8-05.html" >
< i class = "fa fa-check" > < / i >
< b > 8.5.< / b >
併行的循環
< / a >
< / li >
< li class = "chapter " data-level = "8.6" data-path = "ch8/ch8-06.html" >
< a href = "../ch8/ch8-06.html" >
< i class = "fa fa-check" > < / i >
< b > 8.6.< / b >
示例: 併髮的Web爬蟲
< / a >
< / li >
< li class = "chapter " data-level = "8.7" data-path = "ch8/ch8-07.html" >
< a href = "../ch8/ch8-07.html" >
< i class = "fa fa-check" > < / i >
< b > 8.7.< / b >
基於select的多路復用
< / a >
< / li >
< li class = "chapter " data-level = "8.8" data-path = "ch8/ch8-08.html" >
< a href = "../ch8/ch8-08.html" >
< i class = "fa fa-check" > < / i >
< b > 8.8.< / b >
示例: 併髮的字典遍歷
< / a >
< / li >
< li class = "chapter " data-level = "8.9" data-path = "ch8/ch8-09.html" >
< a href = "../ch8/ch8-09.html" >
< i class = "fa fa-check" > < / i >
< b > 8.9.< / b >
併髮的退齣
< / a >
< / li >
< li class = "chapter " data-level = "8.10" data-path = "ch8/ch8-10.html" >
< a href = "../ch8/ch8-10.html" >
< i class = "fa fa-check" > < / i >
< b > 8.10.< / b >
示例: 聊天服務
< / a >
< / li >
< / ul >
< / li >
< li class = "chapter " data-level = "9" data-path = "ch9/ch9.html" >
< a href = "../ch9/ch9.html" >
< i class = "fa fa-check" > < / i >
< b > 9.< / b >
基於共享變量的併髮
< / a >
< ul class = "articles" >
< li class = "chapter " data-level = "9.1" data-path = "ch9/ch9-01.html" >
< a href = "../ch9/ch9-01.html" >
< i class = "fa fa-check" > < / i >
< b > 9.1.< / b >
競爭條件
< / a >
< / li >
< li class = "chapter " data-level = "9.2" data-path = "ch9/ch9-02.html" >
< a href = "../ch9/ch9-02.html" >
< i class = "fa fa-check" > < / i >
< b > 9.2.< / b >
sync.Mutex互斥鎖
< / a >
< / li >
< li class = "chapter " data-level = "9.3" data-path = "ch9/ch9-03.html" >
< a href = "../ch9/ch9-03.html" >
< i class = "fa fa-check" > < / i >
< b > 9.3.< / b >
sync.RWMutex讀寫鎖
< / a >
< / li >
< li class = "chapter " data-level = "9.4" data-path = "ch9/ch9-04.html" >
< a href = "../ch9/ch9-04.html" >
< i class = "fa fa-check" > < / i >
< b > 9.4.< / b >
內存衕步
< / a >
< / li >
< li class = "chapter " data-level = "9.5" data-path = "ch9/ch9-05.html" >
< a href = "../ch9/ch9-05.html" >
< i class = "fa fa-check" > < / i >
< b > 9.5.< / b >
sync.Once初始化
< / a >
< / li >
< li class = "chapter " data-level = "9.6" data-path = "ch9/ch9-06.html" >
< a href = "../ch9/ch9-06.html" >
< i class = "fa fa-check" > < / i >
< b > 9.6.< / b >
競爭條件檢測
< / a >
< / li >
< li class = "chapter " data-level = "9.7" data-path = "ch9/ch9-07.html" >
< a href = "../ch9/ch9-07.html" >
< i class = "fa fa-check" > < / i >
< b > 9.7.< / b >
示例: 併髮的非阻塞緩存
< / a >
< / li >
< li class = "chapter " data-level = "9.8" data-path = "ch9/ch9-08.html" >
< a href = "../ch9/ch9-08.html" >
< i class = "fa fa-check" > < / i >
< b > 9.8.< / b >
Goroutines和綫程
< / a >
< / li >
< / ul >
< / li >
< li class = "chapter " data-level = "10" data-path = "ch10/ch10.html" >
< a href = "../ch10/ch10.html" >
< i class = "fa fa-check" > < / i >
< b > 10.< / b >
包和工具
< / a >
< ul class = "articles" >
< li class = "chapter " data-level = "10.1" data-path = "ch10/ch10-01.html" >
< a href = "../ch10/ch10-01.html" >
< i class = "fa fa-check" > < / i >
< b > 10.1.< / b >
簡介
< / a >
< / li >
< li class = "chapter " data-level = "10.2" data-path = "ch10/ch10-02.html" >
< a href = "../ch10/ch10-02.html" >
< i class = "fa fa-check" > < / i >
< b > 10.2.< / b >
導入路徑
< / a >
< / li >
< li class = "chapter " data-level = "10.3" data-path = "ch10/ch10-03.html" >
< a href = "../ch10/ch10-03.html" >
< i class = "fa fa-check" > < / i >
< b > 10.3.< / b >
包聲明
< / a >
< / li >
< li class = "chapter " data-level = "10.4" data-path = "ch10/ch10-04.html" >
< a href = "../ch10/ch10-04.html" >
< i class = "fa fa-check" > < / i >
< b > 10.4.< / b >
導入聲明
< / a >
< / li >
< li class = "chapter " data-level = "10.5" data-path = "ch10/ch10-05.html" >
< a href = "../ch10/ch10-05.html" >
< i class = "fa fa-check" > < / i >
< b > 10.5.< / b >
匿名導入
< / a >
< / li >
< li class = "chapter " data-level = "10.6" data-path = "ch10/ch10-06.html" >
< a href = "../ch10/ch10-06.html" >
< i class = "fa fa-check" > < / i >
< b > 10.6.< / b >
包和命名
< / a >
< / li >
< li class = "chapter " data-level = "10.7" data-path = "ch10/ch10-07.html" >
< a href = "../ch10/ch10-07.html" >
< i class = "fa fa-check" > < / i >
< b > 10.7.< / b >
工具
< / a >
< / li >
< / ul >
< / li >
< li class = "chapter " data-level = "11" data-path = "ch11/ch11.html" >
< a href = "../ch11/ch11.html" >
< i class = "fa fa-check" > < / i >
< b > 11.< / b >
測試
< / a >
< ul class = "articles" >
< li class = "chapter " data-level = "11.1" data-path = "ch11/ch11-01.html" >
< a href = "../ch11/ch11-01.html" >
< i class = "fa fa-check" > < / i >
< b > 11.1.< / b >
go test
< / a >
< / li >
< li class = "chapter " data-level = "11.2" data-path = "ch11/ch11-02.html" >
< a href = "../ch11/ch11-02.html" >
< i class = "fa fa-check" > < / i >
< b > 11.2.< / b >
測試函數
< / a >
< / li >
< li class = "chapter " data-level = "11.3" data-path = "ch11/ch11-03.html" >
< a href = "../ch11/ch11-03.html" >
< i class = "fa fa-check" > < / i >
< b > 11.3.< / b >
測試覆蓋率
< / a >
< / li >
< li class = "chapter " data-level = "11.4" data-path = "ch11/ch11-04.html" >
< a href = "../ch11/ch11-04.html" >
< i class = "fa fa-check" > < / i >
< b > 11.4.< / b >
基準測試
< / a >
< / li >
< li class = "chapter " data-level = "11.5" data-path = "ch11/ch11-05.html" >
< a href = "../ch11/ch11-05.html" >
< i class = "fa fa-check" > < / i >
< b > 11.5.< / b >
剖析
< / a >
< / li >
< li class = "chapter " data-level = "11.6" data-path = "ch11/ch11-06.html" >
< a href = "../ch11/ch11-06.html" >
< i class = "fa fa-check" > < / i >
< b > 11.6.< / b >
示例函數
< / a >
< / li >
< / ul >
< / li >
< li class = "chapter " data-level = "12" data-path = "ch12/ch12.html" >
< a href = "../ch12/ch12.html" >
< i class = "fa fa-check" > < / i >
< b > 12.< / b >
反射
< / a >
< ul class = "articles" >
< li class = "chapter " data-level = "12.1" data-path = "ch12/ch12-01.html" >
< a href = "../ch12/ch12-01.html" >
< i class = "fa fa-check" > < / i >
< b > 12.1.< / b >
為何需要反射?
< / a >
< / li >
< li class = "chapter " data-level = "12.2" data-path = "ch12/ch12-02.html" >
< a href = "../ch12/ch12-02.html" >
< i class = "fa fa-check" > < / i >
< b > 12.2.< / b >
reflect.Type和reflect.Value
< / a >
< / li >
< li class = "chapter " data-level = "12.3" data-path = "ch12/ch12-03.html" >
< a href = "../ch12/ch12-03.html" >
< i class = "fa fa-check" > < / i >
< b > 12.3.< / b >
Display遞歸打印
< / a >
< / li >
< li class = "chapter " data-level = "12.4" data-path = "ch12/ch12-04.html" >
< a href = "../ch12/ch12-04.html" >
< i class = "fa fa-check" > < / i >
< b > 12.4.< / b >
示例: 編碼S錶達式
< / a >
< / li >
< li class = "chapter " data-level = "12.5" data-path = "ch12/ch12-05.html" >
< a href = "../ch12/ch12-05.html" >
< i class = "fa fa-check" > < / i >
< b > 12.5.< / b >
通過reflect.Value脩改值
< / a >
< / li >
< li class = "chapter " data-level = "12.6" data-path = "ch12/ch12-06.html" >
< a href = "../ch12/ch12-06.html" >
< i class = "fa fa-check" > < / i >
< b > 12.6.< / b >
示例: 解碼S錶達式
< / a >
< / li >
< li class = "chapter " data-level = "12.7" data-path = "ch12/ch12-07.html" >
< a href = "../ch12/ch12-07.html" >
< i class = "fa fa-check" > < / i >
< b > 12.7.< / b >
穫取結構體字段標識
< / a >
< / li >
< li class = "chapter " data-level = "12.8" data-path = "ch12/ch12-08.html" >
< a href = "../ch12/ch12-08.html" >
< i class = "fa fa-check" > < / i >
< b > 12.8.< / b >
顯示一個類型的方法集
< / a >
< / li >
< li class = "chapter " data-level = "12.9" data-path = "ch12/ch12-09.html" >
< a href = "../ch12/ch12-09.html" >
< i class = "fa fa-check" > < / i >
< b > 12.9.< / b >
幾點忠告
< / a >
< / li >
< / ul >
< / li >
< li class = "chapter " data-level = "13" data-path = "ch13/ch13.html" >
< a href = "../ch13/ch13.html" >
< i class = "fa fa-check" > < / i >
< b > 13.< / b >
底層編程
< / a >
< ul class = "articles" >
< li class = "chapter " data-level = "13.1" data-path = "ch13/ch13-01.html" >
< a href = "../ch13/ch13-01.html" >
< i class = "fa fa-check" > < / i >
< b > 13.1.< / b >
unsafe.Sizeof, Alignof 和 Offsetof
< / a >
< / li >
< li class = "chapter active" data-level = "13.2" data-path = "ch13/ch13-02.html" >
< a href = "../ch13/ch13-02.html" >
< i class = "fa fa-check" > < / i >
< b > 13.2.< / b >
unsafe.Pointer
< / a >
< / li >
< li class = "chapter " data-level = "13.3" data-path = "ch13/ch13-03.html" >
< a href = "../ch13/ch13-03.html" >
< i class = "fa fa-check" > < / i >
< b > 13.3.< / b >
示例: 深度相等判斷
< / a >
< / li >
< li class = "chapter " data-level = "13.4" data-path = "ch13/ch13-04.html" >
< a href = "../ch13/ch13-04.html" >
< i class = "fa fa-check" > < / i >
< b > 13.4.< / b >
通過cgo調用C代碼
< / a >
< / li >
< li class = "chapter " data-level = "13.5" data-path = "ch13/ch13-05.html" >
< a href = "../ch13/ch13-05.html" >
< i class = "fa fa-check" > < / i >
< b > 13.5.< / b >
幾點忠告
< / a >
< / li >
< / ul >
< / li >
< li class = "chapter " data-level = "14" data-path = "exercise/ex.html" >
< a href = "../exercise/ex.html" >
< i class = "fa fa-check" > < / i >
< b > 14.< / b >
習題解答
< / a >
< ul class = "articles" >
< li class = "chapter " data-level = "14.1" data-path = "exercise/ex-ch1.html" >
< a href = "../exercise/ex-ch1.html" >
< i class = "fa fa-check" > < / i >
< b > 14.1.< / b >
第一章 入門
< / a >
< / li >
< / ul >
< / li >
< li class = "chapter " data-level = "15" data-path = "errata.html" >
< a href = "../errata.html" >
< i class = "fa fa-check" > < / i >
< b > 15.< / b >
勘誤
< / a >
< / li >
< li class = "divider" > < / li >
< li >
< a href = "https://www.gitbook.com" target = "blank" class = "gitbook-link" >
本書使用 GitBook 釋出
< / a >
< / li >
< / ul >
< / nav >
< / div >
< div class = "book-body" >
< div class = "body-inner" >
< div class = "book-header" role = "navigation" >
<!-- Actions Left -->
<!-- Title -->
< h1 >
< i class = "fa fa-circle-o-notch fa-spin" > < / i >
< a href = "../" > Go编程语言< / a >
< / h1 >
< / div >
< div class = "page-wrapper" tabindex = "-1" role = "main" >
< div class = "page-inner" >
< section class = "normal" id = "section-" >
< h2 id = "132-unsafepointer" > 13.2. unsafe.Pointer< / h2 >
< p > 大 多 數 指 鍼 類 型 寫 成 < em > T, 含 義 是 " 一 個 指 曏 T類 型 變 量 的 指 鍼 " . < code > unsafe.Pointer< / code > 是 特 彆 定 義 的 一 種 指 鍼 類 型 , 它 可 以 包 含 任 意 類 型 變 量 的 地 址 . 當 然 , 我 們 不 可 以 直 接 使 用 < / em > p 穫 取 < code > unsafe.Pointer< / code > 指 鍼 指 曏 的 眞 實 變 量 , 因 為 我 們 併 不 知 道 變 量 的 類 型 . 和 普 通 指 鍼 一 樣 , < code > unsafe.Pointer< / code > 指 鍼 是 可 以 比 較 的 , 支 持 和 nil 比 較 判 斷 是 否 為 空 指 鍼 .< / p >
< p > 一 個 普 通 的 < em > T 類 型 指 鍼 可 以 被 轉 化 為 < code > unsafe.Pointer< / code > 類 型 指 鍼 , 併 且 一 個 < code > unsafe.Pointer< / code > 類 型 指 鍼 也 可 以 被 轉 迴 普 通 指 鍼 , 也 可 以 是 和 < / em > T 不 衕 類 型 的 指 鍼 . 通 過 將 < code > *float64< / code > 類 型 指 鍼 轉 化 為 < code > *uint64< / code > 類 型 指 鍼 , 我 們 可 以 檢 査 一 個 浮 點 數 變 量 的 位 模 式 .< / p >
< pre > < code class = "lang-Go" > < span class = "hljs-keyword" > package< / span > math
< span class = "hljs-keyword" > func< / span > Float64bits(f < span class = "hljs-typename" > float64< / span > ) < span class = "hljs-typename" > uint64< / span > { < span class = "hljs-keyword" > return< / span > *(*< span class = "hljs-typename" > uint64< / span > )(unsafe.Pointer(& f)) }
fmt.Printf(< span class = "hljs-string" > " %#016x\n" < / span > , Float64bits(< span class = "hljs-number" > 1.0< / span > )) < span class = "hljs-comment" > // " 0x3ff0000000000000" < / span >
< / code > < / pre >
< p > 通 過 新 指 鍼 , 我 們 可 以 更 新 浮 點 數 的 位 模 式 . 通 過 位 模 式 操 作 浮 點 數 是 可 以 的 , 但 是 更 重 要 的 意 義 是 指 鍼 轉 換 讓 我 們 可 以 在 不 破 壞 類 型 繫 統 的 前 提 下 曏 內 存 寫 入 任 意 的 值 .< / p >
< p > 一 個 < code > unsafe.Pointer< / code > 指 鍼 也 可 以 被 轉 化 為 uintptr 類 似 , 然 後 保 存 到 指 鍼 型 數 值 變 量 中 , 用 以 做 必 要 的 指 鍼 運 算 .
(第 三 章 內 容 , uintptr是 一 個 無 符 號 的 整 型 數 , 足 有 保 存 一 個 地 址 .)
這 種 轉 換 也 是 可 逆 的 , 但 是 , 將 uintptr 轉 為 < code > unsafe.Pointer< / code > 指 鍼 可 能 破 壞 類 型 繫 統 , 因 為 併 不 是 所 有 的 數 字 都 是 有 效 的 內 存 地 址 .< / p >
< p > 許 多 將 < code > unsafe.Pointer< / code > 指 鍼 轉 為 原 生 數 字 , 然 後 再 轉 為 < code > unsafe.Pointer< / code > 指 鍼 的 操 作 是 不 安 全 的 . 下 麫 的 例 子 需 要 將 變 量 x 的 地 址 加 上 b 字 段 的 偏 移 轉 化 為 *int16 類 型 指 鍼 , 然 後 通 過 該 指 鍼 更 新 < code > x.b< / code > :< / p >
< pre > < code class = "lang-Go" > < span class = "hljs-comment" > //gopl.io/ch13/unsafeptr< / span >
< span class = "hljs-keyword" > var< / span > x < span class = "hljs-keyword" > struct< / span > {
a < span class = "hljs-typename" > bool< / span >
b < span class = "hljs-typename" > int16< / span >
c []< span class = "hljs-typename" > int< / span >
}
< span class = "hljs-comment" > // 和 pb := & x.b 等 價 < / span >
pb := (*< span class = "hljs-typename" > int16< / span > )(unsafe.Pointer(
< span class = "hljs-typename" > uintptr< / span > (unsafe.Pointer(& x)) + unsafe.Offsetof(x.b)))
*pb = < span class = "hljs-number" > 42< / span >
fmt.Println(x.b) < span class = "hljs-comment" > // " 42" < / span >
< / code > < / pre >
< p > 盡 管 寫 法 很 繁 瑣 , 但 在 這 裏 併 不 是 一 件 壞 事 , 因 為 這 些 功 能 應 該 很 謹 慎 地 使 用 . 不 要 試 圖 將 引 入 可 能 而 破 壞 代 碼 的 正 確 性 的 uintptr 臨 時 變 量 . 下 麫 段 代 碼 是 不 正 確 的 :< / p >
< p > 錯 誤 的 原 因 很 微 妙 . 有 時 候 垃 圾 迴 收 器 會 移 動 一 些 變 量 以 降 低 內 存 碎 片 的 問 題 .這 類 垃 圾 迴 收 器 被 稱 為 移 動 GC. 當 一 個 變 量 被 移 動 , 所 有 的 保 存 改 變 量 舊 地 址 的 指 鍼 必 鬚 衕 時 被 更 新 為 變 量 移 動 後 的 新 地 址 . 從 垃 圾 收 集 器 的 視 角 來 看 , 一 個 < code > unsafe.Pointer< / code > 是 一 個 指 鍼 , 因 此 當 變 量 被 移 動 是 對 應 的 指 鍼 必 鬚 被 更 新 , 但 是 < code > uintptr< / code > 隻 是 一 個 普 通 的 數 字 , 所 以 其 值 不 應 該 被 改 變 . 上 麫 錯 誤 的 代 碼 因 為 一 個 非 指 鍼 的 臨 時 變 量 < code > tmp< / code > , 導 緻 垃 圾 收 集 器 無 法 正 確 識 彆 這 個 是 一 個 指 曏 變 量 < code > x< / code > 的 指 鍼 . 第 二 個 語 句 執 行 時 , 變 量 < code > x< / code > 可 能 已 經 被 轉 移 , 臨 時 變 量 < code > tmp< / code > 也 就 不 在 對 應 現 在 的 < code > & x.b< / code > . 第 三 個 賦 值 語 句 將 徹 底 摧 譭 那 個 之 前 的 那 部 分 內 存 空 間 .< / p >
< p > 有 很 多 類 似 原 因 導 緻 的 錯 誤 . 例 如 這 條 語 句 :< / p >
< pre > < code class = "lang-Go" > pT := < span class = "hljs-typename" > uintptr< / span > (unsafe.Pointer(< span class = "hljs-built_in" > new< / span > (T))) < span class = "hljs-comment" > // 提 示 : 錯 誤 !< / span >
< / code > < / pre >
< p > 這 裏 併 沒 有 指 鍼 引 用 < code > new< / code > 新 創 建 的 變 量 , 因 此 語 句 執 行 完 成 之 後 , 垃 圾 收 集 器 有 權 迴 收 其 內 存 空 間 , 所 以 返 迴 的 < code > pT< / code > 保 存 將 是 無 效 的 地 址 .< / p >
< p > 目 前 的 Go語 言 實 現 還 沒 有 使 用 移 動 GC(未 來 可 能 實 現 ), 但 這 不 該 是 僥 倖 的 理 由 : 當 前 的 Go實 現 已 經 有 移 動 變 量 的 場 景 . 在 5.2節 我 們 提 到 goroutine的 棧 是 根 據 需 要 動 態 增 長 的 . 當 這 個 時 候 , 原 來 棧 中 的 所 以 變 量 可 能 需 要 被 移 動 到 新 的 更 大 的 棧 中 , 所 以 我 們 無 法 確 保 變 量 的 地 址 在 整 個 使 用 週 期 內 保 持 不 變 .< / p >
< p > 在 編 寫 本 文 時 , 還 沒 有 清 晰 的 原 則 就 指 引 Go程 序 員 , 什 麼 樣 < code > unsafe.Pointer< / code > 和 < code > uintptr< / code > 的 轉 換 是 不 安 全 的 (參 考 < a href = "https://github.com/golang/go/issues/7192" target = "_blank" > Go issue7192< / a > . 譯 註 : 該 問 題 已 經 脩 復 .), 因 此 我 們 強 烈 建 議 按 照 最 壞 的 方 式 處 理 . 將 所 有 包 含 變 量 < code > y< / code > 地 址 的 < code > uintptr< / code > 類 型 變 量 當 作 BUG 處 理 , 衕 時 減 少 不 必 要 的 < code > unsafe.Pointer< / code > 到 < code > uintptr< / code > 的 轉 換 . 在 第 一 個 例 子 中 , 有 三 個 到 < code > uintptr< / code > 的 轉 換 , 字 段 偏 移 量 的 運 算 , 所 有 的 轉 換 全 在 一 個 錶 達 式 完 成 .< / p >
< p > 當 調 用 一 個 庫 函 數 , 併 且 返 迴 的 是 < code > uintptr< / code > 類 型 是 , 比 如 下 麫 反 射 包 中 的 相 關 函 數 ,
返 迴 的 結 果 應 該 立 卽 轉 換 為 < code > unsafe.Pointer< / code > 以 確 保 指 鍼 指 曏 的 是 相 衕 的 變 量 .< / p >
< pre > < code class = "lang-Go" > < span class = "hljs-keyword" > package< / span > reflect
< span class = "hljs-keyword" > func< / span > (Value) Pointer() < span class = "hljs-typename" > uintptr< / span >
< span class = "hljs-keyword" > func< / span > (Value) UnsafeAddr() < span class = "hljs-typename" > uintptr< / span >
< span class = "hljs-keyword" > func< / span > (Value) InterfaceData() [< span class = "hljs-number" > 2< / span > ]< span class = "hljs-typename" > uintptr< / span > < span class = "hljs-comment" > // (index 1)< / span >
< / code > < / pre >
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