December 2013
Intermediate to advanced
496 pages
8h 57m
Chinese
Content preview from LINUX系統程式設計 第二版
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第七章
避免死結
避免死結很重要，而且唯一安全一致的辦法，是從一開始就把上鎖機制設計到你的多執
行緒化程式。關鍵在於排他鎖要關聯到資料（而非程式碼），而且要有清楚的資料（以
及排他鎖）層級。例如，死結的一個簡單的形式稱為 ABBA 死結或稱
致命擁抱
（
deadly
embrace
）。如果一個執行緒取得排他鎖 A 之後又要取得排他鎖 B，而另一個執行緒取得
排他鎖 B 之後又要取得排他鎖 A（因此是 ABBA），便會發生此情況。若時序正確，這
兩個執行緒都會成功取得它們的頭一個排他鎖：執行緒 1 持有排他鎖 A，而執行緒 2 持
有排他鎖 B。當它們接著去取得其他的排他鎖時，它們會發現該排他鎖已被其他的執行
緒持有，因此它們都會受到阻擋，直到排他鎖被釋出。因為每個持有排他鎖的執行緒也
都在等待對方的排他鎖，所以排他鎖不會被釋出，導致執行緒發生死結。
修正此問題需要明確的規則：取得排他鎖 B 之前必須先取得排他鎖 A。隨著你的程式變
得複雜，它的同步化也會因而變得複雜，導致這些規則的實施變得更為困難。設計程式
的時候需要早點開始加入這些明確的規則。
Pthread
Linux 核心對執行緒的支援僅提供了底層的基礎呼叫，例如
clone()
系統呼叫。任何的執
行緒程式庫有大部分位於用戶空間。許多大型軟體專案都會定義自己的執行緒程式庫：
例如，Android、Apache、GNOME 及 Mozilla 都有提供它們自己的執行緒程式庫；此
外，一些程式語言，像是 C++11 及 Java，也會為執行緒提供標準程式庫的支援 ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.