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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行程管理
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185
如下的調用：
pid_t pgid = getpgrp ();
等效於：
pid_t pgid = getpgid (0);
它們都會傳回「進行叫用之行程的」行程群組識別碼。函式
getpgid()
不會失敗。
監控程式
監控程式
（
daemon
）就是一個在背景中運行的行程，它不會連接到任何控制終端機。
監控程式一般會啟動於開機時期，並且會以
root
或其他特殊用戶（例如
apache
或
postfix
）的身份運行，而且所處理的是系統層級的任務。按慣例，監控程式的名稱通常
會以字母 d 結尾（例如
crond
及
sshd
），但是這並非必要或甚至不具普遍性。
daemon 這個名稱源自於 Maxwell
’
s demon（馬克斯威爾的小妖），這是
物理學家 James Maxwell 於 1867 年所提出的一個想法。demon（精靈）
還是希臘神話中超自然的存在，祂存在於人和神之間並且具有天賦的力量
和神的知識。不同於猶太教和基督教所教導的 demon（惡魔），希臘的
demon（精靈）並不必然是邪惡的。的確，在希臘神話中，demon 往往
會幫奧林匹斯山眾神完成祂們不願意做的事—這與 Unix 的 daemon（監
控程式）為前景用戶完成其想要避免之工作非常像。
監控程式通常需要符合兩項需求：它必須是 init 的一個子行程，以及它不得連接到一個
終端機。
總之，一個程式完成以下步驟就可以變成一個 daemon：
1. 叫用
fork()
。這會建立一個新的行程，這個新行程將變成 daemon。
2. 在父行程中呼叫 ...
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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.