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
|
149
行程識別碼的分配
核心會把行程識別碼的最大值預定為 32768。這是為了相容於較舊的 Unix 系統；它們會
替行程識別碼使用較小的 16 位元資料型別。系統管理者可經由
/proc/sys/kernel/pid_max
將其設定為較高的值，使用較大 pid 空間的代價就是降低相容性。
核心會使用嚴格的線性方式，替行程分配行程識別碼。如果 pid 17 是當前所分配的最高
編號，則 pid 18 將會是下一個要分配的編號，即使在新的行程啟動時，上一個被賦予
pid 17 的行程已不再運行。核心不會重複使用 pid 值，除非超過最大值而回頭使用較小
的值—也就是，除非分配了
/proc/sys/kernel/pid_max
中所指定的值，否則不會使用稍早
用過的值。因此，雖然 Linux 無法長期保證行程識別碼的唯一性，但是它的分配行為至
少能夠替行程識別碼提供短期的穩定性以及唯一性。
行程的階層結構
行程若產生了一個新的行程則稱為
父行程
（
parent
）；這個新的行程稱為
子行程
（
child
）。每個行程均產生自另一個行程（當然，init 行程除外）。因此，每個子行程都
會具有一個父行程。此關係會被記錄在每個行程的
父行程識別碼
（
parent process ID
，
常簡寫為
ppid
）；也就是，子行程之父行程的 pid。
每個行程由一個用戶（user）及一個群組（group）所擁有。此所有權（ownership）可
用於控制資源的存取權限。對核心而言，用戶和群組只是整數值。經由檔案
/etc/passwd ...
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