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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記憶體管理
|
329
匿名記憶體映射
glibc
中的記憶體分配，使用了資料區段及記憶體映射的組合。實現
malloc()
的典型做
法，就是把資料區段分割成一系列 2 的次方之分割區（partition），而且可以透過傳回最
接近所要求大小的分割區，來滿足記憶體分配的需求。欲釋出記憶體只需要把相對應的
分割區標記為「可用」（free）就行了。如果毗鄰的分割區也被標記為可用，則它們會被
合併成一個較大的分割區。如果堆積的頂部整個是可用的，則系統會使用
brk()
調低轉
折點，縮小堆積以及把記憶體傳回核心。
此演算法稱為
伙伴記憶體分配機制
（
buddy memory allocation scheme
）。它的優點是速度
快並且容易實作，但缺點是會導致兩種碎裂（fragmentation）。如果使用比所要求大小還
多的記憶體來滿足一個分配需求，就會發生
內部碎裂
（
internal fragmentation
）的情況。
這會導致可用的記憶體變成無用的結果。如果可用的記憶體能夠滿足所要求的大小，但
是卻被分隔成二或多個非毗鄰的團塊，就會發生
外部碎裂
（
external fragmentation
）的情
況。這會導致記憶體無法使用（因為，也許使用了一個較大、較不適當的區塊）或是記
憶體分配失敗（如果不存在可供選擇的區塊）。
再者，此機制會讓一個記憶體分配“釘住”（pin）另一個，以避免傳統的 C 程式庫把
記憶體釋回核心。假設分配了兩個記憶體區塊：區塊 A 和區塊 B。區塊 A 就在轉折點
上，而區塊 B 位於區塊 A 之下。即使程式釋出區塊 ...
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I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
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.