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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第九章
Linux 會透過兩種方法來優化這些變數。首先，因為 bss 區段只用於存放未初始化的資
料，所以鏈結器（
ld
）實際上不會在目的碼檔（object file）中儲存這些特殊值。如此可
以縮減執行檔的尺寸。其次，當此區段被載入至記憶體時，核心會以寫入時才複製的做
法將此區段映射至一個填滿零的頁面，有效地為這些變數設定預定值。
多數位址空間會包含一些映射檔，例如程式的可執行檔本身、C 與其他的
共享程式庫以及資料檔。檔案
/proc/self/maps
或
pmap
程式的輸出，可以看
到一個行程會包含哪些映射檔。
本章接下來將說明 Linux 所提供「用於取得與傳回記憶體、建立與銷毀新的映射以及其
間一切」的介面。
分配動態記憶體
記憶體還會以自動及靜態變數的形式出現，但是任何記憶體管理系統的基礎是
動態記
憶體
（
dynamic memory
）的分配、使用以及最後的傳回。動態記憶體的分配在執行時期
（而非編譯時期）進行，分配之前它的大小可能是未知的。身為一個開發者，會在以下
情況下用到動態記憶體：需要相當數量的記憶體，或是這些記憶體需要使用多久不一
定，在程式執行之前無法知道。例如，你可能會想要把一個檔案的內容，或是讀取自鍵
盤的輸入資料，存放在記憶體中。因為不知道檔案的大小，而且用戶可能會按下任何數
目的鍵，所以無法確定緩衝區的大小，於是你可能會因為所讀取到的資料越來越多，動
態調大緩衝區的尺寸。
C 語言的變數本身並不會用到動態記憶體。例如，C 語言並未提供任何機制，可用於取
得一個存在於動態記憶體的
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