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
經由匿名映射分配記憶體有以下好處：
• 與碎裂無關。當程式不再需要一個匿名記憶體映射時，映射會被解除，而且記憶體
會被立即釋回系統。
• 匿名記憶體映射的尺寸是可調整的，具有可調整的存取權限，而且可以像一般映射
（見第 4 章）那樣收到警告訊息。
• 每個記憶體分配存在於獨立的記憶體映射中。不需要管理全域性的堆積。
相較於堆積，匿名記憶體映射還有兩項缺點：
• 每個記憶體映射的大小是系統頁面尺寸的整數倍。因此，如果一個記憶體分配的大
小不是頁面尺寸的整數倍，則會造成 slack space
5
的浪費。slack space 與小型的記憶
體分配比較有關，相較於記憶體分配的大小，這會浪費比較多的空間。
• 相較於從堆積把記憶體釋回系統（並未涉及任何核心的互動），建立一個新的記憶體
映射會導致更多的開銷。記憶體分配的規模越小，此現象越顯著。
為了讓優點有發揮的空間，同時避開缺點，
glibc
的
malloc()
實作會使用資料區段來滿足
小型的分配需求，以及使用匿名記憶體映射來滿足大型的分配需求。分配需求是大型或
小型，取決於一個可設定的門檻值（見第 9 章〈進階記憶體分配〉一節），而且不同的
glibc
版本可能會有所差異。當前的門檻值為 128 KB：記憶體分配小於或等於 128 KB
者來自堆積，然而大於 128 KB 的記憶體分配則來自匿名記憶體映射。
建立匿名記憶體映射
或許是因為你想要為特定的分配需求使用記憶體映射（而非堆積）、或許是因為你要撰
寫自己的記憶體分配系統，你可能會想要手動建立你自己的匿名記憶體映射—無論是哪 ...
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