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
用
memset()
，直接使用
calloc()
會比較好：
glibc
知道匿名映射已經被填滿零值，而且
calloc()
會對「一個映射不需要明確填入零值」感到滿意。
系統呼叫
munmap()
會釋出一個匿名映射，並把已分配的記憶體釋回核心：
int ret;
/* 釋出 'p' 之後，會釋回 512 KB 的映射 */
ret = munmap (p, 512 * 1024);
if (ret)
perror ("munmap");
mmap()
、
munmap()
以及映射的相關細節可參考第 4 章。
映射 /dev/zero
其他的 Unix 系統，例如 BSD，並不具備
MAP_ANONYMOUS
旗標。事實上，它們是透過映射
一個特殊的裝置檔，
/dev/zero
，來實作類似的解決方案。此裝置檔為匿名記憶體提供了
一致的語意。一個映射包含了填滿零值的「寫入時才複製」（copy-on-write）頁面；因
此映射
/dev/zero
的行為如同使用匿名記憶體。
Linux 之上總是可以找到一個
/dev/zero
裝置，而且提供映射此檔案與取得填滿零值之記
憶體的能力。的確，引進
MAP_ANONYMOUS
之前，Linux 程式設計者就是採用此做法。為
了向下相容於較舊版的 Linux，或是移植到其他 Unix 系統，開發者仍然可以映射
/dev/
zero
，以便取代建立匿名映射的做法。這與「映射任何其他檔案」並無不同：
void *p;
int fd;
/* 開啟 /dev/zero 以備讀取與寫入 */
fd = open ...
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