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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第十一章
執行成功時，此呼叫會把進行調用之行程與它的子行程所耗用的行程時間填入
buf
所指
向的
tms
結構。所回報的時間會被分解成用戶和系統時間。
用戶時間
（
user time
）就是
在用戶空間中執行程式碼所花的時間。
系統時間
（
system time
）就是在核心空間中執行
程式碼所花的時間—例如，系統呼叫期間或頁面失誤。只有在子行程終止以及父行程對
行程調用
waitpid()
（或一個相關的函式）之後才會回報子行程的時間。此呼叫會傳回自
從過去某一參考點之後單調遞增的時鐘週期數目。這個參考點曾經是系統啟動時間—因
此，
times()
函式會傳回 system uptime（以時鐘週期計）—但是現在這個參考點大約是
系統啟動之前 429 百萬秒。核心開發者會實作此變更，是為了避免核心程式碼無法處理
system uptime 繞回變為零的情況。此函式所傳回的是絕對值（而非相對值）因而毫無用
處；然而，兩次調用之間的相對變化仍是有價值的。
執行失敗時，此呼叫會傳回
-1
，而且會把
errno
設定為適當的值。Linux 之上，只有一
個可能的錯誤碼，
EFAULT
，代表
buf
是一個無效的指標。
設定當前時間
前面所描述的是如何取得時間，然而應用程式偶爾也需要把當前的時間與日期設定為所
指定的值。這幾乎總是可以使用專為此用途而設計的共用程式，例如
date
，來處理。
time()
的時間設定版本為
stime()
：
#define _SVID_SOURCE
#include <time.h>
int stime (time_t *t); ...
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