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
比較位元組
類似於
strcmp()
，
memcmp()
可用於比較兩個記憶體團塊是否相等：
#include <string.h>
int memcmp (const void *s1, const void *s2, size_t n);
此函式會比較
s1
與
s2
之頭
n
個位元組。如果這兩個記憶體區塊相等，則傳回
0
；如果
s1
小於
s2
，則傳回小於零的值；如果
s1
大於
s2
，則傳回大於零的值。
同樣的，BSD 也提供了一個現在已廢棄的介面，用以執行大致相同的工作：
#include <strings.h>
int bcmp (const void *s1, const void *s2, size_t n);
bcmp()
會比較
s1
與
s2
之頭
n
個位元組。如果這兩個記憶體區塊相等，則傳回
0
；如果
它們有所不同，則傳回非零值。
因為結構會遭到補白（見第 9 章〈其他的對齊議題〉一小節），所以經由
memcmp()
或
bcmp()
比較兩個結構是否相等並不可靠。一個結構的兩個實例會因為補白中包含未初始
設定的垃圾而導致不相等。因此，如下的程式碼並不安全：
/* 比較兩艘船是否相同？（不安全） */
int compare_dinghies (struct dinghy *a, struct dinghy *b)
{
return memcmp (a, b, sizeof (struct dinghy));
}
事實上，想要比較兩個結構的程式設計者，應該比較兩個結構中每個成員。此類做法可
以經過優化，但所花的功夫絕對會比不安全的 ...
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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.
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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.