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百度了很多的资料都没有明白qsort 中 cmp 的用处，用Bing 一搜就搜到一篇自认为比其他都解析清楚的文章
直接贴出来算了
comparPointer to a function that compares two elements.
This function is called repeatedly by qsort to compare two elements. It shall follow the following prototype:
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Taking two pointers as arguments (both converted to const void*). The function defines the order of the elements by returning (in a stable and transitive manner):
| return value | meaning |
|---|---|
<0 |
The element pointed by p1 goes before the element pointed by p2 |
0 |
The element pointed by p1 is equivalent to the element pointed by p2 |
>0 |
The element pointed by p1 goes after the element pointed by p2 |
假如你要按升序排列数组，需要处理两个参数P1 ，P2，如果P1 > P2 ，那cmp需要返回的是一个<0的值，所以是P1-P2
若P1<P2 ,那么cmp需要返回一个 >0的值，所以同样是P1-P2，一点都不矛盾。
假如你要按降序排列数组，需要处理两个参数P1 ，P2，如果P1 > P2 ，排序应该是P1在P2前面，那cmp需要返回的是一个<0的值，所以是P2-P1 ,若P1<P2 ,那么cmp需要返回一个 >0的值，所以同样是P2-P1
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原文地址：http://www.cnblogs.com/been/p/4470199.html
