#include <stdlib.h> u_int32_t arc4random(void);
The arc4random() function uses the key stream generator employed by the arc4 cipher, which uses 8*8 8bit S-Boxes. The S-Boxes can be in about (2**1700) states. The arc4random() function returns pseudo-random numbers in the range of 0 to (2**32)-1, and therefore
has twice the range of rand().
arc4random() 可以替代传统的rand(), random(), 返回类型是u_int32_t，不需要srand()去生成种子
//可以产生0 ~ 99的随机数 int index = arc4random() %100;