用法：
#include <math.h>
double ceil ( double x);
double floor ( double x);
double round( double x);
ceil(x)返回不小于x的最小整数值（然后转换为double型）。即，向上取整
向下取整
round(x)返回x的四舍五入整数值。四舍五入
给个例子test.c：
#include <stdio.h>
#include <math.h>
int main( int argc, const char *argv[])
{
float num = 1.4999;
printf ( "ceil(%f) is %f\n" , num, ceil (num));
printf ( "floor(%f) is %f\n" , num, floor (num));
printf ( "round(%f) is %f\n" , num, round(num));
return 0;
}
编译：$cc test.c -lm 执行：$./a.out ceil(1.499900) is 2.000000 floor(1.499900) is 1.000000 round(1.499900) is 1.000000
