math.h头文件引用出错

本例是实现哥德巴赫猜想,但是出现编译时出现了问题,在此标记

 

 

[root@localhost baili]# gcc 18.c -o 18
/tmp/ccl4V3ho.o: In function `main':
18.c:(.text+0xbd): undefined reference to `sqrt'
18.c:(.text+0xcb): undefined reference to `floor'
18.c:(.text+0x11b): undefined reference to `sqrt'
18.c:(.text+0x123): undefined reference to `floor'
collect2: ld returned 1 exit status
我已经包含了头文件 math.h
不知道为什么?

 

#include <math.h>
#include <stdio.h>

int main(int argc, char *argv[])
{
    int i,j,num;
    int p,q,flagp,flagq;
    printf("please intput s plus integer:");
    scanf("%d",&num);
    if(((num%2)!=0)||(num<=4))
        printf("input data erro!\n");
    else
    {
        p=1;
        do
        {
            p = p+1;
            q = num-p;
            flagp=1;
            flagq=1;
            for(i=2;i<=(int)(floor(sqrt((double)(p))));i++)
            {
                if((p%i)==0)
                {
                    flagp = 0;
                    break;
                }
            }
        
        j = 2;
        while (j<=(int)(floor(sqrt((double)(q)))))
        {
            if((q%j)==0)
            {
                flagq = 0;
                break;
            }
            j++;
        }
        }while(flagp*flagq == 0);
    printf("%d = %d +%d \n",num,p,q);
    }

}