题目内容：

undefined reference to `sqrt'
#include
#include
double safe_sqr(double _d_num)
{
printf("%lf\n",_d_num);
double d_temp_value = abs((int)_d_num);
printf("%lf\n",d_temp_value);
return sqrt(d_temp_value);
}
int main(int argc,char *argv[])
{
double d_value = safe_sqr(100.0);
printf("%lf\n",d_value);
return 0;
}