#include <stdio.h>
#include <math.h>
double distance(float x1,float y1,float x2,float y2)
{
     double d;
    float sum;
    sum=(x1-x2)*(x1-x2)+(y1-y2)*(y1-y2);
    d=sqrt(sum);
    return d;

    }
int main()
{
    double d;
    double distance(float,float,float,float);
    float x1,x2,y1,y2;
    printf("\nplease input the coordinate of  point A:");
    scanf("%4f%4f",&x1,&y1);
     printf("\nplease input the coordinate of  point B:");
    scanf("%4f%4f",&x2,&y2);
    if (x1==x2&&y1==y2)
        return 0.0;
    else
        d=distance(x1,y1,x2,y2);
    printf("the distance beween A and B is %f",d);
}   

这样就可以了，把文件名放开头