如何处理分段函数的设计？

#include
#include
int main (void)
{
double x,f(double x);
printf("x=%lf",x);
scanf("%lf",&x);
if(x<0&&(x!=3)) {
f(double x)=xx+x-6;
}
else if(x>=0&&x<10&&(x!=2)&&(x!=3)) {
f(double x)=xx-5x+6;
}
else {
f(double x)=xx-x-1;
}
printf("f(x)=%lf",f(x));
return 0;
}

#include <stdio.h>
#include <math.h>
double f(double x) {
    if(x<0&&x!=-3) 
        return x*x+x-6;
    else if(x>=0&&x<10&&x!=2&&x!=3)
        return x*x-5x+6;
    else 
        return x*x-x-1;
}
int main (void)
{
double x;
printf("x=%lf",x);
scanf("%lf",&x);
printf("f(x)=%lf",f(x));
return 0;
}