how to create a union - c

Question

I made a mistake while using union,and I don't know why.
the problem is in the function goto_xy();
I read it from the book, but it cannot be compiled.
In this function I am trying to locate the cursor, but REGS variable is not declared. I want to know what is its function.
#include<stdio.h>
#include<windows.h>
#include<dos.h>
#include<conio.h>
void goto_xy(int x,int y); //goto is a key word;define the subfunction to creat the original cursor int the coordinate system
void rectangle_clear(int x1,int x2,int y1,int y2); //define the rectangle_clear opening subfunction
void center_clear(int x1,int x2,int y1,int y2); //define the center_clear opening subfunction
void creat(); //define the subfunction of creating the star
int main() //the main function
{
creat();
getch();
center_clear(0,25,0,79);
getch();
}
void center_clear(int x1,int x2,int y1,int y2) //the subfunction which creats the stars while opening the project
{
int x00,y00,x0,y0,i,d;
if((y2-y1)>(x2-x1))
{
d=(x2-x1)/2;
x0=(x1+x2)/2;
y0=y1+d;
y00=y2-d;
for(i=0;i<(d+1);i++)
{
rectangle_clear((x0-i),(x00+i),(y0-i),(y00+i));
}
delay(10); //to delay the dismis of the star
}
else
{
d=(y2-y1)/2;
y0=(y1+y2)/2;
x0=x1+d;
x00=x2-d;
for(i=0;i<d+1;i++)
{
rectangle_clear((x0-i),(x00+i),(y0-i),(y00+i));
}
delay(10);
}
}
void rectangle_clear(int x1,int x2,int y1,int y2) //to creat the star int the shape of a rectangle
{
int i,j;
for(i=y1;i<y2;i++)
{
goto_xy(x1,i);
putchar(' ');
goto_xy(x2,i);
putchar(' ');
delay(10);
}
for(j=x1;j<x2;j++)
{
goto_xy(i,y1);
putchar(' ');
goto_xy(i,y2);
putchar(' ');
delay(10);
}
}
void goto_xy(int x,int y)
{
union REGS r;
r.h.ah=2;
r.h.dl=y;
r.h.dh=x;
r.h.bh=0;
int86(0x10,&r,&r);
}
void creat()
{
int i,j;
for(i=0;i<24;i++)
{
for(j=0;j<79;j++)
{
goto_xy(i,j);
printf("a");
}
}
}

Answer 1

It mostly appears to me that the union REGS must be already present in one of the header files and you are including the same.
As can be seen from your code below, even the members of union like h and the members of h are also present, which means the union is there in some header file and you are including it.
void goto_xy(int x,int y)
{
union REGS r;
r.h.ah=2; //Here you are accessing the member of REGS and even the sub-members of h
r.h.dl=y;
r.h.dh=x;
r.h.bh=0;
int86(0x10,&r,&r);
}
EDIT:
A Google search tells me that UNION REGS will be defined in dos.h and it is some like
union REGS {
struct WORDREGS x;
struct BYTEREGS h;
};
So, you need to include dos.h to solve your problem. But, it appears inspite of you including that, this problem is present. You can as well open dos.h and check if union REGS is present or not.
See here for more details.

Answer 2

To define a union, you need to do the following:
union REGS { some_type h; other_type f; };
Now you can create a variable of type REGS and use the union.