You didn't use code tags around your diagram, so I can't tell much from it. The forum software ruins diagrams if you don't use code tags.
So edit your post, and click on the "Go Advanced" button in the bottom right hand corner, and highlight your diagram. Now click on the # icon just right of top dead center, in the header icon section of the advanced editor page.
Then I can see WTF you're diagram is all about.
I believe this is what you want to do, but not really sure:
Code:
#include <stdio.h>
#define ROW 20
#define COL 20
int main(void){
int r,c,diag=ROW-1;
for(r=0;r<ROW;r++,diag--) {
for(c=0;c<COL;c++) {
if(r==0 || r==ROW-1) {
putchar('*');
}
else if(c==0 || c==COL-1 || c==diag)
putchar('*');
else
putchar(' ');
}
putchar('\n');
}
putchar('\n');
return 0;
}