In this article, I will show you, How to write a C program to print hollow square star pattern with diagonal.  Here, one thing is important to know that all sides of the square must be the same.

Method1:

#include <stdio.h>

int main()
{
    int x = 0,y = 0;
    unsigned int squareSide = 0;


    printf("Enter Side of a Square = ");
    scanf("%u",&squareSide);

    for(x=1; x<=squareSide; ++x)
    {
        for(y=1; y<=squareSide; ++y)
        {
            // Checking boundary conditions and main
            // diagonal and secondary diagonal conditions
            if((x==1) || (x==squareSide) || (y==1) || (y==squareSide) || (x==y)||  (y==(squareSide - x + 1)))
            {
                //Print star
                printf("*");
            }
            else
            {
                //Print space
                printf(" ");
            }
        }
        // Print new line
        printf("\n");
    }
    return 0;
}

Output1:

Method2:

#include <stdio.h>


#define isBoundary(x,y,n) ((x==1) || (x==n) || (y==1) || (y==n) || (x==y)||  (y==(n - x + 1)))


int main()
{
    int x = 0,y = 0;
    unsigned int squareSide = 0;


    printf("Enter Side of a Square = ");
    scanf("%u",&squareSide);

    for(x=1; x<=squareSide; ++x)
    {
        for(y=1; y<=squareSide; ++y)
        {
            isBoundary(x,y,squareSide)? printf("*"):printf(" ");
        }
        // Print new line
        printf("\n");
    }

    return 0;
}

Output2: