Just an exercise example.

box.c

// Just an exercise example.
#include <stdio.h>
#include <stdlib.h>

void crep(int character, unsigned int number);

int main(int argc, char **argv)
{
    if (argc < 2)
    {
        fputs("Error: Please specify the box length. Example: ./box 16\n", stderr);
        return 1;
    }

    unsigned int length = strtol(argv[1], NULL, 10);
    if (length % 2)
    {
        length++;
    }

    unsigned int middle = length / 2;
    char symbol = '*';

    crep(symbol, length);
    for (int x = 1; x < middle - 1; x++)
    {
        printf("%c%*c%*c%*c\n", symbol, x, symbol, length - (x * 2) - 2, symbol, x, symbol);
    }

    printf("%c%*c%*c\n", symbol, middle - 1, symbol, middle - 1, symbol);

    for (int x = middle - 2; x > 0; x--)
    {
        printf("%c%*c%*c%*c\n", symbol, x, symbol, length - (x * 2) - 2, symbol, x, symbol);
    }

    crep(symbol, length);
    return 0;
}

void crep(int character, unsigned int number)
{
    while (--number)
        putchar(character);
    putchar('\n');
}