two loop solution

main.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
char * char_repeat( int n, char c ) {
  char * dest = malloc(n+1);
  memset(dest, c, n);
  dest[n] = '\0';
  return dest;
}
int main()

{
  int i, j, rows;

  printf("Enter number of rows: ");

  scanf("%d",&rows);

  for(i = rows; i >= 1; --i)
  {
    printf("%s", char_repeat(i, '*'));
    printf("\n");
  }
  for(j = 1; j <= rows; ++j)
  {
    printf("%s", char_repeat(j, '*'));
    printf("\n");
  }
  return 0;
}