To help me understand the way of using string.h library

problem10.c

#include<stdio.h>
#include<string.h>

int main()
{
  char c[20];
  char d[20];
  gets(c);
  int i=0;
  int j;
  j=strlen(c);
  for(i;i<strlen(c);i++)
  {
    j--;
    d[j]=c[strlen(c)-i-1];
  }
  int result = strcmp(c, d);
  if (result < 0)
  {
    printf("%d\n",-1);
  } else if (result > 0)
  {
    printf("%d\n", 1);
  } else {
    printf("%d\n", 0);
  }
}