String Manipulation - Substring Search & Replacement C Program

Program:
#include<stdio.h>
#include<string.h>
main()
{
   char str[40],newstr[20],substr[20];
   int len1,len2,i,j,k,t=0,len3,d=0;
   printf("Enter the string:");
   gets(str);
   printf("Enter the substring:");
   gets(substr);
   printf("Enter the new string:");
   gets(newstr);
   len1 = strlen(str);
   len2 = strlen(substr);
   len3 = strlen(newstr);
   for(i=0;i<len1;i++)
   {
     for(j=t=0,k=i;j<len2;j++,k++)
     {
        if(str[k]==substr[j])
            t++;
        else
          break;
     }
     if(t==len2)
         break;
   }
   if(t==len2)
   {
     printf("\nThe substring is found.\n");
     printf("String replaced...\n");
     if(len3>len2)
     {
       for(j=len1;j>=k;j--)
       str[j+len3-len2] = str[j];
     }
     else
     {
       for(j=k;j<=len1;j++)
       str[j-(len2-len3)] = str[j];
     }
     while(newstr[d]!='\0')
     {
       str[i] = newstr[d];
       i++;
       d++;
     }
     puts(str);
   }
   else
   {
     printf("\nSubstring not found.\n");
   }
}

Output:
nn@linuxmint ~ $ gcc c18.c
/tmp/ccqnIqKS.o: In function `main':
c18.c:(.text+0x40): warning: the `gets' function is dangerous and should not be used.
nn@linuxmint ~ $ ./a.out
Enter the string:good night
Enter the substring:night
Enter the new string:morning

The substring is found.
String replaced...
good morning
nn@linuxmint ~ $