STOP and Wait(have to insert timer) /Sliding window

#include<stdio.h>

#include<conio.h>

 struct file

 {

         

          char words[30];

 }fl[6];

 struct file1

 {

           

           char words[30];

 }fl1[6];

 int main()

           {

          FILE*fdata1,*fdata2;

          int index1,index2;

          int n=5;

           {

          fdata1=fopen("D:\\123.txt","a+");

          if(fdata1==NULL)

           {

          printf("ERROR");

           }

          else

               {

          for(index1=0;index1<n;index1++)

           {

          fscanf(fdata1,"%s",&fl[index1].words);

           }

    fclose(fdata1);

          fdata1=fopen("D:\\123.txt","a");

          fdata2=fopen("D:\\456.txt","a");

          if(fdata2==NULL)

          {

                          printf("ERROR");

           }

          else

 {

          for(index2=0;index2<n;index2++)

 {

         

          fprintf(fdata2,"%s\n",fl[index2].words);       

          fprintf(fdata2,"Acknowledgement send\n");

          fprintf(fdata1,"Acknowledgement recieved for \t");

          fprintf(fdata1,"%s\n",fl[index2].words);

         

 }

 }

         

}

fclose(fdata1);

fclose(fdata2);

getch();

}

}