can you help me understand about fopen("contact.dll", "r"); - c

Question

I'm trying to implement phonebook in C using data structure.
I found some source code and I'm trying to understand this code but there is really big problem I've never seen. It is dll. I googled about dll but there is nothing related about this. I know the meaning and purpose of dll, but why do we use fopen contact.dll ?
#include<stdio.h>
#include<conio.h>
#include<string.h>
#include<process.h>
#include<stdlib.h>
#include<dos.h>
struct contact
{
long phone;
char name[20],add[20],email[30];
} list;
char query[20],name[20];
FILE *fp, *ft;
int i,n,ch,l,found;
int main()
{
main:
system("cls"); /* ************Main menu *********************** */
printf("\n\t **** Welcome to Contact Management System ****");
printf("\n\n\n\t\t\tMAIN MENU\n\t\t=====================\n\t\t[1] Add a new Contact\n\t\t[2] List all Contacts\n\t\t[3] Search for contact\n\t\t[4] Edit a Contact\n\t\t[5] Delete a Contact\n\t\t[0] Exit\n\t\t=====================\n\t\t");
printf("Enter the choice:");
scanf("%d",&ch);
switch(ch)
{
case 0:
printf("\n\n\t\tAre you sure you want to exit?");
break;
/* *********************Add new contacts************ */
case 1:
system("cls");
fp=fopen("contact.dll","a");
for (;;)
{
fflush(stdin);
printf("To exit enter blank space in the name input\nName (Use identical):");
scanf("%[^\n]",&list.name);
if(stricmp(list.name,"")==0 || stricmp(list.name," ")==0)
break;
fflush(stdin);
printf("phone:");
scanf("%ld",&list.phone);
fflush(stdin);
printf("address:");
scanf("%[^\n]",&list.add);
fflush(stdin);
printf("email address:");
gets(list.email);
printf("\n");
fwrite(&list,sizeof(list),1,fp);
}
fclose(fp);
break;
/* *********************list of contacts************************* */
case 2:
system("cls");
printf("\n\t\t================================\n\t\t\tLIST OF CONTACTS\n\t\t================================\n\nName\t\tphone No\t Address\t\tE-mail ad.\n=================================================================\n\n");
for(i=97; i<=122; i=i+1)
{
fp=fopen("contact.dll","r");
fflush(stdin);
found=0;
while(fread(&list,sizeof(list),1,fp)==1)
{
if(list.name[0]==i || list.name[0]==i-32)
{
printf("\nName\t: %s\nphone\t: %ld\nAddress\t: %s\nEmail\t: %s\n",list.name,
list.phone,list.add,list.email);
found++;
}
}
if(found!=0)
{
printf("=========================================================== [%c]-(%d)\n\n",i-32,found);
getch();
}
fclose(fp);
}
break;
/* *******************search contacts********************** */
case 3:
system("cls");
do
{
found=0;
printf("\n\n\t..::CONTACT SEARCH\n\t===========================\n\t..::Name of contact to search: ");
fflush(stdin);
scanf("%[^\n]",&query);
l=strlen(query);
fp=fopen("contact.dll","r");
system("cls");
printf("\n\n..::Search result for '%s' \n===================================================\n",query);
while(fread(&list,sizeof(list),1,fp)==1)
{
for(i=0; i<=l; i++)
name[i]=list.name[i];
name[l]='\0';
if(stricmp(name,query)==0)
{
printf("\n..::Name\t: %s\n..::phone\t: %ld\n..::Address\t: %s\n..::Email\t: %s\n",list.name,list.phone,list.add,list.email);
found++;
if (found%4==0)
{
printf("..::Press any key to continue...");
getch();
}
}
}
if(found==0)
printf("\n..::No match found!");
else
printf("\n..::%d match(s) found!",found);
fclose(fp);
printf("\n ..::Try again?\n\n\t[1] Yes\t\t[0] No\n\t");
scanf("%d",&ch);
}
while(ch==1);
break;
/* *********************edit contacts************************/
case 4:
system("cls");
fp=fopen("contact.dll","r");
ft=fopen("temp.dat","w");
fflush(stdin);
printf("..::Edit contact\n===============================\n\n\t..::Enter the name of contact to edit:");
scanf("%[^\n]",name);
while(fread(&list,sizeof(list),1,fp)==1)
{
if(stricmp(name,list.name)!=0)
fwrite(&list,sizeof(list),1,ft);
}
fflush(stdin);
printf("\n\n..::Editing '%s'\n\n",name);
printf("..::Name(Use identical):");
scanf("%[^\n]",&list.name);
fflush(stdin);
printf("..::phone:");
scanf("%ld",&list.phone);
fflush(stdin);
printf("..::address:");
scanf("%[^\n]",&list.add);
fflush(stdin);
printf("..::email address:");
gets(list.email);
printf("\n");
fwrite(&list,sizeof(list),1,ft);
fclose(fp);
fclose(ft);
remove("contact.dll");
rename("temp.dat","contact.dll");
break;
/* ********************delete contacts**********************/
case 5:
system("cls");
fflush(stdin);
printf("\n\n\t..::DELETE A CONTACT\n\t===============================\n\t..::Enter the name of contact to delete:");
scanf("%[^\n]",&name);
fp=fopen("contact.dll","r");
ft=fopen("temp.dat","w");
while(fread(&list,sizeof(list),1,fp)!=0)
if (stricmp(name,list.name)!=0)
fwrite(&list,sizeof(list),1,ft);
fclose(fp);
fclose(ft);
remove("contact.dll");
rename("temp.dat","contact.dll");
break;
default:
printf("Invalid choice");
break;
}
printf("\n\n\n..::Enter the Choice:\n\n\t[1] Main Menu\t\t[0] Exit\n");
scanf("%d",&ch);
switch (ch)
{
case 1:
goto main;
case 0:
break;
default:
printf("Invalid choice");
break;
}
return 0;
}

Answer 1

Judging by the way the file is used, contact.dll is not an actual Windows DLL, but a datafile that this application used to store phonebook entries.
The various cases add, remove, update, and retrieve list entries from the file. The name given to the file is misleading.