Curious

I want to store patient records by a incremental number so that no two records have the same id number. The problem is that when i close the program and have written the data to file, how do i retrieve the last id number to so that if the user which to add another customer, it starts from that point.


void addrecord(void)

{

     system("color 2");    

         

     customer1 = fopen("customer1.txt","a+");

    

     int num=0,x;

     int base=0;

     

     int regis=1000;

     

     printf("\n\n\t\t\t  PLEASE ENTER THE AMOUNT OF CUSTOMER YOU WANT TO ADD:");

     scanf("%d",&num);

     

     printf("\n\n\t\t\t  PLEASE ENTER THE CUSTOMER INFORMATION BELOW");    

        

       

    for(x=0;x<num;x++)

    {    

     printf("\n\n\t\t\t  NAME: ");

     fflush(stdin);

     gets(info.Name);     

     

     printf("\n\n\t\t\t  ADDRESS: ");

     gets(info.address);

     

     printf("\n\n\t\t\t  NUMBER: ");

     gets(info.number);

     

     printf("\n\n\t\t\t  TREATMENT: ");

     gets(info.treatment);

     

     printf("\n\n\t\t\t  ALLERGIES: ");

     gets(info.allergies); 

     

     printf("\n\n\t\t\t  DATE OF BIRTH 12/12/12: "); 

     gets(info.dob);     

        

     printf("\n\n\t\t\t  DATE OF LAST APPOINTMENT 12/12/12: ");

     gets(info.app);

     

         

     system("cls");

     

     system("color 2");

     

     if (customer1==NULL)

            printf("\n\n\t\t\t  FILE WAS NOT FOUND ");

     else

         {

            regis++;                         

            

            fprintf(customer1,"%d %s %s %s %s %s %s %s\n",regis,info.Name,info.address,info.number,info.dob,info.treatment,info.allergies,info.app);

                              

         }

       

       } 

                   

     fclose(customer1);     

     

}

An exapmle of the file is below.

1001 Jason.Roberts 4.Clover.Road .............
1002 ..................................
1003 ................................

I would want to get the 1003 and then begin from there.