Writing a Database System Function

DP4 provides template versions of the standard DP4 database manager functions. You can modify these functions in order to intercept the calls, and then pass the function onto the database manager, or indicate that your code performs the required action.

For example, to intercept the message to fetch the primary key of a record, ftch_prm(), you supply your database system code version of the message, u_ftchprm(), as shown in Figure 58.

int u_ftchprm(flags, data_record, index_record, role, keys)

struct DATAREC *datarec;

{
   if (datarec->typenr==_CUSTOMER)
   {
   customer = (struct CUSTOMER *) datarec;
   ....<Your_code_goes_here>....
   return 0;  /* Record to be read by non-DP4 system */
   }
   return 1;    /* DP4 to read the record */
}

Your code can make full use of the DP4 database access functions. If you are writing code that is native to one operating system, you can use the standard file functions of your operating system. If you are writing object-code portable programs, you should not use functions that are specific to one operating system.

DP4 provides a special function, user_auxdbms(), that lets you communicate with database system code from an application.