Opening and Closing Databases

A database must be opened before it can be accessed. You open a database with the db_open() or dp4_db_open() functions. In DP4 C applications you will often only need the call to db_open() that LIBMAKE generates for you.

When you have finished using a database, you must close it to ensure that it is shut down properly. Another C function, db_close(), is available for closing a database. In C programs LIBMAKE generates a call to close the database in the finish() function, which is always called when the application is terminating.

The Example Code Generated by LIBMAKE contains the lines shown here:

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void init()
{
  df_libtype = 1;
  db_open("SALESORD",0,SHARED+PRIVATE_DB);
  map_load("TEMPLATE");
}
void finish()
{
  db_close();
}

When any DP4 C program using the standard main() function is run, the function init() is called before process(). It contains the following call to db_open on line 40: db_open("SALESORD",0,SHARED+PRIVATE_DB);

• The first parameter, "SALESORD", specifies the database that is to be opened.
• The second parameter, 0, is the generation number. (It will usually be a much bigger number)
• The third parameter contains the flags SHARED and PRIVATE_DB. The SHARED flag means that others users can use the SALESORD database at the same time. The PRIVATE_DB flag means that if you read a record, DP4 scans your pending updates before it scans the actual database. So, you only see the database as it will be when you commit your changes.

When a DP4 C program finishes, the function finish is called last. Therefore, the call to db_close() shown in line 45 closes the SALESORD database. You can open and close databases at any point in your program.