Palm OS Programming, 2nd Edition

Opening, Creating, and Closing Databases

You handle these operations in a straightforward manner in Palm applications.

Opening a Database

You can specify a database to open using one of the following:

• Type and creator

• Name and card number

The type and creator is usually more convenient, but you can run into a problem if more than one database exists with the same type and creator. This might happen, for instance, in a book-reading program, where every downloaded book is a database with the same type and creator, but with a different name. If more than one database exists, one of them (no guarantee which one) will be opened.

An open database is referenced by a DmOpenRef , which is a pointer to information about the database. If an open fails, it returns NULL. Many Data Managers return an error code explicitly. For those that don’t, call DmGetLastErr to determine the last error.

Opening by type and creator

You usually open your database using the type and creator. Example 10-1 shows opening the Memo Pad database (which has a type of DATA and a creator ID of memo) for read-write access.

Example 10-1. Opening the Memo Pad database by type and creator

Err OpenMemopadByTypeCreator(DmOpenRef *refPtr) 
{
  Err       err = errNone;
  UInt16    mode = dmModeReadWrite;
  DmOpenRef ref;
  
  ref = DmOpenDatabaseByTypeCreator('DATA', 'memo', mode);
  if (ref == NULL)
    err = DmGetLastErr(  );
  *refPtr = ref;
  return err;
}

In your application, you’ll usually use a mode of dmModeReadWrite , since you may be modifying records ...