Handles are identifiers for storage areas that are allocated by the Driver
Manager or individual drivers. The three types of handles are: - Environment handle:
- Global information, that includes other handles. One handle is allowed
per application.
- Connection handle:
- Information about connection to a data source. Multiple connection handles
are allowed per environment.
- Statement handle:
- Information about a particular SQL statement. Multiple statement handles
are allowed per connection. Statement handles can be reused for other SQL
statements and long as the statement state is valid.
- Descriptor handle:
- Information about explicit descriptors that are associated with the connection
handle. The application creates these, and asks the driver to use them instead
of the implicit descriptors associated with a statement handle.
Essentially, a handle can be considered as an identifier
for a resource that is recognized by ODBC (an environment, connection or statement).
ODBC provides an identifier (the handle) for this resource that you can use
in your program. Exactly what ODBC stores in the handle (which is held as
a long integer) is not relevant. Be careful not to change the value, and to
assign unique names to the variables that hold the various handles.
Some
APIs set the handle (for example, SQLAllocEnv or SQLAllocHandle with
SQL_HANDLE_ENV handle type), and you must pass in a reference, or pointer
to the variable. Some APIs refer to a handle that previously was set (for
example, SQLExecute), and you must pass in the variable by value.