Open Database Connectivity

Mr. Harvey describes the ODBC open specification for application developers.
Connect to Data Source

Initialize ODBC by calling SQLAllocEnv and SQLAllocConnect, then call SQLConnect.

Create and Execute an SQL Statement

Initialize a statement by calling SQLAllocStmt. Call SQLPrepare to allow the Driver Manager a chance to preprocess the SQL, then call SQLExecute.

Process Results

The simplest way to process results is to call SQLFetch in a loop and SQLGetData for each column in the result set. SQLNumResultCols can be used to find out how many columns are in the result set.

Close Connection

Figure 5. Include Files

Call SQLFreeStmt, SQLDisconnect, SQLFreeConnect and SQLFreeEnv to clean up. Your source should include sqlext.h (see Figure 5) and should link against libodbc.so (see Figure 1).

A Driver's Perspective

The driver and the Driver Manager share 98 percent of their function names. This is not surprising when you consider that the Driver Manager passes off most processing to the driver. An important difference, however, is that the Environment, Connection and Statement handles contain different information. For example, the driver's Connection handle will often contain some database-specific data such as a socket handle, while the Driver Manager's Connection handle will usually contain very little aside from a pointer to the driver's Connection handle. unixODBC contains a driver template which should act as a good starting point for anyone interested in creating a new driver. It should be very easy to port a driver to/from Linux using unixODBC, because unixODBC is designed to support driver code from other platforms. A useful driver will implement, at a minimum, the following functions: SQLAllocConnect, SQLFreeConnect, SQLAllocStmt, SQLFreeStmt, SQLConnect, SQLDisconnect, SQLPrepare, SQLExecute, SQLFetch, SQLGetData, SQLNumResultCols and SQLColAttribute.

Perhaps the best way to learn how a driver works is to look at some driver code. unixODBC contains a number of such examples. Your source will likely implement driver.h and driverextras.h, but at a minimum it should include sqlext.h and odbcinst.h (see Figure 5). Driver code from other platforms may have their own version of driver.h and driverextras.h or may not have them at all; this is okay, because these two includes are implemented in the local source directory and are not used by any other code. You should link against libodbcinst.so and any database-specific libraries (see Figure 1). unixODBC has something for just about every Linux user.

Application developers can now create portable data access code independent of the platform and the data source. Driver programmers can use the unixODBC driver template to get started on a new driver, then use Odbcinst when creating their install script/RPM. All Linux users can easily configure ODBC data sources for their applications with ODBCConfig, then take a look at what resources are available in a data source by browsing in the DataManager. Ease of use and functionality surpass other platforms which have had ODBC for a number of years. Most importantly, widespread adoption of unixODBC in popular Linux distributions will allow application developers to assume a desktop has these features and take advantage of them. Such features are of critical importance in having Linux fully accepted on an average user's desk.

Tips

Resources

Peter Harvey started programming games on the C64 while in the Canadian military and went on to make programming his occupation of choice. He is the founder of CodeByDesign, which recently changed its focus from developing for MS platforms to developing for Linux. Peter can be reached at pharvey@codebydesign.com.

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