How to use Firebird from C?
If you wish to use Firebird from plain old C, you can do that directly via Firebird API. Take a look in 'examples' subdirectory of your Firebird installation for examples. There is also a detailed manual available (written for InterBase 6.0, but almost everything still applies):
Make sure you read Firebird 1.5 and 2.0 release notes to see if some functions have changed.