Drupal - How to create new field type
When I did this a while ago I adapted the excellent
field_example module from the Examples Module which is always the first place to look, as it has a lot of Drupal best practice in it.
As they say in the module comments:
* Providing a field requires: * - Defining a field: * - hook_field_info() * - hook_field_schema() * - hook_field_validate() * - hook_field_is_empty() * * - Defining a formatter for the field (the portion that outputs the field for * display): * - hook_field_formatter_info() * - hook_field_formatter_view() * * - Defining a widget for the edit form: * - hook_field_widget_info() * - hook_field_widget_form()
So, not for the faint hearted, but as others have said, all the information is there to get you started.