Verifying input data before gtk_dialog_run returns

["Jaap Haitsma" <jaap haitsma org>]

Hi,

Is there a recommended way to check if the data in a dialog is valid
when the user  presses the OK button? If the data is not OK I want to
present a modal dialog saying there is an error. If the user closes
that dialog the user can correct the error and press OK again.

The concrete case I'm looking at is in a file save dialog where I want
to check if a user entered a valid file extension when he/she presses
the save button.

Thanks

Jaap