In the most of cases there is not an equivalent of an
API when moving from a platform to another; writing
cross-platform code then mean to write down a series of
#ifdef
directives, dependant to the active platform, so that you make your work in a diferent way depending on the platform API.
To write
cross-platform software is a very complex task; fortunately there are
frameworks that deal with it.
Why don't you have a look to
Qt - A cross-platform application and UI framework[
^] or
wxWidgets[
^]