How to open the default web browser from in C?


We're planning on switching to using HTML help for some of our software ...but I don't know how to open a web browser from within an application. Here's the particulars and the challenge:

We program entirely in C, using GTK+, and release linux AND Windoze releases. Currently, the Linux side opens a window with help text in it (no active links!!) and on the Windoze side, we use the MS help system (compiled help files, .chm etc).

I assume that opening a browser from a Windoze app isn't so hard ...the default browser is probably available from the registry and no big deal to find and then make a system call.

For Linux however, noting that we use RedHat, Ubuntu, Suze, and CentOS, I don't know how to determine what the default web browser (and path) so I can call it with a system call... Anyone? I know that one option is to use a layout engine like GtkHTML, but we'd really like to just call a web browser instead...