connecting a callback to a boolean value

[tc6 <tc6 free fr>]

Hello,

I'm trying to connect a callback function to a boolean value, ie the 
callback has to be called each time a variable equals true...
It would be really helpful if someone has some code example... I spend a 
lot of time tring to use add_watch() with giochannel (or 
Glib::iochannel) from an udp socket of gnet library (if someone manages 
to do something with it, i'm really interested). So I decided to use : 
"gboolean gnet_udp_socket_has_packet 
<http://www.gnetlibrary.org/docs/gnet-udp.html#gnet-udp-socket-has-packet> 
(...)" with gtkmm signal handling ...

Any other solutions are welcome

thank you in advance