Many GNet functions block. That is, they do not return immediately. For example gnet_inetaddr_new performs a DNS lookup that may take several seconds. This is acceptable for many programs. But, it is not tolerable in interactive GUI programs or high-performance servers.

There are two ways to hide blocking. The first is to use threads. GLib 2.0 includes thread support (GThread). The second method is to use asynchronous functions. Asynchronous functions return immediately and call a callback when the operation is completed. For example, gnet_inetaddr_new_async begins an asynchronous DNS lookup and returns immediately. When the lookup is complete, a callback is called with the GInetAddr. Most blocking function in GNet have an asynchronous counterpart.

To use GNet's asynchronous functions, you must also use the GLib main event loop. Most GTK and Gnome program already do this.

Another common blocking operation is reading or writing to a GIOChannel (a GLib object). g_io_channel_read blocks until there is data available to read. g_io_channel_write blocks until there is OS buffer space to write to. To determine when a GIOChannel can be read to or written from without blocking, use GLib's g_io_add_watch to set a watch. A callback is called when the GIOChannel becomes readable or writable. See the GLib documentation for more information and GNet's echoclient-async and echoserver-async for an example.