With gtk3, how do I make a write to the clipboard presistant?

I stink at reading C code.gtk

In the sample clipboard_simple.c

it states

// NOTE: If you test this without a clipboard manager running, the
// new text saved to the clipboard will disappear as soon as this
// program exits.

xclip does not have this issue, and I can't figure out why.

How do I make a (text) write to the clipboard persistent?

Many thanks,