grep -ir  cbcp_start_callback .
./pppd/cbcp.c:static void cbcp_start_callback __P((cbcp_state *us));
./pppd/cbcp.c:  cbcp_start_callback( stop_iface );
./pppd/cbcp.c:void cbcp_start_callback (us)
./pppd/cbcp.c:    dbglog("cbcp_start_callback running");