It seems we lost arguments in the spawn documentation, perhaps it's related to macro handling of gtk-doc.

Compare yourself:

http://developer.gnome.org/glib/stable/glib-Spawning-Processes.html

and

http://developer.gnome.org/glib/2.26/glib-Spawning-Processes.html

#define             g_spawn_async_with_pipes

vs

gboolean            g_spawn_async_with_pipes            (const gchar *working_directory,
                                                         gchar **argv,
                                                         gchar **envp,
                                                         GSpawnFlags flags,
                                                         GSpawnChildSetupFunc child_setup,
                                                         gpointer user_data,
                                                         GPid *child_pid,
                                                         gint *standard_input,
                                                         gint *standard_output,
                                                         gint *standard_error,
                                                         GError **error);