in the file gtk+/demos/gtk-demo/main.c a the line 1173,

the signal "command-line" is linked with the callback command_line:

      g_signal_connect (app, "command-line", G_CALLBACK (command_line), NULL);

And this callback can be found at the line 1076 :

static void
command_line (GApplication            *app,
              GApplicationCommandLine *cmdline)

After looking at the documentation, this callback should return a gint:

https://developer.gnome.org/gio/unstable/GApplication.html#GApplication-command-line

gint
user_function (GApplication            *application,
               GApplicationCommandLine *command_line,
               gpointer                 user_data)


Sorry for bothering you if this is not an error.