Inserting text in a text buffer using a callback function

[Horror Vacui <horrorvacui gmx net>]

I have the following problem: while trying to learn GTK, I decided to 
create a button that should insert text in a scrolled window (more 
exactly: into a text buffer in a scrolled window). I created a callback 
for a function, and it wouldn't work. I transferred parts of code from 
the function to main (so that it is inserted during the realisation of 
the window), and that did work. The function behaves weird: before, the 
program would just segfault when this button is clicked, now it returns 
a Gtk-critical message saying that assertion GTK_IS_TEXT_BUFFER (buffer) 
for the argument I passed to the function failed (I don't understand 
that change either - I didn't change anything of significance). From 
what I learned from learning programming, the error is probably 
something very trivial, but it can take me days to sort out. Here are 
the significant parts of code:

  GtkWidget *view;
  GtkTextBuffer *buffer;

  button=gtk_button_new_with_label("text");   /*the button*/
  g_signal_connect(G_OBJECT (button), "clicked", G_CALLBACK (text), 
buffer);   /*connecting signal to callback*/

  view=gtk_text_view_new();
  buffer=gtk_text_view_get_buffer(GTK_TEXT_VIEW (view));   /*the buffer*/
  gtk_text_buffer_set_text(buffer, "This is the text in the scrolled 
window\n", -1);   /*inserting text, this works*/

void text(GtkTextBuffer *buffer)    /*the callback function*/
{
  GtkTextIter iter;
  gtk_text_buffer_get_iter_at_offset(buffer, &iter, 0);
  gtk_text_buffer_insert(buffer, &iter, "This is the text in the 
scrolled window\n", -1);   /*this doesn't work*/
}

Of course, I've already run the code through a debugger. The callback 
works, the function text is called with pointer "buffer" as argument, it 
is the correct one, it is the correct type inside the function. Yet it 
doesn't work. I've also tried using exactly the same function that works 
in main (gtk_text_buffer_set_text()) in the callback function: nothing. 
I also thought about a conflict between the two variables with the same 
name "buffer" (which should hardly matter) and tried giving them 
different names: nothing. I'm quite stuck, I hope someone can help.

Thanks in advance
Horror Vacui