segfault when inserting text in a textbuffer

[Jc <jcb niluge net>]

anybody an idea why i get a segfault when i insert data in a
textbuffer.
the segfault does not occur just after i call the function
but when gtk is in the main loop, here is the backtrace :

#0  0x4041c008 in mallopt () from /lib/libc.so.6
#1  0x4041ba20 in mallopt () from /lib/libc.so.6
#2  0x4041ace5 in malloc () from /lib/libc.so.6
#3  0x40036b97 in g_malloc () from /usr/lib/libglib-2.0.so.0
#4  0x4036053a in pango_get_mirror_char () from /usr/lib/libpango-1.0.so.0
#5  0x40360c57 in pango_log2vis_get_embedding_levels () from /usr/lib/libpango-1.0.so.0
#6  0x40353f52 in pango_itemize () from /usr/lib/libpango-1.0.so.0
#7  0x40359a95 in pango_layout_get_pixel_size () from /usr/lib/libpango-1.0.so.0
#8  0x40358444 in pango_layout_get_cursor_pos () from /usr/lib/libpango-1.0.so.0
#9  0x403586a3 in pango_layout_get_extents () from /usr/lib/libpango-1.0.so.0
#10 0x401982fc in gtk_text_layout_get_line_display () from /usr/lib/libgtk-x11-2.0.so.0
#11 0x40196cc0 in gtk_text_layout_validate () from /usr/lib/libgtk-x11-2.0.so.0
#12 0x40196297 in gtk_text_layout_wrap () from /usr/lib/libgtk-x11-2.0.so.0
#13 0x40184a17 in _gtk_text_btree_validate_line () from /usr/lib/libgtk-x11-2.0.so.0
#14 0x40196a8a in gtk_text_layout_validate_yrange () from /usr/lib/libgtk-x11-2.0.so.0
#15 0x401a368c in gtk_text_view_place_cursor_onscreen () from /usr/lib/libgtk-x11-2.0.so.0
#16 0x401a374c in gtk_text_view_place_cursor_onscreen () from /usr/lib/libgtk-x11-2.0.so.0
#17 0x401a37d9 in gtk_text_view_place_cursor_onscreen () from /usr/lib/libgtk-x11-2.0.so.0
#18 0x40034246 in g_timeout_add () from /usr/lib/libglib-2.0.so.0
#19 0x40031b29 in g_get_current_time () from /usr/lib/libglib-2.0.so.0
#20 0x40032947 in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#21 0x40032d23 in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#22 0x400333ff in g_main_loop_run () from /usr/lib/libglib-2.0.so.0
#23 0x4012a34f in gtk_main () from /usr/lib/libgtk-x11-2.0.so.0
#24 0x0804f362 in main (argc=1, argv=0xbffff9a4) at gcaratk.c:24