POC for GTK bug 749875, http://stackoverflow.com/questions/30387247

gtktextbuffer-markup-bug.c

#include <gtk/gtk.h>

void main() {
	GtkTextIter start, end, inspos, tagpos;
	gsize length;

	GtkTextBuffer *buffer = gtk_text_buffer_new(NULL);

	GtkTextTag *tag1 = gtk_text_buffer_create_tag(buffer, "tagone", NULL);
	GtkTextTag *tag2 = gtk_text_buffer_create_tag(buffer, "tagtwo", NULL);

	gtk_text_buffer_get_iter_at_mark(buffer, &inspos, gtk_text_buffer_get_insert(buffer));
	gtk_text_buffer_insert_interactive(buffer, &inspos, "tag1", 4, TRUE);
	gtk_text_iter_backward_chars(&inspos, 4);
	gtk_text_buffer_get_iter_at_mark(buffer, &tagpos, gtk_text_buffer_get_insert(buffer));
	gtk_text_buffer_apply_tag(buffer, tag1, &inspos, &tagpos);

	gtk_text_buffer_get_iter_at_mark(buffer, &inspos, gtk_text_buffer_get_insert(buffer));
	gtk_text_buffer_insert_interactive(buffer, &inspos, "tags12", 6, TRUE);
	gtk_text_iter_backward_chars(&inspos, 6);
	gtk_text_buffer_get_iter_at_mark(buffer, &tagpos, gtk_text_buffer_get_insert(buffer));
	gtk_text_buffer_apply_tag(buffer, tag1, &inspos, &tagpos);
	gtk_text_buffer_apply_tag(buffer, tag2, &inspos, &tagpos);

	gtk_text_buffer_get_iter_at_mark(buffer, &inspos, gtk_text_buffer_get_insert(buffer));
	gtk_text_buffer_insert_interactive(buffer, &inspos, "notags", 6, TRUE);

	gtk_text_buffer_get_start_iter(buffer, &start);
	gtk_text_buffer_get_end_iter(buffer, &end);
	GdkAtom format = gtk_text_buffer_register_serialize_tagset(buffer, NULL);
	guint8 *serialized = gtk_text_buffer_serialize(buffer, buffer, format, &start, &end, &length);
	write(1, serialized, length);
}