gunichar gtk_text_iter_get_char ( const GtkTextIter* iter )
The Unicode character at this iterator is returned.
Equivalent to operator* on a C++ iterator. If the element at this iterator is a non-character element, such as an image embedded in the buffer, the Unicode “unknown” character 0xFFFC is returned. If invoked on the end iterator, zero is returned; zero is not a valid Unicode character.
So you can write a loop which ends when this function returns 0.