gtk:text-buffer-iter-at-offset (buffer offset)

buffer -- a gtk:text-buffer object
offset -- an integer with the char offset from the start of the text buffer, counting from 0, or -1

The gtk:text-iter iterator.

Initializes the returned iterator to a position offset chars from the start of the entire text buffer. If the offset argument is -1 or greater than the number of characters in the text buffer, the iterator is initialized to the end iterator, the iterator one past the last valid character in the text buffer.