c_gtk_text_buffer_get_iter_at_line_offset

Undocumented in source but is binding to C. You might be able to learn more by searching the web for its name.
__gshared extern (C)
int function(GtkTextBuffer* buffer, GtkTextIter* iter, int lineNumber, int charOffset) c_gtk_text_buffer_get_iter_at_line_offset;

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