+/**
+ * Add an event source for a file descriptor.
+ *
+ * @param pollfd The GPollFD.
+ * @param timeout Max time to wait before the callback is called, ignored if 0.
+ * @param cb Callback function to add. Must not be NULL.
+ * @param cb_data Data for the callback function. Can be NULL.
+ * @param poll_object TODO.
+ *
+ * @return SR_OK upon success, SR_ERR_ARG upon invalid arguments, or
+ * SR_ERR_MALLOC upon memory allocation errors.
+ */