+ * \return Zero if no data was available, positive if a buffer was read whose
+ * contents compare as equal to the expected string, negative otherwise.
+ * Possible errors: (a) not enough data was read, (b) the buffer contents
+ * compared as non-equal, (c) a read error occurred. In the first two cases,
+ * -E_READ_PATTERN is returned. In the read error case the (negative) return
+ * value of the underlying call to \ref read_nonblock() is returned.