+/**
+ * Read a four-byte hex-number and return its value.
+ *
+ * Each status item sent by para_server is prefixed with such a hex number in
+ * ASCII which describes the size of the status item.
+ *
+ * \param buf The buffer which must be at least four bytes long.
+ *
+ * \return The value of the hex number on success, \p -E_SIZE_PREFIX if the
+ * buffer did not contain only hex digits.
+ */