- * When @ref SP_ERR_FAIL is returned, an error code or string description of
- * the error can be obtained by calling sp_last_error_code() or
- * sp_last_error_message(). The error code or message is that provided by the
- * OS; libserialport does not define any error codes or messages of its own.
+ * - @ref SP_ERR_ARG means that a function was called with invalid
+ * arguments. This implies a bug in the caller. The arguments passed would
+ * be invalid regardless of the underlying OS or serial device involved.
+ *
+ * - @ref SP_ERR_FAIL means that the OS reported a failure. The error code or
+ * message provided by the OS can be obtained by calling sp_last_error_code()
+ * or sp_last_error_message().
+ *
+ * - @ref SP_ERR_SUPP indicates that there is no support for the requested
+ * operation in the current OS, driver or device. No error message is
+ * available from the OS in this case. There is either no way to request
+ * the operation in the first place, or libserialport does not know how to
+ * do so in the current version.
+ *
+ * - @ref SP_ERR_MEM indicates that a memory allocation failed.
+ *
+ * All of these error values are negative.