A shared memory system for providing unidirectional communication across a security threshold defined by producer and consumer of a message at different security levels. A unidirectional shared memory element is accessible for read and write access by the message producer (sender), and the consumer (receiver) has only read access. Transmission logic ensures atomic receipt of the message if it is received, as the receiver cannot issue an acknowledgement to the sender as in conventional electronic interfaces. An arrangement of indices and counters allows asynchronous operation by the sender and receiver, as messages may queue up in the shared memory if the producer exceeds the consumer. In the event of a failure, anomaly, or resource overrun, it is known that messages that were received were received in entirety; in other words, atomicity is preserved.