8 How to use external I/O event loops

The above implementations of an Assuan client and server are synchronous, insofar as the main routines block until a request or client connection is completely processed. In some programs, for example GUI applications, this is undesirable. Instead, Assuan operations should be non-blocking, and the caller should be able to poll all involved file descriptors to determine when the next Assuan function can be invoked without blocking.

To make this possible, client and server have to adhere to some rules:

• Either partner should always write full lines. If partial lines are written, the remainder of the line should be sent without delay.
• Either partner should eagerly receive status messages. While receiving and sending bulk data may be delayed, the status communication channel is different: Both partners may send status messages in blocking mode at any time the protocol allows them to send such status messages. To ensure that these send operations do not actually block the sender, the recipient must be ready to receive without undue delay.
• If descriptor passing is used over a socket, the descriptor must be sent after the corresponding command without undue delay.

Together, these restrictions allow to limit the need for asynchronous I/O operations to bulk data and the inbound status file descriptor.

In addition to the above rules, client and server should adhere to the following implementation guidelines.