How can a variable within a multiprocess be accessed in an API stream-like way by another IO process?

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One way to accomplish this is to use a shared memory object or a message passing system.

A shared memory object allows multiple processes to access the same block of memory, which can be used to share data between processes. The challenge here is to ensure that access to the shared memory object is properly synchronized, so that different processes do not try to update the same data at the same time.

Alternatively, a message passing system can be used to send messages between processes. In this approach, the process that needs to access the variable would send a message requesting its value, and the process that owns the variable would send a message in response with the value. This method is often easier to use, as it provides a clear interface for communicating between processes, and avoids many of the synchronization issues associated with shared memory objects.

Ultimately, the choice between these two approaches will depend on the specifics of your application and the requirements for performance, reliability, and security.