FAQ for C-Core SDK

Check out the frequently asked questions about the PubNub C-Core SDK.

Why is a user ID required?

The user ID uniquely identifies the client (end user, device, or server). Set and persist a stable value for the lifetime of the user or device; without it, the client cannot connect.

Can I publish and subscribe with a single context?

Yes, but not at the same time. To publish and subscribe concurrently, use separate contexts or a pool for publishing.

How do I choose static vs dynamic buffers?

Use static buffers when memory is tight and message sizes are predictable. Use dynamic buffers for large or variable replies. Review PUBNUB_DYNAMIC_REPLY_BUFFER and PUBNUB_REPLY_MAXLEN.

What happens if the transaction timer expires?

The SDK cancels the transaction locally and returns timeout. Publishes that already reached the server are not rolled back.

Is the C-Core SDK thread safe?

Yes when compiled with thread-safety enabled, but you must still synchronize your own application data. Without thread safety, use one thread per context.

How do I reduce memory footprint?

Lower the number of contexts, use static buffers, disable proxy support and other unused modules, and enable PUBNUB_ONLY_PUBSUB if you only need publish/subscribe.

Can I change the origin (DNS host) at runtime?

Yes if PUBNUB_ORIGIN_SETTABLE is enabled. Passing NULL resets the origin to the default.

What are the default retry settings for subscribe?

Exponential backoff with up to 6 retries, increasing delays from 2 to 150 seconds.

Why am I missing subscribe messages?

Static reply buffers may truncate large responses, and long gaps can advance the subscribe cursor. Increase PUBNUB_REPLY_MAXLEN or review your subscribe loop configuration.

Which encryption should I use?

Prefer 256-bit AES-CBC. Legacy 128-bit encryption remains available for backward compatibility. See the Encryption docs.

How do I enable Transport Layer Security (TLS)?

Call the SSL configuration API with TLS enabled to secure all communications with PubNub.

Can I use the old subscribe functions with the event engine enabled?

No. When PUBNUB_USE_SUBSCRIBE_EVENT_ENGINE is ON, do not call pubnub_subscribe or pubnub_subscribe_v2.

How do I use a proxy?

Enable PUBNUB_PROXY_API and configure host, port, and protocol using the proxy APIs.