This documentation is for version v1.3.0 of NNG, but the latest released version is v1.8.0. see the documentation for v1.8.0 for the most up-to-date information.


#include <nng/nng.h>
#include <nng/protocol/pubsub0/sub.h>


The sub protocol is one half of a publisher/subscriber pattern. In this pattern, a publisher sends data, which is broadcast to all subscribers. The subscribing applications only see the data to which they have subscribed.

The sub protocol is the subscriber side, and the pub protocol is the publisher side.

In this implementation, the publisher delivers all messages to all subscribers. The subscribers maintain their own subscriptions, and filter them locally. Thus, this pattern should not be used in an attempt to reduce bandwidth consumption.

The topics that subscribers subscribe to is just the first part of the message body. Applications should construct their messages accordingly.

Socket Operations

The nng_sub0_open() functions create a subscriber socket. This socket may be used to receive messages, but is unable to send them. Attempts to send messages will result in NNG_ENOTSUP.

Protocol Versions

Only version 0 of this protocol is supported. (At the time of writing, no other versions of this protocol have been defined.)

Protocol Options

The following protocol-specific options are available.


This option registers a topic that the subscriber is interested in. The option is write-only, and takes an array of bytes, of arbitrary size. Each incoming message is checked against the list of subscribed topics. If the body begins with the entire set of bytes in the topic, then the message is accepted. If no topic matches, then the message is discarded.

This option is a byte array. Thus if you use nng_setopt_string() the NUL terminator byte will be included in the topic. If that isn’t desired, consider using nng_setopt() and using strlen() of the topic as the topic size.
To receive all messages, an empty topic (zero length) can be used.

This option, also read-only, removes a topic from the subscription list. Note that if the topic was not previously subscribed to with NNG_OPT_SUB_SUBSCRIBE then an NNG_ENOENT error will result.


(bool) This read/write option specifies the behavior of the subscriber when the queue is full. When true (the default), the subscriber will make room in the queue by removing the oldest message. When false, the subscriber will reject messages if the message queue does not have room.

Protocol Headers

The sub protocol has no protocol-specific headers.