NAME

       nng_sub - subscriber protocol

SYNOPSIS

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

DESCRIPTION

       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.

           Note

           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.

       NNG_OPT_SUB_SUBSCRIBE
           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.

               Note

               This option is deprecated. New applications should use nng_sub0_socket_subscribe or
               nng_sub0_ctx_subscribe functions.

               Note

               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.

               Tip

               To receive all messages, an empty topic (zero length) can be used.

       NNG_OPT_SUB_UNSUBSCRIBE
           This option, also write-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. + NOTE:
           This option is deprecated. New applications should use nng_sub0_socket_unsubscribe or
           nng_sub0_ctx_unsubscribe functions.

       NNG_OPT_SUB_PREFNEW
           (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.

SEE ALSO

       nng_sub_open(3), nng_sub_subscribe(3), nng_pub(7), nng(7)

                                                   2025-04-20                                         NNG_SUB(7)