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/transport/tcp/tcp.h>

int nng_tcp_register(void);


The tcp transport provides communication support between nng sockets across a TCP/IP network. Both IPv4 and IPv6 are supported when the underlying platform also supports it.


The nng_tcp transport is generally built-in to the nng core, so no extra steps to use it should be necessary.

URI Format

This transport uses URIs using the scheme tcp://, followed by an IP address or hostname, followed by a colon and finally a TCP port number. For example, to contact port 80 on the localhost either of the following URIs could be used: tcp:// or tcp://localhost:80.

A URI may be restricted to IPv6 using the scheme tcp6://, and may be restricted to IPv4 using the scheme tcp4://.

Specifying tcp6:// may not prevent IPv4 hosts from being used with IPv4-in-IPv6 addresses, particularly when using a wildcard hostname with listeners. The details of this varies across operating systems.
Both tcp6:// and tcp4:// are nng extensions, and might not be understood by other implementations.
We recommend using either numeric IP addresses, or names that are specific to either IPv4 or IPv6 to prevent confusion and surprises.

When specifying IPv6 addresses, the address must be enclosed in square brackets ([]) to avoid confusion with the final colon separating the port.

For example, the same port 80 on the IPv6 loopback address (::1) would be specified as tcp://[::1]:80.

The special value of 0 (INADDR_ANY) can be used for a listener to indicate that it should listen on all interfaces on the host. A short-hand for this form is to either omit the address, or specify the asterisk (*) character. For example, the following three URIs are all equivalent, and could be used to listen to port 9999 on the host:

  1. tcp://

  2. tcp://*:9999

  3. tcp://:9999

The entire URI must be less than NNG_MAXADDRLEN bytes long.

Socket Address

When using an nng_sockaddr structure, the actual structure is either of type nng_sockaddr_in (for IPv4) or nng_sockaddr_in6 (for IPv6).

Transport Options

The following transport options are supported by this transport, where supported by the underlying platform.