# nng_listen(3) Manual Page

## NAME

nng_listen - create and start listener

## SYNOPSIS

    #include <nng/nng.h>

    int nng_listen(nng_socket s, const char *url, nng_listener *lp, int flags);

## DESCRIPTION

The `nng_listen()` function creates a newly initialized `nng_listener` object, associated with socket s, and configured to listen at the address specified by url, and starts it. If the value of lp is not `NULL`, then the newly created listener is stored at the address indicated by lp.

Listeners are used to accept connections initiated by remote dialers. An incoming connection generally results in and `nng_pipe` object being created and attached to the socket s. Unlike dialers, listeners generally can create many pipes, which may be open concurrently.

The flags argument is ignored, but reserved for future use.

| While it is convenient to think of listeners as servers, the relationship between the listener or dialer is orthogonal to any server or client status that might be associated with a given protocol. For example, a req socket might have associated dialers, but might also have associated listeners. It may even have some of each at the same time!

Because the listener is started immediately, it is generally not possible to apply extra configuration; if that is needed applications should consider using `nng_listener_create()` and `nng_listener_start()` instead.

The created listener will continue to accept new connections, associating their pipes with the socket, until either it or the socket s is closed.

## RETURN VALUES

This function returns 0 on success, and non-zero otherwise.

## ERRORS

`NNG_EADDRINUSE` | The address specified by url is already in use.
`NNG_EADDRINVAL` | An invalid url was specified.
`NNG_ECLOSED` | The socket s is not open.
`NNG_EINVAL` | An invalid set of flags or an invalid url was specified.
`NNG_ENOMEM` | Insufficient memory is available.

## SEE ALSO

nng_dial(3), nng_listener_close(3), nng_listener_create(3) nng_listener_start(3), nng_strerror(3), nng_listener(5), nng(7)