= nng_listener_id(3)
//
// Copyright 2018 Staysail Systems, Inc. <info@staysail.tech>
// Copyright 2018 Capitar IT Group BV <info@capitar.com>
//
// This document is supplied under the terms of the MIT License, a
// copy of which should be located in the distribution where this
// file was obtained (LICENSE.txt).  A copy of the license may also be
// found online at https://opensource.org/licenses/MIT.
//

== NAME

nng_listener_id - return numeric listener identifier

== SYNOPSIS

[source, c]
----
#include <nng/nng.h>

int nng_listener_id(nng_listener l);
----

== DESCRIPTION

The `nng_listener_id()` function returns a positive identifier for the
listener _l_, if it is valid.
Otherwise it returns `-1`.

NOTE: A listener is considered valid if it was ever created with the
xref:nng_listener_create.3.adoc[`nng_listener_create()`] or
xref:nng_listen.3.adoc[`nng_listen()`] functions.
listeners that are allocated on the stack or statically should be
initialized with the macro
xref:nng_ctx.5.adoc#NNG_LISTENER_INITIALIZER[`NNG_LISTENER_INITIALIZER`] to ensure that
they cannot be confused with a valid listener before they are created properly.

== RETURN VALUES

This function returns the positive value for the listener identifier, or
`-1` if the listener is invalid.

== ERRORS

None.

== SEE ALSO

[.text-left]
xref:nng_listener.5.adoc[nng_listener(5)],
xref:nng.7.adoc[nng(7)]