= nng_send(3)
//
// Copyright 2025 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_send - send data

== SYNOPSIS

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

int nng_send(nng_socket s, void *data, size_t size, int flags);
----

== DESCRIPTION

The `nng_send()` function sends a message containing the _data_ of
length _size_ using the xref:nng_socket.5.adoc[socket] _s_.

NOTE: The semantics of what sending a message means vary from protocol to
protocol, so examination of the protocol documentation is encouraged.
(For example, with an xref:nng_pub.7.adoc[_pub_] socket the data is broadcast, so that
any peers who have a suitable subscription will be able to receive it using
xref:nng_recv.3.adoc[`nng_recv()`] or a similar function.)
Furthermore, some protocols may not support sending data (such as
xref:nng_sub.7.adoc[_sub_]) or may require other conditions.
(For example, xref:nng_rep.7.adoc[_rep_] sockets cannot normally send data,
which are responses to requests, until they have first received a request.)

The _flags_ may contain either of (or neither of) the following values:

`NNG_FLAG_NONBLOCK`::
    The function returns immediately, regardless of whether
    the socket is able to accept the data or not.  If the socket is unable
    to accept the data (such as if backpressure exists because the peers
    are consuming messages too slowly, or no peer is present), then the
    function will return with `NNG_EAGAIN`.  If this flag is not specified,
    then the function will block if such a condition exists.

`NNG_FLAG_ALLOC`::
    The _data_ was allocated using xref:nng_alloc.3.adoc[`nng_alloc()`], or was
    obtained from a call to xref:nng_recv.3.adoc[`nng_recv()`] with
    the `NNG_FLAG_ALLOC` flag.
    If this function returns success, then the _data_ is "owned" by the
    function, and it will assume responsibility for calling
    xref:nng_free.3.adoc[`nng_free()`] when it is no longer needed.
    In the absence of this flag, the _data_ is copied by the implementation
    before the function returns to the caller.

NOTE: The `NNG_FLAG_ALLOC` flag is deprecated and may be removed in a future release.
Applications should use xref:nng_sendmsg.3.adoc[`nng_sendmsg()`] for zero copy performance
improvements.

NOTE: Regardless of the presence or absence of `NNG_FLAG_NONBLOCK`, there may
be queues between the sender and the receiver.
Furthermore, there is no guarantee that the message has actually been delivered.
Finally, with some protocols, the semantic is implicitly `NNG_FLAG_NONBLOCK`,
such as with xref:nng_pub.7.adoc[_pub_] sockets, which are best-effort delivery only.

IMPORTANT: When using `NNG_FLAG_ALLOC`, it is important that the value of _size_
match the actual allocated size of the data.
Using an incorrect size results
in unspecified behavior, which may include heap corruption, program crashes,
or trans-dimensional mutation of the program's author.

== RETURN VALUES

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

== ERRORS

[horizontal]
`NNG_EAGAIN`:: The operation would block, but `NNG_FLAG_NONBLOCK` was specified.
`NNG_ECLOSED`:: The socket _s_ is not open.
`NNG_EINVAL`:: An invalid set of _flags_ was specified.
`NNG_EMSGSIZE`:: The value of _size_ is too large.
`NNG_ENOMEM`:: Insufficient memory is available.
`NNG_ENOTSUP`:: The protocol for socket _s_ does not support sending.
`NNG_ESTATE`:: The socket _s_ cannot send data in this state.
`NNG_ETIMEDOUT`:: The operation timed out.

== SEE ALSO

[.text-left]
xref:nng_alloc.3.adoc[nng_alloc(3)],
xref:nng_free.3.adoc[nng_free(3)],
xref:nng_recv.3.adoc[nng_recv(3)],
xref:nng_sendmsg.3.adoc[nng_sendmsg(3)],
xref:nng_strerror.3.adoc[nng_strerror(3)],
xref:nng_socket.5.adoc[nng_socket(5)],
xref:nng.7.adoc[nng(7)]
