= nng_recv(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_recv - recv data

== SYNOPSIS

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

int nng_recv(nng_socket s, void *data, size_t *sizep, int flags);
----

== DESCRIPTION

The `nng_recv()` receives a message.

The _flags_ is a bit mask that may contain any of the following values:

`NNG_FLAG_NONBLOCK`::
  The function returns immediately, even if no message is available.
  Without this flag, the function will wait until a message is received
  by the socket _s_, or any configured timer expires.

`NNG_FLAG_ALLOC`::
  If this flag is present, then a ((zero-copy)) mode is used.
  In this case the caller must set the value of _data_ to the location
  of another pointer (of type `void *`), and the _sizep_ pointer must be set
  to a location to receive the size of the message body.
  The function will then allocate a message buffer
  (as if by xref:nng_alloc.3.adoc[`nng_alloc()`]), fill it with
  the message body, and store it at the address referenced by _data_, and update
  the size referenced by _sizep_.
  The caller is responsible for disposing of the received buffer either by
  the xref:nng_free.3.adoc[`nng_free()`] function or passing the message (also
  with the `NNG_FLAG_ALLOC` flag) in a call to xref:nng_send.3.adoc[`nng_send()`].

If the special flag `NNG_FLAG_ALLOC` (see above) is not specified, then the
caller must set _data_ to a buffer to receive the message body content,
and must store the size of that buffer at the location pointed to by _sizep_.
When the function returns, if it is successful, the size at _sizep_ will be
updated with the actual message body length copied into _data_.

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

NOTE: The semantics of what receiving a message means vary from protocol to
protocol, so examination of the protocol documentation is encouraged.
(For example, with a xref:nng_req.7.adoc[_req_] socket a message may only be received
after a request has been sent, and a xref:nng_sub.7.adoc[_sub_] socket
may only receive messages corresponding to topics to which it has subscribed.)
Furthermore, some protocols may not support receiving data at all, such as
xref:nng_pub.7.adoc[_pub_].

== 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 received message did not fit in the size provided.
`NNG_ENOMEM`:: Insufficient memory is available.
`NNG_ENOTSUP`:: The protocol for socket _s_ does not support receiving.
`NNG_ESTATE`:: The socket _s_ cannot receive 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_recvmsg.3.adoc[nng_recvmsg(3)],
xref:nng_send.3.adoc[nng_send(3)],
xref:nng_strerror.3.adoc[nng_strerror(3)],
xref:nng.7.adoc[nng(7)]
