= nng_recv(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_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.

If the special flag `NNG_FLAG_ALLOC` 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_.

If the special flag `NNG_FLAG_ALLOC` 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 <<nng_alloc#,nng_alloc(3)>>), fill it with
the message body, and store it at the address referenced by _data_, and update
the size referenced by _sizep_.  When this flag is present, the caller assumes
responsibility for disposing of the received buffer either by the function
<<nng_free#,nng_free(3)>> or reusing the message for sending (with the same
size) via <<nng_send#,nng_send(3)>>.

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 an <<nng_req#,nng_req(7)>> socket a message may only be received
after a request has been sent, and an <<nng_sub#,nng_sub(7)>> 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
<<nng_pub#,nng_pub(7)>>.

TIP: The `NNG_FLAG_ALLOC` flag can be used to reduce data copies, thereby
increasing performance, particularly if the buffer is reused to send
a response using the same flag.

== RETURN VALUES

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

== ERRORS

`NNG_EAGAIN`:: The socket _s_ cannot accept data for sending.
`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.

== SEE ALSO

<<nng_alloc#,nng_alloc(3)>>,
<<nng_free#,nng_free(3)>>,
<<nng_recvmsg#,nng_recvmsg(3)>>,
<<nng_send#,nng_send(3)>>,
<<nng_strerror#,nng_strerror(3)>>,
<<nng#,nng(7)>>