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= 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.3#,`nng_alloc()`>>), 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.3#,`nng_free()`>> or reusing the message for sending (with the same
size) in a call to <<nng_send.3#,`nng_send()`>>.
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 <<nng_req.7#,_req_>> socket a message may only be received
after a request has been sent, and a <<nng_sub.7#,_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
<<nng_pub.7#,_pub_>>.
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.3#,nng_alloc(3)>>,
<<nng_free.3#,nng_free(3)>>,
<<nng_recvmsg.3#,nng_recvmsg(3)>>,
<<nng_send.3#,nng_send(3)>>,
<<nng_strerror.3#,nng_strerror(3)>>,
<<nng.7#,nng(7)>>
|
