= nng_recv(3)
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//
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// Copyright 2018 Staysail Systems, Inc. <info@staysail.tech>
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// Copyright 2018 Capitar IT Group BV <info@capitar.com>
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//
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// This document is supplied under the terms of the MIT License, a
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// copy of which should be located in the distribution where this
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// file was obtained (LICENSE.txt). A copy of the license may also be
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// found online at https://opensource.org/licenses/MIT.
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//
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== NAME
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nng_recv - recv data
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== SYNOPSIS
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[source, c]
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----
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#include <nng/nng.h>
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int nng_recv(nng_socket s, void *data, size_t *sizep, int flags);
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----
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== DESCRIPTION
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The `nng_recv()` receives a message.
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The _flags_ is a bit mask that may contain any of the following values:
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`NNG_FLAG_NONBLOCK`::
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The function returns immediately, even if no message is available.
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Without this flag, the function will wait until a message is received
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by the socket _s_, or any configured timer expires.
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`NNG_FLAG_ALLOC`::
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If this flag is present, then a ((zero-copy)) mode is used.
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In this case the caller must set the value of _data_ to the location
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of another pointer (of type `void *`), and the _sizep_ pointer must be set
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to a location to receive the size of the message body.
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The function will then allocate a message buffer
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(as if by xref:nng_alloc.3.adoc[`nng_alloc()`]), fill it with
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the message body, and store it at the address referenced by _data_, and update
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the size referenced by _sizep_.
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The caller is responsible for disposing of the received buffer either by
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the xref:nng_free.3.adoc[`nng_free()`] function or passing the message (also
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with the `NNG_FLAG_ALLOC` flag) in a call to xref:nng_send.3.adoc[`nng_send()`].
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If the special flag `NNG_FLAG_ALLOC` (see above) is not specified, then the
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caller must set _data_ to a buffer to receive the message body content,
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and must store the size of that buffer at the location pointed to by _sizep_.
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When the function returns, if it is successful, the size at _sizep_ will be
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updated with the actual message body length copied into _data_.
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NOTE: The semantics of what receiving a message means vary from protocol to
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protocol, so examination of the protocol documentation is encouraged.
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(For example, with a xref:nng_req.7.adoc[_req_] socket a message may only be received
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after a request has been sent, and a xref:nng_sub.7.adoc[_sub_] socket
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may only receive messages corresponding to topics to which it has subscribed.)
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Furthermore, some protocols may not support receiving data at all, such as
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xref:nng_pub.7.adoc[_pub_].
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TIP: The `NNG_FLAG_ALLOC` flag can be used to reduce data copies, thereby
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increasing performance, particularly if the buffer is reused to send
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a response using the same flag.
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== RETURN VALUES
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This function returns 0 on success, and non-zero otherwise.
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== ERRORS
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[horizontal]
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`NNG_EAGAIN`:: The operation would block, but `NNG_FLAG_NONBLOCK` was specified.
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`NNG_ECLOSED`:: The socket _s_ is not open.
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`NNG_EINVAL`:: An invalid set of _flags_ was specified.
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`NNG_EMSGSIZE`:: The received message did not fit in the size provided.
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`NNG_ENOMEM`:: Insufficient memory is available.
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`NNG_ENOTSUP`:: The protocol for socket _s_ does not support receiving.
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`NNG_ESTATE`:: The socket _s_ cannot receive data in this state.
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`NNG_ETIMEDOUT`:: The operation timed out.
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== SEE ALSO
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[.text-left]
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xref:nng_alloc.3.adoc[nng_alloc(3)],
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xref:nng_free.3.adoc[nng_free(3)],
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xref:nng_recvmsg.3.adoc[nng_recvmsg(3)],
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xref:nng_send.3.adoc[nng_send(3)],
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xref:nng_strerror.3.adoc[nng_strerror(3)],
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xref:nng.7.adoc[nng(7)]
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