= nn_recvmsg(3compat)
//
// 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

nn_recvmsg - receive message (compatible API)

== SYNOPSIS

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

int nn_recvmsg(int sock, struct nn_msghdr *hdr, int flags);
----

== DESCRIPTION

The `nn_recvmsg()` function receives a message into the header described by
_hdr_ using the socket _sock_.

NOTE: This function is provided for API
<<nng_compat.3compat#,compatibility>> with legacy _libnanomsg_.
Consider using the relevant <<libnng.3#,modern API>> instead.

The _flags_ field may contain the special flag `NN_DONTWAIT`.
In this case, if no message is ready for receiving on _sock_,
the operation shall not block, but instead will fail with the error `EAGAIN`.

The _hdr_ points to a structure of type `struct nn_msghdr`, which has the
following definition:

[source, c]
----
struct nn_iovec {
    void * iov_base;
    size_t iov_len;
};

struct nn_msghdr {
    struct nn_iovec *msg_iov;
    int              msg_iovlen;
    void *           msg_control;
    size_t           msg_controllen;
};
----

The `msg_iov` is an array of scatter items, permitting the message
to be spread into different memory blocks.
There are `msg_iovlen` elements in this array, each of which
has the base address (`iov_base`) and length (`iov_len`) indicated.

The last member of this array may have the `iov_len` field set to `NN_MSG`,
in which case the function shall allocate a message buffer, and store the
pointer to it at the address indicated by `iov_base`.
This can help save an extra copy operation.
The buffer should be deallocated by `<<nn_freemsg.3compat#,nn_freemsg()>>`
or similar when it is no longer needed.

The values of `msg_control` and `msg_controllen` describe a buffer
of ancillary data associated with the message.
This is currently only useful to obtain the message headers
used with <<nng.7#raw_mode,raw mode>> sockets.
In all other circumstances these fields should be zero.
Details about this structure are covered in
`<<nn_cmsg.3compat#,nn_cmsg(3compat)>>`.

== RETURN VALUES

This function returns the number of bytes received on success, and -1 on error.

== ERRORS

[horizontal]
`EAGAIN`:: The operation would block.
`EBADF`:: The socket _sock_ is not open.
`EFSM`:: The socket cannot receive in this state.
`EINVAL`:: The _hdr_ is invalid.
`ENOTSUP`:: This protocol cannot receive.
`ETIMEDOUT`:: Operation timed out.

== SEE ALSO

[.text-left]
<<nn_cmsg.3compat#,nn_cmsg(3compat)>>,
<<nn_errno.3compat#,nn_errno(3compat)>>,
<<nn_recv.3compat#,nn_recv(3compat)>>,
<<nn_send.3compat#,nn_send(3compat)>>,
<<nn_socket.3compat#,nn_socket(3compat)>>,
<<nng_compat.3compat#,nn_compat(3compat)>>,
<<nng.7#,nng(7)>>
