= nn_poll(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_poll - poll sockets (compatible API)

== SYNOPSIS

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

#define NN_POLLIN  1
#define NN_POLLOUT 2

struct nn_pollfd {
    int      fd;
    uint16_t events;
    uint16_t revents;
};

int nn_poll(struct nn_pollfd *pfds, int npfd, int timeout);
----

== DESCRIPTION

The `nn_poll()` function polls a group of sockets for readiness to send or receive.

NOTE: This function is provided for API
xref:nng_compat.3compat.adoc[compatibility] with legacy _libnanomsg_.
Consider using the relevant xref:libnng.3.adoc[modern API] instead.

The array of _nfds_ sockets to poll for are passed into _pfds_.
Each member of this array is initialized with the `fd` field set to
the socket, and the `events` field set to a mask that can contain either or both
of the flags `NN_POLLIN` and `NN_POLLOUT`.

The flag `NN_POLLIN` indicates that a socket is ready for receiving without
blocking (a message is available on the socket), and the flag `NN_POLLOUT`
indicates that a socket is ready for sending without blocking.

Upon success, the function returns the number of updates the `revents`
field of each member of the _pfds_ array, setting it to indicate
whether the requested status is true or not.

NOTE: The `revents` field will only have a flag set if the corresponding
flag was also set in the `events` field.

If the _timeout_ field is positive, then this function will wait for
up the that many milliseconds.
If none of the requested events occurs before that timeout occurs, then
the function will return -1 and set the error to `ETIMEDOUT`.

If the _timeout_ is zero, then this function will return immediately,
after updating the current status of the sockets.

If the _timeout_ is -1, then the function waits forever, or until one of the
requested events occurs.

IMPORTANT: This function is only suitable for use with sockets obtained with the
xref:nn_socket.3compat.adoc[`nn_socket()`] function, and is not compatible
with file descriptors obtained via any other means.
This includes file descriptors obtained using the `NN_SNDFD` or `NN_RCVFD`
options with xref:nn_getsockopt.3compat.adoc[`nn_getsockopt()`]

NOTE: This function is significantly less efficient than other polling
or asynchronous I/O mechanisms, and is provided for API compatibility only.
It's use is discouraged.

NOTE: This function is *not* supported on systems other than POSIX derived
platforms and Windows.

== RETURN VALUES

This function returns the number of sockets with events on success, or -1 on error.

== ERRORS

[horizontal]
`ENOMEM`:: Insufficient memory available.
`EBADF`:: One of the sockets is not open.
`ETIMEDOUT`:: Operation timed out.
`ENOTSUP`:: This function is not supported on this platform.

== SEE ALSO

[.text-left]
xref:nn_errno.3compat.adoc[nn_errno(3compat)],
xref:nn_recv.3compat.adoc[nn_recv(3compat)],
xref:nn_send.3compat.adoc[nn_send(3compat)],
xref:nn_socket.3compat.adoc[nn_socket(3compat)],
xref:nng_compat.3compat.adoc[nn_compat(3compat)],
xref:nng.7.adoc[nng(7)]