= zmq_poll(3)


== NAME
zmq_poll - input/output multiplexing


== SYNOPSIS

*int zmq_poll (zmq_pollitem_t '*items', int 'nitems', long 'timeout');*


== DESCRIPTION
The _zmq_poll()_ function provides a mechanism for applications to multiplex
input/output events in a level-triggered fashion over a set of sockets. Each
member of the array pointed to by the 'items' argument is a *zmq_pollitem_t*
structure. The 'nitems' argument specifies the number of items in the 'items'
array. The *zmq_pollitem_t* structure is defined as follows:

["literal", subs="quotes"]
typedef struct
{
    void '*socket';
    zmq_fd_t 'fd';
    short 'events';
    short 'revents';
} zmq_pollitem_t;

For each *zmq_pollitem_t* item, _zmq_poll()_ shall examine either the 0MQ
socket referenced by 'socket' *or* the standard socket specified by the file
descriptor 'fd', for the event(s) specified in 'events'. If both 'socket' and
'fd' are set in a single *zmq_pollitem_t*, the 0MQ socket referenced by
'socket' shall take precedence and the value of 'fd' shall be ignored.

For each *zmq_pollitem_t* item, _zmq_poll()_ shall first clear the 'revents'
member, and then indicate any requested events that have occurred by setting the
bit corresponding to the event condition in the 'revents' member.

If none of the requested events have occurred on any *zmq_pollitem_t* item,
_zmq_poll()_ shall wait 'timeout' milliseconds for an event to occur on
any of the requested items. If the value of 'timeout' is `0`, _zmq_poll()_
shall return immediately. If the value of 'timeout' is `-1`, _zmq_poll()_ shall
block indefinitely until a requested event has occurred on at least one
*zmq_pollitem_t*.

The 'events' and 'revents' members of *zmq_pollitem_t* are bit masks constructed
by OR'ing a combination of the following event flags:

*ZMQ_POLLIN*::
For 0MQ sockets, at least one message may be received from the 'socket' without
blocking. For standard sockets this is equivalent to the 'POLLIN' flag of the
_poll()_ system call and generally means that at least one byte of data may be
read from 'fd' without blocking.

*ZMQ_POLLOUT*::
For 0MQ sockets, at least one message may be sent to the 'socket' without
blocking. For standard sockets this is equivalent to the 'POLLOUT' flag of the
_poll()_ system call and generally means that at least one byte of data may be
written to 'fd' without blocking.

*ZMQ_POLLERR*::
For standard sockets, this flag is passed through _zmq_poll()_ to the
underlying _poll()_ system call and generally means that some sort of error
condition is present on the socket specified by 'fd'. For 0MQ sockets this flag
has no effect if set in 'events', and shall never be returned in 'revents' by
_zmq_poll()_.

*ZMQ_POLLPRI*::
For 0MQ sockets this flags is of no use. For standard sockets this means there
is urgent data to read. Refer to the POLLPRI flag for more information.
For file descriptor, refer to your use case: as an example, GPIO interrupts
are signaled through a POLLPRI event.
This flag has no effect on Windows.

NOTE: The _zmq_poll()_ function may be implemented or emulated using operating
system interfaces other than _poll()_, and as such may be subject to the limits
of those interfaces in ways not defined in this documentation.

== THREAD SAFETY
The *zmq_pollitem_t* array must only be used by the thread which 
will/is calling _zmq_poll_.

If a socket is contained in multiple *zmq_pollitem_t* arrays, each owned by a
different thread, the socket itself needs to be thread-safe (Server, Client, ...).
Otherwise, behaviour is undefined.


== RETURN VALUE
Upon successful completion, the _zmq_poll()_ function shall return the number
of *zmq_pollitem_t* structures with events signaled in 'revents' or `0` if no
events have been signaled. Upon failure, _zmq_poll()_ shall return `-1` and set
'errno' to one of the values defined below.


== ERRORS
*ETERM*::
At least one of the members of the 'items' array refers to a 'socket' whose
associated 0MQ 'context' was terminated.
*EFAULT*::
The provided 'items' was not valid (NULL).
*EINTR*::
The operation was interrupted by delivery of a signal before any events were
available.


== EXAMPLE
.Polling indefinitely for input events on both a 0MQ socket and a standard socket.
----
zmq_pollitem_t items [2];
/* First item refers to 0MQ socket 'socket' */
items[0].socket = socket;
items[0].events = ZMQ_POLLIN;
/* Second item refers to standard socket 'fd' */
items[1].socket = NULL;
items[1].fd = fd;
items[1].events = ZMQ_POLLIN;
/* Poll for events indefinitely */
int rc = zmq_poll (items, 2, -1);
assert (rc >= 0);
/* Returned events will be stored in items[].revents */
----


== SEE ALSO
* xref:zmq_socket.adoc[zmq_socket]
* xref:zmq_send.adoc[zmq_send]
* xref:zmq_recv.adoc[zmq_recv]
* xref:zmq.adoc[zmq]

Your operating system documentation for the _poll()_ system call.


== AUTHORS
This page was written by the 0MQ community. To make a change please
read the 0MQ Contribution Policy at <https://zeromq.org/how-to-contribute/>.
