ZMQ_POLL

Section: 0MQ Manual (3)
Updated: 01/30/2021
Page Index

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:

typedef struct
{
    void *socket;
    int 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 informations. 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 thead-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 */

AUTHORS

This page was written by the 0MQ community. To make a change please read the 0MQ Contribution Policy at m[blue]http://www.zeromq.org/docs:contributingm[].