zmq_poll(3)

# zmq_poll(3) Manual Page

## NAME

zmq_ppoll - input/output multiplexing with signal mask

## SYNOPSIS

int zmq_ppoll (zmq_pollitem_t *items, int nitems, long timeout, const sigset_t *sigmask);

## DESCRIPTION

The relationship between zmq_poll() and zmq_ppoll() is analogous to the relationship between poll(2) and ppoll(2) and between select(2) and pselect(2): zmq_ppoll() allows an application to safely wait until either a file descriptor becomes ready or until a signal is caught.

When using zmq_ppoll() with sigmask set to NULL, its behavior is identical to that of zmq_poll(). See zmq_poll(3) for more on this.

To make full use of zmq_ppoll(), a non-NULL pointer to a signal mask must be constructed and passed to sigmask. See sigprocmask(2) for more details. When this is done, inside the actual ppoll() (or pselect(), see note below) system call, an atomic operation consisting of three steps is performed: 1. The current signal mask is replaced by the one pointed to by sigmask. 2. The actual poll() call is done. 3. The original signal mask is restored. Because these operations are done atomically, there is no opportunity for race conditions in between the calls changing the signal mask and the poll/select system call. This means that only during this (atomic) call, we can unblock certain signals, so that they can be handled at that time only, not outside of the call. This means that effectively, we extend our poller into a function that not only watches sockets for changes, but also watches the "POSIX signal socket" for incoming signals. At other times, these signals will be blocked, and we will not have to deal with interruptions in system calls at these other times.

Note | The zmq_ppoll() function may be implemented or emulated using operating system interfaces other than ppoll(), and as such may be subject to the limits of those interfaces in ways not defined in this documentation.

Note | There is no ppoll or pselect on Windows, so zmq_ppoll() is not supported in Windows builds. It is still callable, but its sigmask has void pointer type (because sigset_t is also not available on Windows) and zmq_ppoll() will return with an error (see error section below).

## THREAD SAFETY

The zmq_pollitem_t array must only be used by the thread which will/is calling zmq_ppoll.

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_ppoll() 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_ppoll() 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 ØMQ 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.

EINTR

  The operation was interrupted by delivery of a signal before any events were available.

ENOTSUP

  zmq_ppoll() was not activated in this build.

## EXAMPLE

Polling indefinitely for input events on both a ØMQ socket and a standard socket.

See the example section of zmq_poll(3). One only needs to replace the zmq_poll call with zmq_ppoll and add a NULL argument for the sigmask parameter.

Handle SIGTERM during zmq_ppoll (and block it otherwise).

    // simple global signal handler for SIGTERM
    static bool sigterm_received = false;
    void handle_sigterm (int signum) {
        sigterm_received = true;
    }

    // set up signal mask and install handler for SIGTERM
    sigset_t sigmask, sigmask_without_sigterm;
    sigemptyset(&sigmask);
    sigaddset(&sigmask, SIGTERM);
    sigprocmask(SIG_BLOCK, &sigmask, &sigmask_without_sigterm);
    struct sigaction sa;
    memset(&sa, '\0', sizeof(sa));
    sa.sa_handler = handle_sigterm;

    // poll
    zmq_pollitem_t items [1];
    // Just one item, which refers to 0MQ socket 'socket' */
    items[0].socket = socket;
    items[0].events = ZMQ_POLLIN;
    // Poll for events indefinitely, but also exit on SIGTERM
    int rc = zmq_poll (items, 2, -1, &sigmask_without_sigterm);
    if (rc < 0 && errno == EINTR && sigterm_received) {
      // do your SIGTERM business
    } else {
      // do your non-SIGTERM error handling
    }

## SEE ALSO

zmq_poll(3) zmq_socket(3) zmq_send(3) zmq_recv(3) zmq(7)

Your operating system documentation for the poll() system call.

## AUTHORS

This page was written by the ØMQ community. To make a change please read the ØMQ Contribution Policy at http://www.zeromq.org/docs:contributing.