NAME¶

zmq_poller - input/output multiplexing

SYNOPSIS¶

void *zmq_poller_new (void);

int zmq_poller_destroy (void **poller_p);

int zmq_poller_size (void *poller);

int zmq_poller_add (void *poller, void *socket, void *user_data, short events);

int zmq_poller_modify (void *poller, void *socket, short events);

int zmq_poller_remove (void *poller, void *socket);

int zmq_poller_add_fd (void *poller, int fd, void *user_data, short events);

int zmq_poller_modify_fd (void *poller, int fd, short events);

int zmq_poller_remove_fd (void *poller, int fd);

int zmq_poller_wait (void *poller, zmq_poller_event_t *event, long timeout);

int zmq_poller_wait_all (void *poller, zmq_poller_event_t *events, int n_events, long timeout);

int zmq_poller_fd (void *poller, zmq_fd_t *fd);

DESCRIPTION¶

The zmq_poller*_ functions provide a mechanism for applications to multiplex input/output events in a level-triggered fashion over a set of sockets.

zmq_poller_new and zmq_poller_destroy manage the lifetime of a poller instance. zmq_poller_new creates and returns a new poller instance, while zmq_poller_destroy destroys it. A pointer to a valid poller must be passed as the poller_p argument of zmq_poller_destroy. In particular, zmq_poller_destroy may not be called multiple times for the same poller instance. zmq_poller_destroy sets the passed pointer to NULL in case of a successful execution. zmq_poller_destroy implicitly unregisters all registered sockets and file descriptors.

zmq_poller_size queries the number of sockets or file descriptors registered with a poller. The initial size of a poller is 0, a successful add operation increases the size by 1 and a successful remove operation decreases the size by 1. The size is unaffected by the events specified.

zmq_poller_add, zmq_poller_modify and zmq_poller_remove manage the 0MQ sockets registered with a poller.

zmq_poller_add registers a new socket with a given poller. Both poller and socket must point to valid 0MQ objects. The events parameter specifies which event types the client wants to subscribe to. It is legal to specify no events (i.e. 0), and activate them later with zmq_poller_modify. In addition, user_data may be specified, which is not used by the poller, but passed back to the caller when an event was signalled in a call to zmq_poller_wait or zmq_poller_wait_all. user_data may be NULL. If it is not NULL, it must be a valid pointer. Otherwise, behaviour is undefined. You must only add a socket to a single poller instance once (unless zmq_poller_remove has been called for that socket before). You may add a socket to multiple poller instances, if the socket itself is explicitly thread-safe (Server, Client, ...). If the socket is not, you may invoke undefined behavior.

zmq_poller_modify modifies the subscribed events for a socket. It is legal to specify no events (i.e. 0) to disable events temporarily, and reactivate them later with another call to zmq_poller_modify.

zmq_poller_remove removes a socket registration completely. zmq_poller_remove must be called before a socket is closed with zmq_close.

Note that it is not necessary to call zmq_poller_remove for any socket before calling zmq_poller_destroy.

Also note that calling zmq_poller_remove is not equivalent to calling zmq_poller_modify with no events. zmq_poller_modify does not free resources associated with the socket registration, and requires that the socket remains valid.

zmq_poller_add_fd, zmq_poller_modify_fd and zmq_poller_remove_fd are analogous to the previous functions but manage regular file descriptors registered with a poller. On Windows, these functions can only be used with WinSock sockets.

In the following, 0MQ sockets added with zmq_poller_add and file descriptors added with zmq_poller_add_fd are referred to as registered objects.

The zmq_poller_event_t structure is defined as follows:

typedef struct
{


    void *socket;


    zmq_fd_t fd;


    void *user_data;


    short events;
} zmq_poller_event_t;

For each registered object, zmq_poller_wait_all() shall examine the registered objects for the event(s) currently registered.

If none of the registered events have occurred, zmq_poller_wait_all shall wait timeout milliseconds for an event to occur on any of the registered objects. If the value of timeout is 0, zmq_poller_wait_all shall return immediately. If the value of timeout is -1, zmq_poller_wait_all shall block indefinitely until one event has occurred on any of the registered objects.

The events argument zmq_poller_wait_all must be a pointer to an array of at least n_events elements. Behaviour is undefined if events does not point to an array of at least n_events elements.

zmq_poller_wait_all returns at most n_events events. If more than n_events events were signalled, only an unspecified subset of the signalled events is returned through events.

A caller is advised to ensure that n_events is equal to the number of registered objects. Otherwise, a livelock situation may result: If more than n_events registered objects have an active event on each call to zmq_poller_wait_all, it might happen that the same subset of registered objects is always returned, and the caller never notices the events on the others. The number of objects registered can be queried with zmq_poller_size.

zmq_poller_wait_all returns the number of valid elements. The valid elements are placed in positions 0 to n_events - 1 in the events array. All members of a valid element are set to valid values by zmq_poller_wait_all. For socket events socket is non-null and fd is an operating system specific value for an invalid socket (-1 or INVALID_SOCKET). For fd events socket is NULL and fd is a valid file descriptor. The client does therefore not need to initialize the contents of the events array before a call to zmq_poller_wait_all. It is unspecified whether the the remaining elements of events are written to by zmq_poller_wait_all.

zmq_poller_fd queries the file descriptor associated with the zmq_poller, and stores it in the address pointer to by fd. The zmq_poller is only guaranteed to have a file descriptor if at least one thread-safe socket is currently registered.

Note that closing a socket that is registered in a poller leads to undefined behavior. The socket must be unregistered first.

EVENT TYPES¶

The events parameter of zmq_poller_add and zmq_poller_modify, and the events member of the zmq_poller_event_t structure are bit masks constructed by OR’ing a combination of the following event flags:

ZMQ_POLLIN

ZMQ_POLLOUT

ZMQ_POLLERR

ZMQ_POLLPRI

THREAD SAFETY¶

Like most other 0MQ objects, a poller is not thread-safe. All operations must be called from the same thread. Otherwise, behaviour is undefined.

In addition to that, if you want to add a socket to multiple existing poller instances, the socket itself needs to be thread-safe (Server, Client, ...). Otherwise, behaviour is undefined.

RETURN VALUE¶

zmq_poller_new returns a valid pointer to a poller, or NULL in case of a failure.

All functions that return an int, return -1 in case of a failure. In that case, zmq_errno() can be used to query the type of the error as described below.

zmq_poller_wait_all returns the number of events signalled and returned in the events array. It never returns 0.

All other functions return 0 in case of a successful execution.

ERRORS¶

On zmq_poller_new:

ENOMEM

On zmq_poller_destroy:

EFAULT

On zmq_poller_size:

EFAULT

On zmq_poller_add, zmq_poller_modify and zmq_poller_remove:

EFAULT

ENOTSOCK

On zmq_poller_add:

EMFILE

On zmq_poller_add or zmq_poller_add_fd:

ENOMEM

EINVAL

On zmq_poller_modify, zmq_poller_modify_fd, zmq_poller_remove or zmq_poller_remove_fd:

EINVAL

On zmq_poller_add_fd, zmq_poller_modify_fd and zmq_poller_remove_fd:

EBADF

On zmq_poller_wait and zmq_poller_wait_all:

ENOMEM

ETERM

EFAULT

EINTR

EAGAIN

On zmq_poller_fd:

EINVAL

EFAULT

EXAMPLE¶

Polling indefinitely for input events on both a 0MQ socket and a standard socket..

void *poller = zmq_poller_new ();
/* First item refers to 0MQ socket 'socket' */
zmq_poller_add (poller, socket, NULL, ZMQ_POLLIN);
/* Second item refers to standard socket 'fd' */
zmq_poller_add_fd (poller, fd, NULL, ZMQ_POLLIN);
zmq_poller_event_t events [2];
/* Poll for events indefinitely */
int rc = zmq_poller_wait_all (poller, events, 2, -1);
assert (rc >= 0);
/* Returned events will be stored in 'events' */
for (int i = 0; i < 2; ++i) {


    if (events[i].socket == socket && events[i].events & ZMQ_POLLIN) {


        // ...


    } else if (events[i].fd == fd && events[i].events & ZMQ_POLLIN)) {


        // ...


    }
}
zmq_poller_destroy (&poller);

SEE ALSO¶

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

AUTHORS¶

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