ZMQ_POLLER(3) 0MQ Manual ZMQ_POLLER(3)
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
descriptiors 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
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 0MQ sockets this flag has no effect on the zmq_poller_add and
zmq_poller_modify functions, and is never set in the events member
of the zmq_poller_event_t structure. For standard sockets, this
flag is passed through zmq_poller_wait_all to the underlying poll()
system call and generally means that some sort of error condition
is present on the socket specified by fd.
ZMQ_POLLPRI
For 0MQ sockets this flag has no effect on the zmq_poller_add and
zmq_poller_modify functions, and is never set in the events member
of the zmq_poller_event_t structure. For standard sockets this
means there is urgent data to read. Refer to the POLLPRI flag for
more informations. For a file descriptor, refer to your OS
documentation: as an example, GPIO interrupts are signaled through
a POLLPRI event. This flag has no effect on Windows.
Note
The zmq_poller*_ functions 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
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:: A new poller could not be allocated
successfully.
On zmq_poller_destroy: EFAULT:: poller_p did not point to a valid
poller. Note that passing an invalid pointer (e.g. pointer to
deallocated memory) may cause undefined behaviour (e.g. an access
violation).
On zmq_poller_size: EFAULT:: poller did not point to a valid poller.
Note that passing an invalid pointer (e.g. pointer to deallocated
memory) may cause undefined behaviour (e.g. an access violation).
On zmq_poller_add, zmq_poller_modify and zmq_poller_remove: EFAULT::
poller did not point to a valid poller. Note that passing an invalid
pointer (e.g. pointer to deallocated memory) may cause undefined
behaviour (e.g. an access violation). ENOTSOCK:: socket did not point
to a valid socket. Note that passing an invalid pointer (e.g. pointer
to deallocated memory) may cause undefined behaviour (e.g. an access
violation).
On zmq_poller_add: EMFILE:: TODO
On zmq_poller_add or zmq_poller_add_fd: ENOMEM:: Necessary resources
could not be allocated. EINVAL:: socket resp. fd was already registered
with the poller.
On zmq_poller_modify, zmq_poller_modify_fd, zmq_poller_remove or
zmq_poller_remove_fd: EINVAL:: socket resp. fd was not registered with
the poller.
On zmq_poller_add_fd, zmq_poller_modify_fd and zmq_poller_remove_fd:
EBADF*: The fd specified was the retired fd.
On zmq_poller_wait and zmq_poller_wait_all: ENOMEM:: Necessary
resources could not be allocated. ETERM:: At least one of the
registered objects is a socket whose associated 0MQ context was
terminated. EFAULT:: The provided events was NULL, or poller did not
point to a valid poller, or there are no registered objects or all
event subscriptions are disabled and timeout was negative. EINTR:: The
operation was interrupted by delivery of a signal before any events
were available. EAGAIN:: No registered event was signalled before the
timeout was reached.
On zmq_poller_fd: EINVAL:: The poller has no associated file
descriptor. EFAULT:: The provided poller did not point to a valid
poller.
EXAMPLE
Polling indefinitely for input events on both a 0MQ socket and a
standard socket..
void *poller = zmq_poller_new ();
zmq_poller_event_t events [2];
/* 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);
/* Poll for events indefinitely */
int rc = zmq_poller_wait_all (poller, events, 2, -1);
assert (rc >= 0);
/* Returned events will be stored in 'events' */
zmq_poller_destroy (poller);
SEE ALSO
zmq_socket(3) zmq_send(3) zmq_recv(3) zmq(7)
AUTHORS
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0MQ 4.3.4 01/18/2023 ZMQ_POLLER(3)
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