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nn_poll - Man Page

poll a set of SP sockets for readability and/or writability


#include <nanomsg/nn.h>

int nn_poll (struct nn_pollfd *fds, int nfds, int timeout);


The function checks a set of SP socket and reports whether it’s possible to send a message to the socket and/or receive a message from each socket.

fds argument is an array of nn_pollfd structures with nfds argument specifying the size of the array:

struct nn_pollfd {
    int fd;
    short events;
    short revents;

Each entry in the array represents an SP socket to check. events field specifies which events to check for. The value is a bitwise combination of the following values:


Check whether at least one message can be received from the fd socket without blocking.


Check whether at least one message can be sent to the fd socket without blocking.

After the function returns, revents field contains bitwise combination of NN_POLLIN and NN_POLLOUT according to whether the socket is readable or writable.

timeout parameter specifies how long (in milliseconds) should the function block if there are no events to report.

Return Value

Upon successful completion, the number of nn_pollfds structures with events signaled is returned. In case of timeout, return value is 0. In case of error, -1 is returned and errno is set the one of the values below.



Some of the provided sockets are invalid.


The operation was interrupted by delivery of a signal before the message was sent.


The library is terminating.


nn_poll is a convenience function. You can achieve same behaviour by using NN_RCVFD and NN_SNDFD socket options. However, using the socket options allows for usage that’s not possible with nn_poll, such as simultaneous polling for both SP and OS-level sockets, integration of SP sockets with external event loops etc.


struct nn_pollfd pfd [2];
pfd [0].fd = s1;
pfd [0].events = NN_POLLIN | NN_POLLOUT;
pfd [1].fd = s2;
pfd [1].events = NN_POLLIN;
rc = nn_poll (pfd, 2, 2000);
if (rc == 0) {
    printf ("Timeout!");
    exit (1);
if (rc == -1) {
    printf ("Error!");
    exit (1);
if (pfd [0].revents & NN_POLLIN) {
    printf ("Message can be received from s1!");
    exit (1);

See Also

nn_socket(3) nn_getsockopt(3) nanomsg(7)


Martin Sustrik

Referenced By


2024-01-25 nanomsg 1.1.5