## nn_poll

Poll sockets (compatible API).

### Synopsis

```c
#include <nanomsg/nn.h>

#define NN_POLLIN  1
#define NN_POLLOUT 2

struct nn_pollfd {
    int      fd;
    uint16_t events;
    uint16_t revents;
};

int nn_poll(struct nn_pollfd *pfds, int npfd, int timeout);
```

### Description

The `nn_poll` function polls a group of sockets for readiness to send or receive.

The array of _nfds_ sockets to poll for are passed into _pfds_.
Each member of this array is initialized with the `fd` field set to the socket, and the `events` field set to a mask that can contain either or both of the flags `NN_POLLIN` and `NN_POLLOUT`.

The flag `NN_POLLIN` indicates that a socket is ready for receiving without blocking (a message is available on the socket), and the flag `NN_POLLOUT` indicates that a socket is ready for sending without blocking.

Upon success, the function returns the number of updates the `revents` field of each member of the _pfds_ array, setting it to indicate whether the requested status is true or not.

NOTE: The `revents` field will only have a flag set if the corresponding flag was also set in the `events` field.

If the _timeout_ field is positive, then this function will wait for up the that many milliseconds.
If none of the requested events occurs before that timeout occurs, then the function will return -1 and set the error to `ETIMEDOUT`.

If the _timeout_ is zero, then this function will return immediately, after updating the current status of the sockets.

If the _timeout_ is -1, then the function waits forever, or until one of the requested events occurs.

IMPORTANT: This function is only suitable for use with sockets obtained with the xref:nn_socket.adoc[`nn_socket`] function, and is not compatible with file descriptors obtained via any other means.
This includes file descriptors obtained using the `NN_SNDFD` or `NN_RCVFD`
options with xref:nn_getsockopt.adoc[`nn_getsockopt`]

NOTE: This function is significantly less efficient than other polling or asynchronous I/O mechanisms, and is provided for API compatibility only.
It's use is discouraged.

NOTE: This function is *not* supported on systems other than POSIX derived platforms and Windows.

### Return Values

This function returns the number of sockets with events on success, or -1 on error.

### Errors

[horizontal]
`ENOMEM`:: Insufficient memory available.
`EBADF`:: One of the sockets is not open.
`ETIMEDOUT`:: Operation timed out.
`ENOTSUP`:: This function is not supported on this platform.

### See Also

xref:nn_errno.adoc[nn_errno],
xref:nn_recv.adoc[nn_recv],
xref:nn_send.adoc[nn_send],
xref:nn_socket.adoc[nn_socket]
