POLL(2)

NAME
poll - synchronous I/O multiplexing

SYNOPSIS
#include <poll.h>
int
poll(struct pollfd *fds, int nfds, int timeout);

DESCRIPTION
poll() provides a mechanism for reporting I/O conditions across a set of file descriptors.

The arguments are as follows:

fds Points to an array of pollfd structures, which are defined as:

        struct pollfd {
        	int fd;
        	short events;
        	short revents;
        };

    The fd member is an open file descriptor. The events and revents members are bitmasks of conditions to monitor and conditions found, respectively.

nfds
    The number of pollfd structures in the array.

timeout
    Maximum interval to wait for the poll to complete, in milliseconds. If this value is 0, then poll() will return immediately. If this value is INFTIM (-1), poll() will block indefinitely until a condition is found.

The calling process sets the events bitmask and poll() sets the revents bitmask. Each call to poll() resets the revents bitmask for accuracy. The condition flags in the bitmasks are defined as:

POLLIN
    Data is available on the file descriptor for reading.

POLLNORM
    Same as POLLIN.

POLLPRI
    Same as POLLIN.

POLLOUT
    Data can be written to the file descriptor without blocking.

POLLERR
    This flag is not used in this implementation and is provided only for source code compatibility.

POLLHUP
    The file descriptor was valid before the polling process and invalid after. Presumably, this means that the file descriptor was closed sometime during the poll.

POLLNVAL
    The corresponding file descriptor is invalid.

POLLRDNORM
    Same as POLLIN.

POLLRDBAND
    Same as POLLIN.

POLLWRNORM
    Same as POLLOUT.

POLLWRBAND
    Same as POLLOUT.

POLLMSG
    This flag is not used in this implementation and is provided only for source code compatibility.

All flags except POLLIN, POLLOUT, and their synonyms are for use only in the revents member of the pollfd structure. An attempt to set any of these flags in the events member will generate an error condition.

In addition to I/O multiplexing, poll() can be used to generate simple timeouts. This functionality may be achieved by passing a null pointer for fds.

WARNINGS
The POLLHUP flag is only a close approximation and may not always be accurate.

RETURN VALUES
Upon error, poll() returns a -1 and sets the global variable errno to indicate the error. If the timeout interval was reached before any events occurred, a 0 is returned. Otherwise, poll() returns the number of file descriptors for which revents is non-zero.

ERRORS
poll() will fail if:

[EINVAL]
    nfds was either a negative number or greater than the number of available file descriptors.

[EINVAL]
    An invalid flags was set in the events member of the pollfd structure.

[EINVAL]
    The timeout passed to poll() was too large.

[EAGAIN]
    Resource allocation failed inside of poll(). Subsequent calls to poll() may succeed.

[EINTR]
    poll() caught a signal during the polling process.

SEE ALSO
poll(2), select(2), sysconf(3)

HISTORY
A poll() system call appeared in AT&T UNIX.