poll() and
pollts() examine a set of file descriptors to see if some of them are ready for I/O. The
fds argument is a pointer to an array of pollfd structures as defined in
<poll.h> (shown below). The
nfds argument determines the size of the
fds array.
struct pollfd {
int fd; /* file descriptor */
short events; /* events to look for */
short revents; /* events returned */
};
The fields of
struct pollfd are as follows:
fd
File descriptor to poll. If the value in fd is negative, the file descriptor is ignored and revents is set to 0.
events
Events to poll for. (See below.)
revents
Events which may occur. (See below.)
The event bitmasks in
events and
revents have the following bits:
POLLIN
Data other than high priority data may be read without blocking.
POLLRDNORM
Normal data may be read without blocking.
POLLRDBAND
Data with a non-zero priority may be read without blocking.
POLLPRI
High priority data may be read without blocking.
POLLOUT
Normal data may be written without blocking.
POLLWRNORM
Equivalent to POLLOUT.
POLLWRBAND
Data with a non-zero priority may be written without blocking.
POLLERR
An exceptional condition has occurred on the device or socket. This flag is always checked, even if not present in the events bitmask.
POLLHUP
The device or socket has been disconnected. This flag is always checked, even if not present in the events bitmask. Note that POLLHUP and POLLOUT should never be present in the revents bitmask at the same time. If the remote end of a socket is closed, poll() returns a POLLIN event, rather than a POLLHUP.
POLLNVAL
The file descriptor is not open. This flag is always checked, even if not present in the events bitmask.
If
timeout is neither zero nor INFTIM (-1), it specifies a maximum interval to wait for any file descriptor to become ready, in milliseconds. If
timeout is INFTIM (-1), the poll blocks indefinitely. If
timeout is zero, then
poll() will return without blocking.
If
ts is a non-null pointer, it references a timespec structure which specifies a maximum interval to wait for any file descriptor to become ready. If
ts is a null pointer,
pollts() blocks indefinitely. If
ts is a non-null pointer, referencing a zero-valued timespec structure, then
pollts() will return without blocking.
If
sigmask is a non-null pointer, then the
pollts() function shall replace the signal mask of the caller by the set of signals pointed to by
sigmask before examining the descriptors, and shall restore the signal mask of the caller before returning.