The poll(3) function 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 that refers to the
file /proc/procID/ctl, where procID is
the process ID of a process running on the system. (The poll
function is not supported for use with other files.) The
events and revents members are bitmasks of conditions
to monitor and conditions found, respectively.
nfds
The number of pollfd structures in the fds
array.
timeout
Maximum interval to wait for the poll to complete, in
milliseconds. If this value is 0, the poll() will return
immediately. If this value is -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 other than high priority data can be read without
blocking.
POLLRDNORM
Normal data can be read without blocking.
POLLRDBAND
Data with a nonzero priority can be read without blocking.
POLLPRI
High priority data can be read without blocking.
POLLOUT
Data other than high priority data can be written without
blocking.
POLLWRNORM
Normal data can be written without blocking.
POLLWRBAND
Data with a nonzero priority can 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.
POLLINVAL
The file descriptor is not open. This flag is always checked,
even if it is not present in the events bitmask.
If timeout is neither zero nor -1, it specifies a maximum
interval to wait for any file descriptor to become ready, in
milliseconds. If timeout is -1, the poll blocks
indefinitely. If timeout is zero, the poll() returns
without blocking.
The poll() function returns the number of descriptors
that are ready for I/O, or -1 if an error occurred. If the time
limit expires, poll() returns 0. If poll() returns
with an error, including one due to an interrupted call, the
fds array will be unmodified.
ERRORS
[EFAULT]
The fds argument points outside the allocated address
space of the process.
[EINTR]
A signal was delivered before the time limit expired and before
any of the selected events occurred.
[EINVAL]
The specified time limit is negative.
NOTE
On Interix, poll() is supported only for use with
ctl files (located in /proc/procID). No other
file types are allowed.