int poll(fds, nfds, timeout) struct pollfd *fds; unsigned long nfds; int timeout;
poll() provides users with a mechanism for multiplexing input/output over a set of file descriptors (see intro.2 poll() identifies those file descriptors on which a user can send or receive messages, or on which certain events have occurred. A user can receive messages using read.2v or getmsg.2 and can send messages using write.2v and putmsg.2 Certain ioctl.2 calls, such as I_RECVFD and I_SENDFD (see streamio.4 can also be used to receive and send messages on streams.
fds specifies the file descriptors to be examined and the events of interest for each file descriptor. It is a pointer to an array with one element for each open file descriptor of interest. The array's elements are pollfd structures which contain the following members:
int fd; /* file descriptor */ short events; /* requested events */ short revents; /* returned events */
where fd specifies an open file descriptor and events and revents are bitmasks constructed by ORing any combination of the following event flags:
For each element of the array pointed to by fds, poll() examines the given file descriptor for the event(s) specified in events. The number of file descriptors to be examined is specified by nfds. If nfds exceeds the system limit of open files (see getdtablesize.2 poll() will fail.
If the value
is less than zero,
is ignored and
is set to 0 in that entry on return from
The results of the poll() query are stored in the revents field in the pollfd structure. Bits are set in the revents bitmask to indicate which of the requested events are true. If none are true, none of the specified bits is set in revents when the poll() call returns. The event flags POLLHUP, POLLERR, and POLLNVAL are always set in revents if the conditions they indicate are true; this occurs even though these flags were not present in events.
If none of the defined events have occurred on any selected file descriptor, poll() waits at least timeout milliseconds for an event to occur on any of the selected file descriptors. On a computer where millisecond timing accuracy is not available, timeout is rounded up to the nearest legal value available on that system. If the value timeout is 0, poll() returns immediately. If the value of timeout is -1, poll() blocks until a requested event occurs or until the call is interrupted. poll() is not affected by the O_NDELAY flag.
poll() returns a non-negative value on success. A positive value indicates the total number of file descriptors that has been selected (for instance, file descriptors for which the revents field is non-zero). 0 indicates the call timed out and no file descriptors have been selected. On failure, poll() returns -1 and sets errno to indicate the error.
nfds is greater than the system limit of open files.
Created by unroff & hp-tools. © by Hans-Peter Bischof. All Rights Reserved (1997).
Last modified 21/April/97