POLL(2) — SYSTEM CALLS

NAME

poll − STREAMS I/O multiplexing

SYNOPSIS

#include <stropts.h>
#include <poll.h>

int poll(fds, nfds, timeout)
struct pollfd ∗fds;
unsigned long nfds;
int timeout;

DESCRIPTION

poll() provides users with a mechanism for multiplexing input/output over a set of file descriptors that reference open streams (see intro(2)). poll() identifies those streams 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.

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:

POLLIN A non-priority or file descriptor passing message (see I_RECVFD) is present on the stream head read queue. This flag is set even if the message is of zero length. In revents, this flag is mutually exclusive with POLLPRI.

POLLPRI A priority message is present on the stream head read queue. This flag is set even if the message is of zero length. In revents, this flag is mutually exclusive with POLLIN.

POLLOUT The first downstream write queue in the stream is not full. Priority control messages can be sent (see putmsg(2)) at any time.

POLLERR An error message has arrived at the stream head. This flag is only valid in the revents bitmask; it is not used in the events field.

POLLHUP A hangup has occurred on the stream. This event and POLLOUT are mutually exclusive; a stream can never be writable if a hangup has occurred. However, this event and POLLIN or POLLPRI are not mutually exclusive. This flag is only valid in the revents bitmask; it is not used in the events field.

POLLNVAL The specified fd value does not belong to an open stream. This flag is only valid in the revents field; it is not used in the events field.

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 fd is less than zero, events is ignored and revents is set to 0 in that entry on return from poll().

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.

RETURN VALUE

Upon successful completion, a non-negative value is returned. 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). A value of 0 indicates that the call timed out and no file descriptors have been selected. Upon failure, a value of −1 is returned and errno is set to indicate the error.

ERRORS

poll() fails if one or more of the following are true:

EAGAIN Allocation of internal data structures failed, but the request should be attempted again.

EFAULT Some argument points outside the allocated address space.

EINTR A signal was caught during the poll() system call.

EINVAL The argument nfds is less than zero, or nfds is greater than the system limit of open files.

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