poll(2) SYSTEM CALLS poll(2)
NAME
poll - input/output multiplexing
SYNOPSIS
#include <stropts.h>
#include <poll.h>
int poll(struct poll *fds, sizet nfds, int timeout);
DESCRIPTION
poll provides users with a mechanism for multiplexing
input/output over a set of file descriptors that reference
open files. poll identifies those files on which a user can
send or receive messages, or on which certain events have
occurred. 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 */
fd specifies an open file descriptor and events and revents
are bitmasks constructed by an OR of any combination of the
following event flags:
POLLIN Data other than high priority data may be
read without blocking. For STREAMS, this
flag is set even if the message is of zero
length.
POLLRDNORM Normal data (priority band = 0) may be read
without blocking. For STREAMS, this flag is
set even if the message is of zero length.
POLLRDBAND Data from a non-zero priority band may be
read without blocking For STREAMS, this flag
is set even if the message is of zero length.
POLLPRI High priority data may be received without
blocking. For STREAMS, this flag is set even
if the message is of zero length.
POLLOUT Normal data may be written without blocking.
POLLWRNORM The same as POLLOUT.
POLLWRBAND Priority data (priority band > 0) may be
written.
POLLMSG An MSIG or MPCSIG message containing the
SIGPOLL signal has reached the front of the
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poll(2) SYSTEM CALLS poll(2)
stream head read queue.
POLLERR An error has occured on the device or stream.
This flag is only valid in the revents bit-
mask; 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,
POLLRDNORM, POLLRDBAND, 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 file. 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 exam-
ines the given file descriptor for the event(s) specified in
events. The number of file descriptors to be examined is
specified by nfds.
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 mil-
lisecond timing accuracy is not available, timeout is
rounded up to the nearest legal value available on that sys-
tem. If the value timeout is 0, poll returns immediately.
If the value of timeout is INFTIM (or -1), poll blocks until
a requested event occurs or until the call is interrupted.
poll is not affected by the ONDELAY and ONONBLOCK flags.
poll fails if one or more of the following are true:
EAGAIN Allocation of internal data structures
failed, but the request may be attempted
again.
EFAULT Some argument points outside the allocated
address space.
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poll(2) SYSTEM CALLS poll(2)
EINTR A signal was caught during the poll system
call.
EINVAL The argument nfds is greater than {OPENMAX}.
SEE ALSO
intro(2), getmsg(2), getrlimit(2), putmsg(2), read(2),
write(2)
Programmer's Guide: STREAMS
DIAGNOSTICS
Upon successful completion, a non-negative value is
returned. A positive value indicates the total number of
file descriptors that has been selected (i.e., file descrip-
tors 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.
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