poll(2) poll(2)
NAME [Toc] [Back]
poll - monitor I/O conditions on multiple file descriptors
SYNOPSIS [Toc] [Back]
#include <poll.h>
int poll(
struct pollfd fds[],
nfds_t nfds,
int timeout
);
DESCRIPTION [Toc] [Back]
poll() provides a general mechanism for reporting I/O conditions
associated with a set of file descriptors and for waiting until one or
more specified conditions becomes true. Specified conditions include
the ability to read or write data without blocking, and error
conditions.
Arguments [Toc] [Back]
fds Points to an array of pollfd structures, one for
each file descriptor of interest.
nfds Specifies the number of pollfd structures in the
fds array.
timeout Specifies the maximum length of time (in
milliseconds) to wait for at least one of the
specified conditions to occur.
Each pollfd structure includes the following members:
int fd File descriptor
short events Requested conditions
short revents Reported conditions
The fd member of each pollfd structure specifies an open file
descriptor. The poll() function uses the events member to determine
what conditions to report for this file descriptor. If one or more of
these conditions is true, poll() sets the associated revents member.
poll() ignores any pollfd structure whose fd member is negative. If
the fd member of all pollfd structures is negative, poll() returns 0
and has no other results.
The events and revents members of the pollfd structure are bit masks.
The calling process sets the events bit mask, and poll() sets the
revents bit masks. These bit masks contain ORed combinations of
condition flags. The following condition flags are defined:
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poll(2) poll(2)
POLLIN Data can be read without blocking. For
streams, this flag means that a message that
is not high priority is at the front of the
stream head read queue. This message can be
of zero length.
POLLNORM Synonym for POLLIN
POLLPRI A high priority message is available. For
streams, this message can be of zero length.
POLLOUT Data can be written without blocking. For
streams, this flag specifies that normal data
(not high priority or priority band > 0) can
be written without being blocked by flow
control. This flag is not used for high
priority data, because it can be written even
if the stream is flow controlled.
POLLERR An error has occurred on the file descriptor.
POLLHUP The device has been disconnected. For
streams, this flag in revents is mutually
exclusive with POLLOUT, since a stream cannot
be written to after a hangup occurs. This
flag and POLLIN, POLLPRI, POLLRDNORM,
POLLRDBAND, and POLLMSG are not mutually
exclusive.
POLLNVAL fd is not a valid file descriptor.
POLLRDNORM A non-priority message is available. For
streams, this flag means that a normal
message (not high priority or priority band >
0) is at the front of the stream head read
queue. This message can be of zero length.
POLLRDBAND A priority message (priority band > 0) is at
the front of the stream head read queue.
This message can be read without blocking.
The message can be of zero length.
POLLWRNORM Same as POLLOUT
POLLWRBAND Priority data (priority band > 0) can be
written without being blocked by flow
control. Only previously written bands are
checked.
POLLMSG A M_SIG or M_PCSIG message specifying SIGPOLL
has reached the front of the stream head read
queue.
The conditions indicated by POLLNORM and POLLOUT are true if and only
if at least one byte of data can be read or written without blocking.
The exception is regular files, which always poll true for POLLNORM
and POLLOUT. Also, streams return POLLNORM in revents even if the
available message is of zero length.
The condition flags POLLERR, POLLHUP, and POLLNVAL are always set in
revents if the conditions they indicate are true for the specified
file descriptor, whether or not these flags are set in events.
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poll(2) poll(2)
For each call to poll(), the set of reportable conditions for each
file descriptor consists of those conditions that are always reported,
together with any further conditions for which flags are set in
events. If any reportable condition is true for any file descriptor,
poll() returns with flags set in revents for each true condition for
that file descriptor.
If no reportable condition is true for any of the file descriptors,
poll() waits up to timeout milliseconds for a reportable condition to
become true. If, in that time interval, a reportable condition
becomes true for any of the file descriptors, poll() reports the
condition in the file descriptor's associated revents member and
returns. If no reportable condition becomes true, poll() returns
without setting any revents bit masks.
If the timeout parameter is a value of -1, poll() does not return
until at least one specified event has occurred. If the value of the
timeout parameter is 0, poll() does not wait for an event to occur but
returns immediately, even if no specified event has occurred. The
behavior of poll() is not affected by whether the O_NONBLOCK flag is
set on any of the specified file descriptors.
RETURN VALUES [Toc] [Back]
Upon successful completion, poll() returns a nonnegative value. If
the call returns 0, poll() has timed out and has not set any of the
revents bit masks. A positive value indicates the number of file
descriptors for which poll() has set the revents bit mask. If poll()
fails, it returns -1 and sets errno to indicate the error.
ERRORS [Toc] [Back]
poll() fails if any of the following conditions are encountered:
[EAGAIN] Allocation of internal data structures failed. A
later call to poll() may complete successfully.
[EINTR] A signal was delivered before any of the selected
for conditions occurred or before the time limit
expired.
[EINVAL] timeout is a negative number other than -1.
[EFAULT] The fds parameter in conjunction with the nfds
parameter addresses a location outside of the
allocated address space of the process. Reliable
detection of this error is implementationdependent.
EXAMPLES [Toc] [Back]
Wait for input on file descriptor 0:
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poll(2) poll(2)
#include <poll.h>
struct pollfd fds;
fds.fd = 0;
fds.events = POLLNORM;
poll(&fds, 1, -1);
Wait for input on ifd1 and ifd2, output on ofd, giving up after 10
seconds:
#include <poll.h>
struct pollfd fds[3];
int ifd1, ifd2, ofd, count;
fds[0].fd = ifd1;
fds[0].events = POLLNORM;
fds[1].fd = ifd2;
fds[1].events = POLLNORM;
fds[2].fd = ofd;
fds[2].events = POLLOUT;
count = poll(fds, 3, 10000);
if (count == -1) {
perror("poll failed");
exit(1);
}
if (count==0)
printf("No data for reading or writing\n");
if (fds[0].revents & POLLNORM)
printf("There is data for reading fd %d\n", fds[0].fd);
if (fds[1].revents & POLLNORM)
printf("There is data for reading fd %d\n", fds[1].fd);
if (fds[2].revents & POLLOUT)
printf("There is room to write on fd %d\n", fds[2].fd);
Check for input or output on file descriptor 5 without waiting:
#include <poll.h>
struct pollfd fds;
fds.fd = 5;
fds.events = POLLNORM|POLLOUT;
poll(&fds, 1, 0);
if (fds.revents & POLLNORM)
printf("There is data available on fd %d\n", fds.fd);
if (fds.revents & POLLOUT)
printf("There is room to write on fd %d\n", fds.fd);
Wait 3.5 seconds:
#include <stdio.h>
#include <poll.h>
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poll(2) poll(2)
poll((struct pollfd *) NULL, 0, 3500);
Wait for a high priority, priority, or normal message on streams file
descriptor 0:
#include <poll.h>
struct pollfd fds;
fds.fd = 0;
fds.events = POLLIN|POLLPRI;
poll(&fds, 1, -1);
SEE ALSO [Toc] [Back]
read(2), write(2), select(2), getmsg(2), putmsg(2), streamio(7).
STANDARDS CONFORMANCE [Toc] [Back]
poll(): AES, SVID2, SVID3
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