POLL(2) Linux Programmer's Manual POLL(2)NAME
poll, ppoll - wait for some event on a file descriptor
SYNOPSIS
#include <poll.h>
int poll(struct pollfd *fds, nfds_t nfds, int timeout);
#define _GNU_SOURCE
#include <poll.h>
int ppoll(struct pollfd *fds, nfds_t nfds,
const struct timespec *timeout, const sigset_t *sigmask);
DESCRIPTIONpoll() performs a similar task to select(2): it waits for one of a set
of file descriptors to become ready to perform I/O.
The set of file descriptors to be monitored is specified in the fds
argument, which is an array of nfds structures of the following form:
struct pollfd {
int fd; /* file descriptor */
short events; /* requested events */
short revents; /* returned events */
};
The field fd contains a file descriptor for an open file.
The field events is an input parameter, a bit mask specifying the
events the application is interested in.
The field revents is an output parameter, filled by the kernel with the
events that actually occurred. The bits returned in revents can
include any of those specified in events, or one of the values POLLERR,
POLLHUP, or POLLNVAL. (These three bits are meaningless in the events
field, and will be set in the revents field whenever the corresponding
condition is true.)
If none of the events requested (and no error) has occurred for any of
the file descriptors, then poll() blocks until one of the events
occurs.
The timeout argument specifies an upper limit on the time for which
poll() will block, in milliseconds. Specifying a negative value in
timeout means an infinite timeout.
The bits that may be set/returned in events and revents are defined in
<poll.h>:
POLLIN There is data to read.
POLLPRI
There is urgent data to read (e.g., out-of-band data on
TCP socket; pseudo-terminal master in packet mode has
seen state change in slave).
POLLOUT
Writing now will not block.
POLLRDHUP (since Linux 2.6.17)
Stream socket peer closed connection, or shut down writ‐
ing half of connection. The _GNU_SOURCE feature test
macro must be defined in order to obtain this definition.
POLLERR
Error condition (output only).
POLLHUP
Hang up (output only).
POLLNVAL
Invalid request: fd not open (output only).
When compiling with _XOPEN_SOURCE defined, one also has the following,
which convey no further information beyond the bits listed above:
POLLRDNORM
Equivalent to POLLIN.
POLLRDBAND
Priority band data can be read (generally unused on
Linux).
POLLWRNORM
Equivalent to POLLOUT.
POLLWRBAND
Priority data may be written.
Linux also knows about, but does not use POLLMSG.
ppoll()
The relationship between poll() and ppoll() is analogous to the rela‐
tionship between select(2) and pselect(2): like pselect(2), ppoll()
allows an application to safely wait until either a file descriptor
becomes ready or until a signal is caught.
Other than the difference in the timeout argument, the following
ppoll() call:
ready = ppoll(&fds, nfds, timeout, &sigmask);
is equivalent to atomically executing the following calls:
sigset_t origmask;
sigprocmask(SIG_SETMASK, &sigmask, &origmask);
ready = poll(&fds, nfds, timeout);
sigprocmask(SIG_SETMASK, &origmask, NULL);
See the description of pselect(2) for an explanation of why ppoll() is
necessary.
If the sigmask argument is specified as NULL, then no signal mask
manipulation is performed (and thus ppoll() differs from poll() only in
the precision of the timeout argument).
The timeout argument specifies an upper limit on the amount of time
that ppoll() will block. This argument is a pointer to a structure of
the following form:
struct timespec {
long tv_sec; /* seconds */
long tv_nsec; /* nanoseconds */
};
If timeout is specified as NULL, then ppoll() can block indefinitely.
RETURN VALUE
On success, a positive number is returned; this is the number of struc‐
tures which have non-zero revents fields (in other words, those
descriptors with events or errors reported). A value of 0 indicates
that the call timed out and no file descriptors were ready. On error,
-1 is returned, and errno is set appropriately.
ERRORS
EBADF An invalid file descriptor was given in one of the sets.
EFAULT The array given as argument was not contained in the calling
program's address space.
EINTR A signal occurred before any requested event; see signal(7).
EINVAL The nfds value exceeds the RLIMIT_NOFILE value.
ENOMEM There was no space to allocate file descriptor tables.
VERSIONS
The poll() system call was introduced in Linux 2.1.23. The poll()
library call was introduced in libc 5.4.28 (and provides emulation
using select(2) if your kernel does not have a poll() system call).
The ppoll() system call was added to Linux in kernel 2.6.16. The
ppoll() library call was added in glibc 2.4.
CONFORMING TOpoll() conforms to POSIX.1-2001. ppoll() is Linux-specific.
NOTES
Some implementations define the non-standard constant INFTIM with the
value -1 for use as a timeout. This constant is not provided in glibc.
Linux Notes
The Linux ppoll() system call modifies its timeout argument. However,
the glibc wrapper function hides this behavior by using a local vari‐
able for the timeout argument that is passed to the system call. Thus,
the glibc ppoll() function does not modify its timeout argument.
BUGS
See the discussion of spurious readiness notifications under the BUGS
section of select(2).
SEE ALSOselect(2), select_tut(2), feature_test_macros(7), time(7)COLOPHON
This page is part of release 3.15 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2008-04-23 POLL(2)
