poll, ppoll — wait for some event on a file descriptor
#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); |
poll() 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;short events;short revents;};
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>
POLLINThere is data to read.
POLLPRIThere 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).
POLLOUTWriting now will not block.
POLLRDHUP(since Linux 2.6.17)Stream socket peer closed connection, or shut down writing half of connection. The
_GNU_SOURCEfeature test macro must be defined in order to obtain this definition.POLLERRError condition (output only).
POLLHUPHang up (output only).
POLLNVALInvalid request:
fdnot open (output only).
When compiling with _XOPEN_SOURCE defined, one also has the
following, which convey no further information beyond the
bits listed above:
POLLRDNORMEquivalent to
POLLIN.POLLRDBANDPriority band data can be read (generally unused on Linux).
POLLWRNORMEquivalent to
POLLOUT.POLLWRBANDPriority data may be written.
Linux also knows about, but does not use POLLMSG.
The relationship between poll() and ppoll() is analogous to the relationship
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.
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;long tv_nsec;};
If timeout is
specified as NULL, then ppoll() can block indefinitely.
On success, a positive number is returned; this is the
number of structures which have nonzero 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.
An invalid file descriptor was given in one of the sets.
The array given as argument was not contained in the calling program's address space.
A signal occurred before any requested event.
The nfds
value exceeds the RLIMIT_NOFILE value.
There was no space to allocate file descriptor tables.
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.
Some implementations define the non-standard constant
INFTIM with the value −1
for use as a timeout.
This constant is not provided in glibc.
This page is part of release 2.79 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/.
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Copyright (C) 1997 Andries Brouwer (aebcwi.nl) and Copyright (C) 2006, Michael Kerrisk <mtk.manpagesgmail.com> Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. Additions from Richard Gooch <rgoochatnf.CSIRO.AU> and aeb, 971207 2006-03-13, mtk, Added ppoll() + various other rewordings 2006-07-01, mtk, Added POLLRDHUP + various other wording and formatting changes. FIXME . 2.6.17 has a definition for POLLREMOVE, but this flag is not used in the code. Check later to see if it does get a use. 2.6.21 still shows no use. |