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 ; |
/* 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 bitmask 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 writing 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
.
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
;/* seconds */ long tv_nsec
;/* nanoseconds */ };
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 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.
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.
select(2), select_tut(2), feature_test_macros(7)
|