poll
NAME
poll, ppoll - wait for some event on a file descriptor
SYNOPSIS
#include <poll.h> I int poll(struct pollfd * fds , nfds_t nfds , int timeout ); #define _GNU_SOURCE #include <poll.h> I int ppoll(struct pollfd * fds , nfds_t nfds , I const struct timespec * timeout , const sigset_t * sigmask );
DESCRIPTION
R 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
R events ,
or one of the values
R POLLERR ,
R POLLHUP ,
or
R 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
R poll ()
blocks until one of the events occurs.
The
timeout
argument specifies an upper limit on the time for which
R 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.
R 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
R POLLIN .
POLLRDBAND
Priority band data can be read (generally unused on Linux).
POLLWRNORM
Equivalent to
R POLLOUT .
POLLWRBAND
Priority data may be written.
Linux also knows about, but does not use
R POLLMSG .
ppoll()
The relationship between R poll () and R ppoll () is analogous to the relationship between select(2) and pselect(2): like pselect(2), R 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
R 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
R ppoll ()
is necessary.
The
timeout
argument specifies an upper limit on the amount of time that
R 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
R ppoll ()
can block indefinitely.
RETURN VALUE
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.
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.
EINVAL
The
nfds
value exceeds the
RLIMIT_NOFILE
value.
ENOMEM
There was no space to allocate file descriptor tables.
VERSIONS
The
R poll ()
system call was introduced in Linux 2.1.23.
The
R poll ()
library call was introduced in libc 5.4.28
(and provides emulation using select(2) if your kernel does not
have a
R poll ()
system call).
The
R ppoll ()
system call was added to Linux in kernel 2.6.16.
The
R ppoll ()
library call was added in glibc 2.4.
CONFORMING TO
R poll ()
conforms to POSIX.1-2001.
R ppoll ()
is Linux specific.
NOTES
Some implementations define the non-standard constant
INFTIM
with the value -1 for use as a
R timeout .
This constant is not provided in glibc.
Linux Notes
The Linux R ppoll () system call modifies its timeout argument. However, the glibc wrapper function hides this behavior by using a local variable for the timeout argument that is passed to the system call. Thus, the glibc R ppoll () function does not modify its timeout argument.BUGS
See the discussion of spurious readiness notifications under the
BUGS section of
select(2).
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