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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 /* See feature_test_macros(7) */
#include <signal.h>
#include <poll.h>
int ppoll(struct pollfd *fds, nfds_t nfds,
const struct timespec *tmo_p, const sigset_t *sigmask);
DESCRIPTION
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 structures of the following form:
struct pollfd {
int fd; /* file descriptor */
short events; /* requested events */
short revents; /* returned events */
};
The caller should specify the number of items in the fds array in nfds.
The field fd contains a file descriptor for an open file. If this field is negative, then
the corresponding events field is ignored and the revents field returns zero. (This
provides an easy way of ignoring a file descriptor for a single poll() call: simply negate
the fd field. Note, however, that this technique can't be used to ignore file descriptor
0.)
The field events is an input parameter, a bit mask specifying the events the application
is interested in for the file descriptor fd. This field may be specified as zero, in
which case the only events that can be returned in revents are POLLHUP, POLLERR, and
POLLNVAL (see below).
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 the number of milliseconds that poll() should block waiting
for a file descriptor to become ready. The call will block until either:
* a file descriptor becomes ready;
* the call is interrupted by a signal handler; or
* the timeout expires.
Note that the timeout interval will be rounded up to the system clock granularity, and
kernel scheduling delays mean that the blocking interval may overrun by a small amount.
Specifying a negative value in timeout means an infinite timeout. Specifying a timeout of
zero causes poll() to return immediately, even if no file descriptors are ready.
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 some exceptional condition on the file descriptor. Possibilities include:
* There is out-of-band data on a TCP socket (see tcp(7)).
* A pseudoterminal master in packet mode has seen a state change on the slave (see
ioctl_tty(2)).
* A cgroup.events file has been modified (see cgroups(7)).
POLLOUT
Writing is now possible, though a write larger that the available space in a socket
or pipe will still block (unless O_NONBLOCK is set).
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 (before including any header files)
in order to obtain this definition.
POLLERR
Error condition (only returned in revents; ignored in events). This bit is also
set for a file descriptor referring to the write end of a pipe when the read end
has been closed.
POLLHUP
Hang up (only returned in revents; ignored in events). Note that when reading from
a channel such as a pipe or a stream socket, this event merely indicates that the
peer closed its end of the channel. Subsequent reads from the channel will return
0 (end of file) only after all outstanding data in the channel has been consumed.
POLLNVAL
Invalid request: fd not open (only returned in revents; ignored in events).
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 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 precision of the timeout argument, the following ppoll()
call:
ready = ppoll(&fds, nfds, tmo_p, &sigmask);
is equivalent to atomically executing the following calls:
sigset_t origmask;
int timeout;
timeout = (tmo_p == NULL) ? -1 :
(tmo_p->tv_sec * 1000 + tmo_p->tv_nsec / 1000000);
pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
ready = poll(&fds, nfds, timeout);
pthread_sigmask(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 tmo_p 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 tmo_p is specified as NULL, then ppoll() can block indefinitely.
RETURN VALUE
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.
ERRORS
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. On older kernels that lack this
system call, the glibc (and the old Linux libc) poll() wrapper function provides emulation
using select(2).
The ppoll() system call was added to Linux in kernel 2.6.16. The ppoll() library call was
added in glibc 2.4.
CONFORMING TO
poll() conforms to POSIX.1-2001 and POSIX.1-2008. ppoll() is Linux-specific.
NOTES
On some other UNIX systems, poll() can fail with the error EAGAIN if the system fails to
allocate kernel-internal resources, rather than ENOMEM as Linux does. POSIX permits this
behavior. Portable programs may wish to check for EAGAIN and loop, just as with EINTR.
Some implementations define the nonstandard constant INFTIM with the value -1 for use as a
timeout for poll(). This constant is not provided in glibc.
For a discussion of what may happen if a file descriptor being monitored by poll() is
closed in another thread, see select(2).
C library/kernel differences
The Linux ppoll() system call modifies its tmo_p 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 ppoll() function does not modify its tmo_p
argument.
The raw ppoll() system call has a fifth argument, size_t sigsetsize, which specifies the
size in bytes of the sigmask argument. The glibc ppoll() wrapper function specifies this
argument as a fixed value (equal to sizeof(kernel_sigset_t)). See sigprocmask(2) for a
discussion on the differences between the kernel and the libc notion of the sigset.
BUGS
See the discussion of spurious readiness notifications under the BUGS section of
select(2).
SEE ALSO
restart_syscall(2), select(2), select_tut(2), epoll(7), time(7)
COLOPHON
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