ppoll (2)
Leading comments
Copyright (C) 1997 Andries Brouwer (aeb@cwi.nl) and Copyright (C) 2006, Michael Kerrisk <mtk.manpages@gmail.com> %%%LICENSE_START(VERBATIM) 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 ...
NAME
poll, ppoll - wait for some event on a file descriptorSYNOPSIS
#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 urgent data to read (e.g., out-of-band data on TCP socket; pseudoterminal master in packet mode has seen state change in slave).
- 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).
- 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
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(sigset_t)).