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          /* See feature_test_macros(7) */
#include <poll.h>
int ppoll( struct pollfd *fds,
  nfds_t nfds,
  const struct timespec *timeout_ts,
  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 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.)

The field events is an input parameter, a bit mask specifying the events the application is interested in for the file descriptor fd. If this field is specified as zero, then all events are ignored for fd and revents returns zero.

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. This 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>


There is data to read.


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).


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 (before including any header files) in order to obtain this definition.


Error condition (output only).


Hang up (output only).


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:


Equivalent to POLLIN.


Priority band data can be read (generally unused on Linux).


Equivalent to POLLOUT.


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 precision of the timeout argument, the following ppoll() call:

    ready = ppoll(&fds, nfds, timeout_ts, &sigmask);

is equivalent to atomically executing the following calls:

    sigset_t origmask;
    int timeout;

    timeout = (timeout_ts == NULL) ? −1 :
              (timeout_ts.tv_sec * 1000 + timeout_ts.tv_nsec / 1000000);
    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.

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 timeout_ts 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_ts 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.



The array given as argument was not contained in the calling program's address space.


A signal occurred before any requested event; see signal(7).


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. 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.


poll() conforms to POSIX.1-2001. ppoll() is Linux-specific.


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).

Linux notes

The Linux ppoll() system call modifies its timeout_ts 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 timeout_ts argument.


See the discussion of spurious readiness notifications under the BUGS section of select(2).


restart_syscall(2), select(2), select_tut(2), time(7)


This page is part of release 3.54 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at−pages/.

  Copyright (C) 1997 Andries Brouwer (
and Copyright (C) 2006, Michael Kerrisk <>

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

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.