poll(2)

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 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

   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(sigset_t)).

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

   This  page  is  part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at
   https://www.kernel.org/doc/man-pages/.

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