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poll

Name
     poll, ppoll - input/output multiplexing

Synopsis
     #include <poll.h>

     int poll(struct pollfd * fds, nfds_t nfds, int timeout);


     int ppoll(struct pollfd *restrict fds, nfds_t nfds,
     const struct timespec *restrict timeout,
     const sigset_t *restrict sigmask);

Description
     The poll() function provides applications with  a  mechanism
     for  multiplexing  input/output  over a set of file descrip-
     tors.  For each member of  the  array  pointed  to  by  fds,
     poll()  examines  the given file descriptor for the event(s)
     specified in events. The number of pollfd structures in  the
     fds  array is specified by nfds. The poll() function identi-
     fies those file descriptors on which an application can read
     or write data, or on which certain events have occurred.


     The fds argument specifies the file descriptors to be  exam-
     ined  and  the  events of interest for each file descriptor.
     It is a pointer to an array with one member  for  each  open
     file descriptor of interest.  The array's members are pollfd
     structures, which contain the following members:

       int     fd;        /* file descriptor */
       short   events;    /* requested events */
       short   revents;   /* returned events */



     The fd member specifies an  open  file  descriptor  and  the
     events  and  revents  members  are bitmasks constructed by a
     logical OR operation of any  combination  of  the  following
     event flags:

     POLLIN
                   Data other than high priority data may be read
                   without  blocking.  For  streams, this flag is
                   set in revents even if the message is of  zero
                   length.


     POLLRDNORM
                   Normal data (priority band equals  0)  may  be
                   read  without blocking. For streams, this flag
                   is set in revents even if the  message  is  of
                   zero length.

     POLLRDBAND
                   Data from a non-zero priority band may be read
                   without  blocking.  For  streams, this flag is
                   set in revents even if the message is of  zero
                   length.


     POLLPRI
                   High priority data  may  be  received  without
                   blocking.  For  streams,  this  flag is set in
                   revents even if the message is of zero length.


     POLLOUT
                   Normal data (priority band equals  0)  may  be
                   written without blocking.


     POLLWRNORM
                   The same as  POLLOUT.


     POLLWRBAND
                   Priority data (priority band > 0) may be writ-
                   ten.  This event only examines bands that have
                   been written to at least once.


     POLLERR
                   An error has occurred on the device or stream.
                   This  flag  is  only valid in the revents bit-
                   mask; it is not used in the events member.


     POLLHUP
                   A hangup has  occurred  on  the  stream.  This
                   event  and   POLLOUT are mutually exclusive; a
                   stream can never be writable if a  hangup  has
                   occurred.  However,  this  event  and  POLLIN,
                   POLLRDNORM, POLLRDBAND,  or  POLLPRI  are  not
                   mutually exclusive. This flag is only valid in
                   the revents bitmask; it is  not  used  in  the
                   events member.


     POLLNVAL
                   The specified fd value does not belong  to  an
                   open  file.  This  flag  is  only valid in the
                   revents member; it is not used in  the  events
                   member.



     If the value fd is  less  than  0,  events  is  ignored  and
     revents is set to 0 in that entry on return from poll().


     The results of the poll() query are stored  in  the  revents
     member  in the pollfd structure. Bits are set in the revents
     bitmask to indicate which of the requested events are  true.

     If  none  are  true,  none  of the specified bits are set in
     revents when  the  poll()  call  returns.  The  event  flags
     POLLHUP,  POLLERR,  and  POLLNVAL are always  set in revents
     if the conditions they indicate are true; this  occurs  even
     though these flags were not present in events.


     If none of the defined events have occurred on any  selected
     file  descriptor, poll() waits at least timeout milliseconds
     for an event to occur on any of the selected  file  descrip-
     tors. On a computer where millisecond timing accuracy is not
     available, timeout is rounded up to the nearest legal  value
     available  on that system. If the value timeout is 0, poll()
     returns immediately. If the value of timeout is  -1,  poll()
     blocks  until  a requested event occurs or until the call is
     interrupted.  The poll() function is  not  affected  by  the
     O_NDELAY and  O_NONBLOCK flags.


     The poll() function supports  regular  files,  terminal  and
     pseudo-terminal  devices,  streams-based  files,  FIFOs  and
     pipes.  The behavior of poll() on elements of fds that refer
     to other types of file is unspecified.


     The poll() function supports sockets.


     A file descriptor for a socket that is listening for connec-
     tions  will indicate that it is ready for reading, once con-
     nections are available.  A file descriptor for a socket that
     is  connecting asynchronously will indicate that it is ready
     for writing, once a connection has been established.


     Regular files always poll() TRUE for reading and writing.


     The relationship between poll() and ppoll() is analogous  to
     the  relationship  between  select(3C) and pselect(3C): like
     pselect(), ppoll() allows  an  application  to  safely  wait
     until either a file descriptor becomes ready or until a sig-
     nal is caught.


     Other than the difference in the timeout argument, the  fol-
     lowing 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 = ppoll(&fds, nfds, timeout);
        sigprocmask(SIG_SETMASK, &origmask, NULL);



     If sigmask is not a null pointer, then the  pselect()  func-
     tion  replaces  the signal mask of the process by the set of
     signals pointed to by sigmask before examining the  descrip-
     tors,  and  restores  the  signal mask of the process before
     returning.


     The timeout 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 is specified as NULL, ppoll() can block  indefin-
     itely.

Return Values
     Upon  successful  completion,  a   non-negative   value   is
     returned.  A  positive  value  indicates the total number of
     file descriptors that  has  been  selected  (that  is,  file
     descriptors  for  which  the  revents member is non-zero). A
     value of 0 indicates that the call timed  out  and  no  file
     descriptors have been selected. Upon failure, -1 is returned
     and errno is set to indicate the error.

Errors
     The poll() and ppoll() functions will fail if:

     EAGAIN
               Allocation of internal data structures failed, but
               the request may be attempted again.


     EFAULT
               Some argument points to an illegal address.


     EINTR
               A signal was caught during the poll() function.

     EINVAL
               The argument nfds is greater than  {OPEN_MAX},  or
               one of the fd members refers to a stream or multi-
               plexer that is  linked  (directly  or  indirectly)
               downstream from a multiplexer.


     ENSOSY
               There is no poll() interface for doorfs.

Attributes
     See attributes(5) for descriptions of the  following  attri-
     butes:



     tab() box; cw(2.75i) |cw(2.75i) lw(2.75i) |lw(2.75i)  ATTRI-
     BUTE  TYPEATTRIBUTE  VALUE  _ Interface StabilityCommitted _
     MT-LevelMT-Safe _ StandardSee standards(5).

See Also
     Intro(2),  getmsg(2),  getrlimit(2),   putmsg(2),   read(2),
     write(2),     select(3C),    attributes(5),    standards(5),
     chpoll(9E)


     STREAMS Programming Guide

Notes
     Non-STREAMS drivers use  chpoll(9E) to implement  poll()  on
     these devices.
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