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poll(2)

poll(2)                          System Calls                          poll(2)



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 multi‐
       plexing input/output over a set of file descriptors. 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 identifies
       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 examined 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 mem‐
       bers:



         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 with‐
                     out blocking.


       POLLWRNORM    The same as POLLOUT.


       POLLWRBAND    Priority data (priority band > 0) may  be  written.  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 bitmask; it is not used in
                     the events member.


       POLLHUP       A hangup has occurred on the stream. This event and POLL‐
                     OUT  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  speci‐
       fied  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 descriptors. On a computer where mil‐
       lisecond 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-termi‐
       nal  devices,  streams-based  files,  FIFOs  and pipes. The behavior of
       poll() on elements of fds that refer to other types of file is unspeci‐
       fied.


       The poll() function supports sockets.


       A  file  descriptor for a socket that is listening for connections will
       indicate that it is ready for reading, once connections 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 rela‐
       tionship 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 signal is caught.


       Other than the  difference  in  the  timeout  argument,  the  following
       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() function replaces
       the signal mask of the process by the set of signals pointed to by sig‐
       mask  before examining the descriptors, 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 indefinitely.

RETURN VALUES
       Upon successful completion, a non-negative value is returned.  A  posi‐
       tive 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 multiplexer that is linked
                 (directly or indirectly) downstream from a multiplexer.


       ENOSYS    There is no poll() interface for doorfs.


ATTRIBUTES
       See attributes(7) for descriptions of the following attributes:


       tab() box; cw(2.75i) |cw(2.75i) lw(2.75i) |lw(2.75i)


       ATTRIBUTE TYPEATTRIBUTE VALUE _ Interface StabilityCommitted _ MT-Leve‐
       lAsync-Signal-Safe _ StandardSee standards(7).


SEE ALSO
       getmsg(2),   getrlimit(2),   Intro(2),  putmsg(2),  read(2),  write(2),
       select(3C), attributes(7), standards(7), chpoll(9E)


       STREAMS Programming Guide

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



Oracle Solaris 11.4               27 Nov 2017                          poll(2)
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