POLL(2)                    Linux Programmer's Manual                   POLL(2)

       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 <signal.h>
       #include <poll.h>

       int ppoll(struct pollfd *fds, nfds_t nfds,
               const struct timespec *tmo_p, 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.
       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

       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

       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 inter-
       val  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

              POLLIN 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 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  writ-
                     ing  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  (only  returned  in revents; ignored in

                     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  out-
                     standing data in the channel has been consumed.

                     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:

                     Equivalent to POLLIN.

                     Priority  band  data  can  be  read  (generally unused on

                     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  rela-
       tionship  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

       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.

       On success, a positive number is returned; this is the number of struc-
       tures which have nonzero revents fields (in other words, those descrip-
       tors  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.

       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.

       The poll() system call was introduced in Linux 2.1.23.  On  older  ker-
       nels  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 and POSIX.1-2008.  ppoll() is Linux-

       Some implementations define the nonstandard constant  INFTIM  with  the
       value  -1  for  use as a timeout for poll().  This constant is not pro-
       vided in glibc.

       For a discussion of what may happen if a file  descriptor  being  moni-
       tored 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  vari-
       able 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)).

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

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

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Linux                             2015-12-28                           POLL(2)
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