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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 some exceptional condition on the file descriptor.  Possibilities include:

              *  There is out-of-band data on a TCP socket (see tcp(7)).

              *  A pseudoterminal master in packet mode has seen a state change on the slave (see
                 ioctl_tty(2)).

              *  A cgroup.events file has been modified (see cgroups(7)).

       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).  This bit is also
              set for a file descriptor referring to the write end of a pipe when  the  read  end
              has been closed.

       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(kernel_sigset_t)).  See sigprocmask(2) for a
       discussion on the differences between the kernel and the libc notion of the sigset.

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

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       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at https://www.kernel.org/doc/man-pages/.