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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 nearly 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);

       The above code segment is described as nearly equivalent because whereas a  negative  timeout  value  for
       poll()  is  interpreted  as an infinite timeout, a negative value expressed in *tmo_p results in an error
       from ppoll().

       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.

       EINVAL (ppoll()) The timeout value expressed in *ip is invalid (negative).

       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

       The operation of poll() and ppoll() is not affected by the O_NONBLOCK flag.

       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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       information   about   reporting   bugs,   and   the  latest  version  of  this  page,  can  be  found  at
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