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



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 <poll.h>

       int ppoll(struct pollfd *fds, nfds_t nfds,
               const struct timespec *timeout_ts, 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  pro-
       vides  an  easy  way of ignoring a file descriptor for a single poll() call: simply negate
       the fd field.)

       The field events is an input parameter, a bit mask specifying the events  the  application
       is interested in for the file descriptor fd.  If this field is specified as zero, then all
       events are ignored for fd and revents returns zero.

       The field revents is an output parameter, filled by the kernel with the events that  actu-
       ally 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 condi-
       tion is true.)

       If none of the events requested (and no error) has occurred for any of the  file  descrip-
       tors, then poll() blocks until one of the events occurs.

       The  timeout argument specifies the minimum number of milliseconds that poll() will block.
       (This 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 nega-
       tive 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  urgent  data to read (e.g., out-of-band data on TCP socket; pseu-
                     doterminal master in packet mode has seen state change in slave).

              POLLOUT
                     Writing now will not block.

              POLLRDHUP (since Linux 2.6.17)
                     Stream socket peer closed connection, or shut down writing half  of  connec-
                     tion.   The _GNU_SOURCE feature test macro must be defined (before including
                     any header files) in order to obtain this definition.

              POLLERR
                     Error condition (output only).

              POLLHUP
                     Hang up (output only).

              POLLNVAL
                     Invalid request: fd not open (output only).

       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, timeout_ts, &sigmask);

       is equivalent to atomically executing the following calls:

           sigset_t origmask;
           int timeout;

           timeout = (timeout_ts == NULL) ? -1 :
                     (timeout_ts.tv_sec * 1000 + timeout_ts.tv_nsec / 1000000);
           sigprocmask(SIG_SETMASK, &sigmask, &origmask);
           ready = poll(&fds, nfds, timeout);
           sigprocmask(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  per-
       formed  (and  thus  ppoll() differs from poll() only in the precision of the timeout argu-
       ment).

       The timeout_ts 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_ts 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.  ppoll() is Linux-specific.

NOTES
       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).

   Linux notes
       The Linux ppoll() system call modifies its timeout_ts argument.  However, the glibc  wrap-
       per  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  time-
       out_ts argument.

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), time(7)

COLOPHON
       This page is part of release 3.53 of the Linux man-pages project.  A  description  of  the
       project,     and    information    about    reporting    bugs,    can    be    found    at
       http://www.kernel.org/doc/man-pages/.



Linux                                       2013-07-30                                    POLL(2)

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