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
       provides 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
       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.  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 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 urgent  data  to  read  (e.g.,  out-of-band  data  on  TCP  socket;
                     pseudoterminal 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
                     connection.  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
       performed (and thus ppoll() differs from poll() only  in  the  precision  of  the  timeout
       argument).

       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
       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
       timeout_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 3.54 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/.