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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
       #include <poll.h>

       int ppoll(struct pollfd *fds, nfds_t nfds,
               const struct timespec *timeout, 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 nfds structures of the following form:

           struct pollfd {
               int   fd;         /* file descriptor */
               short events;     /* requested events */
               short revents;    /* returned events */

       The field fd contains a file descriptor for an open file.

       The field events is an input parameter, a bit mask specifying the events the appli-
       cation is interested in.

       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  an upper limit on the time for which poll() will
       block, in milliseconds.  Specifying a negative value in timeout means  an  infinite

       The bits that may be set/returned in events and revents are defined in <poll.h>:

              POLLIN There is data to read.

                     There  is  urgent data to read (e.g., out-of-band data on TCP socket;
                     pseudo-terminal master in  packet  mode  has  seen  state  change  in

                     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  in
                     order to obtain this definition.

                     Error condition (output only).

                     Hang up (output only).

                     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:

                     Equivalent to POLLIN.

                     Priority band data can be read (generally unused on Linux).

                     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  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

       Other than the difference in the timeout argument, the following ppoll() call:

           ready = ppoll(&fds, nfds, timeout, &sigmask);

       is equivalent to atomically executing the following calls:

           sigset_t origmask;

           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  time-
       out argument).

       The  timeout  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 is specified as NULL, then ppoll() can block indefinitely.

       On success, a positive number is returned; this is the number of  structures  which
       have  non-zero  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.

       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.  The poll() library call was
       introduced in libc 5.4.28 (and provides emulation using select(2)  if  your  kernel
       does not have a poll() system call).

       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.  ppoll() is Linux-specific.

       Some implementations define the non-standard constant INFTIM with the value -1  for
       use as a timeout.  This constant is not provided in glibc.

   Linux Notes
       The  Linux  ppoll()  system call modifies its timeout 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 argument.

       See the discussion of spurious readiness notifications under the  BUGS  section  of

       select(2), select_tut(2), feature_test_macros(7), time(7)

       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the project, and information about reporting bugs, can be found at  http://www.ker-

Linux                             2009-06-02                           POLL(2)

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