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 bitmask specifying the events
       the application 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

       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 timeout.

       The  bits that may be set/returned in events and revents are defined in

              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 slave).

                     Writing now will not block.

              POLLRDHUP (since Linux 2.6.17)
                     Stream socket peer closed connection, or shut down  writ-
                     ing  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

                     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 rela-
       tionship between select() and pselect(): like pselect(), 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  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 = ppoll(&fds, nfds, timeout);
           sigprocmask(SIG_SETMASK, &origmask, NULL);

       See  the description of pselect(2) for an explanation of why ppoll() is

       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 struc-
       tures  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.

       EBADF  An invalid file descriptor was given in one of the sets.

       EFAULT The array given as argument was not  contained  in  the  calling
              program's address space.

       EINTR  A signal occurred before any requested event.

       EINVAL The nfds value exceeds the RLIMIT_NOFILE value.

       ENOMEM There was no space to allocate file descriptor tables.

       The  Linux ppoll() system call modifies its timeout argument.  However,
       the glibc wrapper function hides this behaviour by using a local  vari-
       able 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).

       poll() conforms to POSIX.1-2001.  ppoll() is Linux specific.

       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() 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.

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

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

Linux 2.6.16                      2006-03-13                           POLL(2)