#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);

       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.

       The  field  events  is  an  input  parameter, a bit mask 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

                     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(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

       If the sigmask argument is specified  as  NULL,  then  no  signal  mask
       manipulation is performed (and thus ppoll() differs from poll() only in

       On success, a positive number is returned; this is the number of struc-
       tures which have nonzero revents fields (in other words, those descrip-
       tors  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 nonstandard constant INFTIM with the
       value -1 for use as a timeout for poll().  This constant  is  not  pro-
       vided in glibc.

   Linux Notes
       The  Linux  ppoll() system call modifies its timeout_ts argument.  How-
       ever, 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  argu-

       See  the  discussion of spurious readiness notifications under the BUGS
       section of select(2).

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

       This page is part of release 3.35 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://man7.org/linux/man-pages/.
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