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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.

       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
       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 timeout.

       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.   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.

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.

   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

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

COLOPHON

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