poll


SYNOPSIS
       #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);

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
                     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  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  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 timeout 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
       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 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 vari-
       able for the timeout argument that is passed to the system call.  Thus,
       the glibc ppoll() function does not modify its timeout argument.

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

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

COLOPHON
       This page is part of release 3.23 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/.



Linux                             2009-09-15                           POLL(2)
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