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POLL(2)                    Linux Programmer's Manual                   POLL(2)



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
       #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 nfds  struc-
       tures 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 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 when-
       ever 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.  Speci-
       fying 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;  pseudo-terminal  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 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 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 the following form:

           struct timespec {
               long    tv_sec;         /* seconds */
               long    tv_nsec;        /* nanoseconds */
           };

       If timeout 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 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.

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 con-
       stant 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.

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 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-06-02                           POLL(2)