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_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 fol-
       lowing 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; 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 pse-
       lect(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 pro-
       vides 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), feature_test_macros(7), time(7)

COLOPHON
       This  page  is  part of release 3.25 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                                                      2010-06-12                                                    POLL(2)