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Linux 2.6 - man page for ppoll (linux section 2)

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

       poll, ppoll - wait for some event on a file descriptor

       #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);

       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  actu-
       ally 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 condi-
       tion is true.)

       If none of the events requested (and no error) has occurred for any of the  file  descrip-
       tors, 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.

		     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).

		     Writing now will not block.

	      POLLRDHUP (since Linux 2.6.17)
		     Stream  socket  peer closed connection, or shut down writing half of connec-
		     tion.  The _GNU_SOURCE feature test macro must be defined (before	including
		     any header files) in order to obtain this definition.

		     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 Linux).

		     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  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()

	   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 per-
       formed (and thus ppoll() differs from poll() only in the precision of  the  timeout  argu-

       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.

       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.

       EFAULT The  array  given  as  argument  was not contained in the calling program's address

       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 intro-
       duced  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 provided in glibc.

   Linux Notes
       The Linux ppoll() system call modifies its timeout_ts argument.	However, the glibc  wrap-
       per  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  time-
       out_ts argument.

       See  the  discussion  of  spurious  readiness  notifications  under  the  BUGS  section of

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

       This page is part of release 3.27 of the Linux man-pages project.  A  description  of  the
       project,   and	information  about  reporting  bugs,  can  be  found  at  http://www.ker-

Linux					    2010-09-20					  POLL(2)

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