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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	   /* 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);

       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.  If this
       field is negative, then the corresponding events field is  ignored  and
       the revents field returns zero.	(This provides an easy way of ignoring
       a file descriptor for a	single	poll()	call:  simply  negate  the  fd

       The  field  events  is  an  input  parameter, a bit mask specifying the
       events the application is interested in for  the	 file  descriptor  fd.
       This field may be specified as zero, in which case the only events that
       can be returned in revents are  POLLHUP,	 POLLERR,  and	POLLNVAL  (see

       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

       The timeout argument specifies the number of milliseconds  that	poll()
       should  block  waiting for a file descriptor to become ready.  The call
       will block until either:

       *  a file descriptor becomes ready;

       *  the call is interrupted by a signal handler; or

       *  the timeout expires.

       Note that the timeout interval will be rounded up to the	 system	 clock
       granularity, and kernel scheduling delays mean that the blocking inter‐
       val may overrun by a small amount.   Specifying	a  negative  value  in
       timeout means an infinite timeout.  Specifying a timeout of zero causes
       poll() to return immediately, even if no file descriptors are ready.

       The bits that may be set/returned in events and revents are defined  in

	      POLLIN There is data to read.

		     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).

		     Writing now will not block.

	      POLLRDHUP (since Linux 2.6.17)
		     Stream  socket peer closed connection, or shut down writ‐
		     ing half of connection.   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

		     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	 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 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

       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 indefi‐

       On success, a positive number is returned; this is the number of struc‐
       tures which have nonzero revents fields (in other words, those descrip‐
       tors 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 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.

       The  poll()  system call was introduced in Linux 2.1.23.	 On older ker‐
       nels that lack this system call, the glibc (and	the  old  Linux	 libc)
       poll() wrapper function provides emulation using select(2).

       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 pro‐
       vided in glibc.

       For a discussion of what may happen if a file  descriptor  being	 moni‐
       tored by poll() is closed in another thread, see select(2).

   Linux notes
       The  Linux  ppoll() system call modifies its timeout_ts argument.  How‐
       ever, 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	 argu‐

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

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

       This page is part of release 3.65 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				  2014-01-31			       POLL(2)

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