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



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

SYNOPSIS
       #include 

       int poll(struct pollfd *fds, nfds_t nfds, int timeout);

       #define _GNU_SOURCE
       #include 

       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 structures 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 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
       :

	      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	 writ-
		     ing  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 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
       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 struc-
       tures  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 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 3.22 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				  2009-06-02			       POLL(2)