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	   /* See feature_test_macros(7) */
       #include <signal.h>
       #include <poll.h>

       int ppoll(struct pollfd *fds, nfds_t nfds,
	       const struct timespec *tmo_p, 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 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 field.
       Note,  however,	that  this  technique  can't  be  used	to ignore file
       descriptor 0.)

       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
       below).

       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 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
       <poll.h>:

	      POLLIN There is data to read.

	      POLLPRI
		     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).

	      POLLOUT
		     Writing is now possible, though a write larger  that  the
		     available	space  in  a  socket  or pipe will still block
		     (unless O_NONBLOCK is set).

	      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.

	      POLLERR
		     Error  condition  (only  returned	in revents; ignored in
		     events).

	      POLLHUP
		     Hang up (only returned in revents;	 ignored  in  events).
		     Note that when reading from a channel such as a pipe or a
		     stream socket, this event merely indicates that the  peer
		     closed its end of the channel.  Subsequent reads from the
		     channel will return 0 (end of file) only after  all  out-
		     standing data in the channel has been consumed.

	      POLLNVAL
		     Invalid  request:	fd not open (only returned in revents;
		     ignored in events).

       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 precision of the timeout argument, the
       following ppoll() call:

	   ready = ppoll(&fds, nfds, tmo_p, &sigmask);

       is equivalent to atomically executing the following calls:

	   sigset_t origmask;
	   int timeout;

	   timeout = (tmo_p == NULL) ? -1 :
		     (tmo_p->tv_sec * 1000 + tmo_p->tv_nsec / 1000000);
	   pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
	   ready = poll(&fds, nfds, timeout);
	   pthread_sigmask(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  tmo_p 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 tmo_p 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 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.

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.  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.

CONFORMING TO
       poll()  conforms	 to  POSIX.1-2001 and POSIX.1-2008.  ppoll() is Linux-
       specific.

NOTES
       On some other UNIX systems, poll() can fail with the  error  EAGAIN  if
       the  system  fails  to  allocate kernel-internal resources, rather than
       ENOMEM as Linux does.  POSIX permits this behavior.  Portable  programs
       may wish to check for EAGAIN and loop, just as with EINTR.

       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).

   C library/kernel differences
       The Linux ppoll() system call modifies its  tmo_p  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 tmo_p argument.

       The  raw	 ppoll()  system call has a fifth argument, size_t sigsetsize,
       which specifies the size in bytes of the sigmask argument.   The	 glibc
       ppoll()	wrapper	 function  specifies  this  argument  as a fixed value
       (equal to sizeof(kernel_sigset_t)).  See sigprocmask(2) for  a  discus-
       sion  on	 the differences between the kernel and the libc notion of the
       sigset.

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

SEE ALSO
       restart_syscall(2), select(2), select_tut(2), epoll(7), time(7)

COLOPHON
       This  page  is  part of release 4.10 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at
       https://www.kernel.org/doc/man-pages/.



Linux				  2017-03-13			       POLL(2)