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 <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 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.)

       The  field  events is an input parameter, a bit mask specifying the events the application is interested in for the file descriptor fd.	If
       this field is specified as zero, then all events are ignored for fd and revents returns zero.

       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 minimum number of milliseconds that poll() will block.  (This interval will be rounded up to the system
       clock granularity, and kernel scheduling delays mean that the blocking interval 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 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 (before including any header files) 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  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 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.  On older kernels 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.  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  pro-
       vided in glibc.

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

   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 argu-
       ment.

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), time(7)

COLOPHON

       This  page is part of release 3.53 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								    2013-07-30								   POLL(2)