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
       #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.

       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 <poll.h>:

	      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 writing 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 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.  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 nonstandard constant INFTIM with the value -1 for use as a timeout for poll()..  This constant is not pro-
       vided in glibc.

   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

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

COLOPHON

       This page is part of release 3.25 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								    2010-06-12								   POLL(2)