poll

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



NAME
       poll - wait for some event on a file descriptor

SYNOPSIS
       #include <sys/poll.h>

       int poll(struct pollfd *ufds, unsigned int nfds, int timeout);

DESCRIPTION
       poll  is a variation on the theme of select.  It specifies an array of
       nfds structures of type
	       struct pollfd {
		       int fd;		 /* file descriptor */
		       short events;	 /* requested events */
		       short revents;	 /* returned events */
	       };
       and a timeout in milliseconds. A negative value means  infinite	time-
       out.   The  field fd contains a file descriptor for an open file.  The
       field events is an input parameter, a bitmask  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, either of the type requested, or of one of the types POLLERR
       or POLLHUP or POLLNVAL.	(These three  bits  are	 meaningless  in  the
       events field, and will be set in the revents field whenever the corre-
       sponding condition is true.)  If none of the events requested (and  no
       error)  has occurred for any of the file descriptors, the kernel waits
       for timeout milliseconds for one of these events to occur.   The	 fol-
       lowing possible bits in these masks are defined in <sys/poll.h>
	   #define POLLIN      0x0001	 /* There is data to read */
	   #define POLLPRI     0x0002	 /* There is urgent data to read */
	   #define POLLOUT     0x0004	 /* Writing now will not block */
	   #define POLLERR     0x0008	 /* Error condition */
	   #define POLLHUP     0x0010	 /* Hung up */
	   #define POLLNVAL    0x0020	 /* Invalid request: fd not open */
       When compiling XPG4.2 source one also has
       #ifdef _XOPEN_SOURCE
	   #define POLLRDNORM  0x0040	 /* Normal data may be read */
	   #define POLLRDBAND  0x0080	 /* Priority data may be read */
	   #define POLLWRNORM  0x0100	 /* Writing now will not block */
	   #define POLLWRBAND  0x0200	 /* Priority data may be written */
       #endif
       Finally, Linux knows about
       #ifdef _GNU_SOURCE
	   #define POLLMSG     0x0400
       #endif

RETURN VALUE
       On  success,  a positive number is returned, where the number returned
       is the number of structures 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
       have  been selected. On error, -1 is returned, and errno is set appro-
       priately.

ERRORS
       EBADF  An invalid file descriptor was given in one of the sets.

       ENOMEM There was no space to allocate file descriptor tables.

       EFAULT The array given as argument was not contained  in	 the  calling
	      program’s address space.

       EINTR  A signal occurred before any requested event.

       EINVAL The nfds value exceeds the RLIMIT_NOFILE value.

CONFORMING TO
       XPG4-UNIX.

AVAILABILITY
       The  poll()  systemcall	was  introduced	 in Linux 2.1.23.  The poll()
       library call was introduced in libc  5.4.28  (and  provides  emulation
       using select if your kernel does not have a poll syscall).

SEE ALSO
       select(2), select_tut(2)



Linux 2.1.23			  1997-12-07			      POLL(2)