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

       poll - wait for some event on a file descriptor

       #include <sys/poll.h>

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

       poll  is  a variation on the theme of select.  It specifies an array of nfds structures of
	       struct pollfd {
		       int fd;		 /* file descriptor */
		       short events;	 /* requested events */
		       short revents;	 /* returned events */
       and a timeout in milliseconds. A negative value means infinite timeout.	The field fd con-
       tains  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 out-
       put  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
       corresponding 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 following possible bits in these masks are  defined  in
	   #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 */
       In <asm/poll.h> also the values POLLRDNORM, POLLRDBAND, POLLWRNORM, POLLWRBAND and POLLMSG
       are defined.

       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 appropriately.

       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

       EINTR  A signal occurred before any requested event.


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

       select(2), select_tut(2)

Linux 2.1.23				    1997-12-07					  POLL(2)
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