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       poll - wait for some event on a file descriptor


       #include <sys/poll.h>

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


       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 timeout.
       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 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 <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 */

       Finally, Linux knows about:

           #ifdef _GNU_SOURCE
           #define POLLMSG     0x0400


       On  success,  a  positive  number  is  returned;  this 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 were ready.  On  error,
       -1 is returned, and errno is set appropriately.


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

       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.

       ENOMEM There was no space to allocate file descriptor tables.


       See the BUGS section of select(2).




       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 if your kernel does not have a poll() system call).


       Some implementations define the non-standard constant INFTIM  with  the
       value -1 for use as a timeout.  This constant is not provided in glibc.


       select(2), select_tut(2)