poll - wait for some event on a file descriptor


       #include <sys/poll.h>

       int poll(struct pollfd *ufds, unsigned int nfds, int time­


       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  inter­
       ested  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
           #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  num­
       ber  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


       ENOMEM There was no  space  to  allocate  file  descriptor
              the calling program's address space.

       EINTR  A signal occurred before any requested event.




       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).