poll - phpMan

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 struc-
       tures 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 */
       #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 appropriately.

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)

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