huschi.net

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

NAME
       poll, ppoll - wait for some event on a file descriptor
SYNOPSIS
       #include <poll.h>
       int poll(struct pollfd *fds, nfds_t nfds, int timeout);
       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <poll.h>
       int ppoll(struct pollfd *fds, nfds_t nfds,
               const struct timespec *timeout_ts, const sigset_t *sigmask);
DESCRIPTION
       poll()  performs a similar task to select(2): it waits for one of a set
       of file descriptors to become ready to perform I/O.
       The set of file descriptors to be monitored is  specified  in  the  fds
       argument, which is an array of structures of the following form:
           struct pollfd {
               int   fd;         /* file descriptor */
               short events;     /* requested events */
               short revents;    /* returned events */
           };
       The caller should specify the number of items in the fds array in nfds.
       The  field  fd  contains  a  file descriptor for an open file.  If this
       field is negative, then the corresponding events field is  ignored  and
       the revents field returns zero.  (This provides an easy way of ignoring
       a file descriptor for a  single  poll()  call:  simply  negate  the  fd
       field.)
       The  field  events  is  an  input  parameter, a bit mask specifying the
       events the application is interested in for the file descriptor fd.  If
       this field is specified as zero, then all events are ignored for fd and
       revents returns zero.
       The field revents is an output parameter, filled by the kernel with the
       events  that  actually  occurred.   The  bits  returned  in revents can
       include any of those specified in events, or one of the values POLLERR,
       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, then  poll()  blocks  until  one  of  the  events
       occurs.
       The  timeout argument specifies the minimum number of milliseconds that
       poll() will block.  (This interval will be rounded  up  to  the  system
       clock  granularity, and kernel scheduling delays mean that the blocking
       interval may overrun by a small amount.)  Specifying a  negative  value
       in  timeout  means  an  infinite timeout.  Specifying a timeout of zero
       causes poll() to return immediately, even if no  file  descriptors  are
       ready.
       The  bits that may be set/returned in events and revents are defined in
       <poll.h>:
              POLLIN There is data to read.
              POLLPRI
                     There is urgent data to read (e.g., out-of-band  data  on
                     TCP socket; pseudoterminal master in packet mode has seen
                     state change in slave).
              POLLOUT
                     Writing now will not block.
              POLLRDHUP (since Linux 2.6.17)
                     Stream socket peer closed connection, or shut down  writ-
                     ing  half  of  connection.   The _GNU_SOURCE feature test
                     macro must be defined (before including any header files)
                     in order to obtain this definition.
              POLLERR
                     Error condition (output only).
              POLLHUP
                     Hang up (output only).
              POLLNVAL
                     Invalid request: fd not open (output only).
       When  compiling with _XOPEN_SOURCE defined, one also has the following,
       which convey no further information beyond the bits listed above:
              POLLRDNORM
                     Equivalent to POLLIN.
              POLLRDBAND
                     Priority band data  can  be  read  (generally  unused  on
                     Linux).
              POLLWRNORM
                     Equivalent to POLLOUT.
              POLLWRBAND
                     Priority data may be written.
       Linux also knows about, but does not use POLLMSG.
   ppoll()
       The  relationship  between poll() and ppoll() is analogous to the rela-
       tionship between select(2) and  pselect(2):  like  pselect(2),  ppoll()
       allows  an  application  to  safely wait until either a file descriptor
       becomes ready or until a signal is caught.
       Other than the difference in the precision of the timeout argument, the
       following ppoll() call:
           ready = ppoll(&fds, nfds, timeout_ts, &sigmask);
       is equivalent to atomically executing the following calls:
           sigset_t origmask;
           int timeout;
           timeout = (timeout_ts == NULL) ? -1 :
                     (timeout_ts.tv_sec * 1000 + timeout_ts.tv_nsec / 1000000);
           sigprocmask(SIG_SETMASK, &sigmask, &origmask);
           ready = poll(&fds, nfds, timeout);
           sigprocmask(SIG_SETMASK, &origmask, NULL);
       See  the description of pselect(2) for an explanation of why ppoll() is
       necessary.
       If the sigmask argument is specified  as  NULL,  then  no  signal  mask
       manipulation is performed (and thus ppoll() differs from poll() only in
       the precision of the timeout argument).
       The timeout_ts argument specifies an upper limit on the amount of  time
       that  ppoll() will block.  This argument is a pointer to a structure of
       the following form:
           struct timespec {
               long    tv_sec;         /* seconds */
               long    tv_nsec;        /* nanoseconds */
           };
       If timeout_ts is specified as NULL,  then  ppoll()  can  block  indefi-
       nitely.
RETURN VALUE
       On success, a positive number is returned; this is the number of struc-
       tures which have nonzero revents fields (in other words, those descrip-
       tors  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.
ERRORS
       EFAULT The  array  given  as  argument was not contained in the calling
              program's address space.
       EINTR  A signal occurred before any requested event; see signal(7).
       EINVAL The nfds value exceeds the RLIMIT_NOFILE value.
       ENOMEM There was no space to allocate file descriptor tables.
VERSIONS
       The poll() system call was introduced in Linux 2.1.23.  On  older  ker-
       nels  that  lack  this  system call, the glibc (and the old Linux libc)
       poll() wrapper function provides emulation using select(2).
       The ppoll() system call was added  to  Linux  in  kernel  2.6.16.   The
       ppoll() library call was added in glibc 2.4.
CONFORMING TO
       poll() conforms to POSIX.1-2001.  ppoll() is Linux-specific.
NOTES
       Some  implementations  define  the nonstandard constant INFTIM with the
       value -1 for use as a timeout for poll().  This constant  is  not  pro-
       vided in glibc.
       For  a  discussion  of what may happen if a file descriptor being moni-
       tored by poll() is closed in another thread, see select(2).
   Linux notes
       The Linux ppoll() system call modifies its timeout_ts  argument.   How-
       ever,  the  glibc wrapper function hides this behavior by using a local
       variable for the timeout argument that is passed to  the  system  call.
       Thus,  the  glibc ppoll() function does not modify its timeout_ts argu-
       ment.
BUGS
       See the discussion of spurious readiness notifications under  the  BUGS
       section of select(2).
SEE ALSO
       restart_syscall(2), select(2), select_tut(2), time(7)
COLOPHON
       This  page  is  part of release 3.53 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux                             2013-07-30                           POLL(2)