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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.
       This field may be specified as zero, in which case the only events that
       can be returned in revents are  POLLHUP,  POLLERR,  and  POLLNVAL  (see
       below).

       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 number of milliseconds  that  poll()
       should  block  waiting for a file descriptor to become ready.  The call
       will block until either:

       *  a file descriptor becomes ready;

       *  the call is interrupted by a signal handler; or

       *  the timeout expires.

       Note that the timeout 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
                     writing 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
       relationship 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
       indefinitely.

RETURN VALUE

       On success, a positive number  is  returned;  this  is  the  number  of
       structures  which  have  nonzero  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.

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
       kernels 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
       provided in glibc.

       For a discussion  of  what  may  happen  if  a  file  descriptor  being
       monitored by poll() is closed in another thread, see select(2).

   Linux notes
       The  Linux  ppoll()  system  call  modifies  its  timeout_ts  argument.
       However, 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
       argument.

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 3.65 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/.



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