poll(2)



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 <signal.h>
       #include <poll.h>

       int ppoll(struct pollfd *fds, nfds_t nfds,
               const struct timespec *tmo_p, 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 ar-
       gument, 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.
       Note,  however,  that  this  technique can't be used to ignore file de-
       scriptor 0.)

       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 be-
       low).

       The field revents is an output parameter, filled by the kernel with the
       events  that  actually  occurred.  The bits returned in revents can in-
       clude 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  oc-
       curs.

       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 inter-
       val  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 some exceptional  condition  on  the  file  descriptor.
              Possibilities include:

              *  There is out-of-band data on a TCP socket (see tcp(7)).

              *  A  pseudoterminal  master  in  packet  mode  has seen a state
                 change on the slave (see ioctl_tty(2)).

              *  A cgroup.events file has been modified (see cgroups(7)).

       POLLOUT
              Writing is now possible, though a write larger that  the  avail-
              able  space  in a socket or pipe will still block (unless O_NON-
              BLOCK is set).

       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 de-
              fined (before including any header files)  in  order  to  obtain
              this definition.

       POLLERR
              Error  condition  (only returned in revents; ignored in events).
              This bit is also set for a  file  descriptor  referring  to  the
              write end of a pipe when the read end has been closed.

       POLLHUP
              Hang  up  (only  returned  in revents; ignored in events).  Note
              that when reading from a channel such as  a  pipe  or  a  stream
              socket, this event merely indicates that the peer closed its end
              of the channel.  Subsequent reads from the channel will return 0
              (end of file) only after all outstanding data in the channel has
              been consumed.

       POLLNVAL
              Invalid request: fd not open (only returned in revents;  ignored
              in events).

       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() al-
       lows an application to safely wait until either a file  descriptor  be-
       comes 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, tmo_p, &sigmask);

       is equivalent to atomically executing the following calls:

           sigset_t origmask;
           int timeout;

           timeout = (tmo_p == NULL) ? -1 :
                     (tmo_p->tv_sec * 1000 + tmo_p->tv_nsec / 1000000);
           pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
           ready = poll(&fds, nfds, timeout);
           pthread_sigmask(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 ma-
       nipulation is performed (and thus ppoll() differs from poll()  only  in
       the precision of the timeout argument).

       The  tmo_p 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 tmo_p is specified as NULL, then ppoll() can block indefinitely.

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 re-
       turned, 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 and POSIX.1-2008.  ppoll() is Linux-
       specific.

NOTES
       On some other UNIX systems, poll() can fail with the  error  EAGAIN  if
       the  system  fails  to  allocate kernel-internal resources, rather than
       ENOMEM as Linux does.  POSIX permits this behavior.  Portable  programs
       may wish to check for EAGAIN and loop, just as with EINTR.

       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).

   C library/kernel differences
       The Linux ppoll() system call modifies its  tmo_p  argument.   However,
       the  glibc  wrapper function hides this behavior by using a local vari-
       able for the timeout argument that is passed to the system call.  Thus,
       the glibc ppoll() function does not modify its tmo_p argument.

       The  raw  ppoll()  system call has a fifth argument, size_t sigsetsize,
       which specifies the size in bytes of the sigmask argument.   The  glibc
       ppoll()  wrapper  function  specifies  this  argument  as a fixed value
       (equal to sizeof(kernel_sigset_t)).  See sigprocmask(2) for  a  discus-
       sion  on  the differences between the kernel and the libc notion of the
       sigset.

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), epoll(7), time(7)

COLOPHON
       This  page  is  part of release 4.16 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-09-15                           POLL(2)

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