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       epoll_wait, epoll_pwait - wait for an I/O event on an epoll file descriptor


       #include <sys/epoll.h>

       int epoll_wait(int epfd, struct epoll_event *events,
                      int maxevents, int timeout);
       int epoll_pwait(int epfd, struct epoll_event *events,
                      int maxevents, int timeout,
                      const sigset_t *sigmask);


       The  epoll_wait() system call waits for events on the epoll(7) instance referred to by the
       file descriptor epfd.  The memory area pointed to by events will contain the  events  that
       will  be  available  for  the  caller.  Up to maxevents are returned by epoll_wait().  The
       maxevents argument must be greater than zero.

       The timeout argument specifies the number of milliseconds that  epoll_wait()  will  block.
       Time is measured against the CLOCK_MONOTONIC clock.  The call will block until either:

       *  a file descriptor delivers an event;

       *  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  timeout of -1 causes epoll_wait() to block indefinitely, while specifying a
       timeout equal to zero cause epoll_wait() to return immediately,  even  if  no  events  are

       The struct epoll_event is defined as:

           typedef union epoll_data {
               void    *ptr;
               int      fd;
               uint32_t u32;
               uint64_t u64;
           } epoll_data_t;

           struct epoll_event {
               uint32_t     events;    /* Epoll events */
               epoll_data_t data;      /* User data variable */

       The  data  field of each returned structure contains the same data as was specified in the
       most recent call to epoll_ctl(2) (EPOLL_CTL_ADD, EPOLL_CTL_MOD) for the corresponding open
       file description.  The events field contains the returned event bit field.

       The  relationship  between epoll_wait() and epoll_pwait() is analogous to the relationship
       between select(2) and pselect(2): like pselect(2), epoll_pwait() allows an application  to
       safely wait until either a file descriptor becomes ready or until a signal is caught.

       The following epoll_pwait() call:

           ready = epoll_pwait(epfd, &events, maxevents, timeout, &sigmask);

       is equivalent to atomically executing the following calls:

           sigset_t origmask;

           pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
           ready = epoll_wait(epfd, &events, maxevents, timeout);
           pthread_sigmask(SIG_SETMASK, &origmask, NULL);

       The  sigmask  argument may be specified as NULL, in which case epoll_pwait() is equivalent
       to epoll_wait().


       When successful, epoll_wait() returns  the  number  of  file  descriptors  ready  for  the
       requested  I/O,  or  zero  if no file descriptor became ready during the requested timeout
       milliseconds.   When  an  error  occurs,  epoll_wait()  returns  -1  and  errno   is   set


       EBADF  epfd is not a valid file descriptor.

       EFAULT The memory area pointed to by events is not accessible with write permissions.

       EINTR  The call was interrupted by a signal handler before either (1) any of the requested
              events occurred or (2) the timeout expired; see signal(7).

       EINVAL epfd is not an epoll file descriptor, or maxevents is less than or equal to zero.


       epoll_wait() was added to the kernel in version 2.6.  Library support is provided in glibc
       starting with version 2.3.2.

       epoll_pwait()  was  added to Linux in kernel 2.6.19.  Library support is provided in glibc
       starting with version 2.6.


       epoll_wait() is Linux-specific.


       While one thread is blocked in a call to epoll_wait(), it is possible for  another  thread
       to  add  a  file descriptor to the waited-upon epoll instance.  If the new file descriptor
       becomes ready, it will cause the epoll_wait() call to unblock.

       If more than maxevents file descriptors  are  ready  when  epoll_wait()  is  called,  then
       successive  epoll_wait() calls will round robin through the set of ready file descriptors.
       This behavior helps avoid starvation scenarios, where  a  process  fails  to  notice  that
       additional file descriptors are ready because it focuses on a set of file descriptors that
       are already known to be ready.

       Note that it is possible to call epoll_wait() on an epoll instance whose interest list  is
       currently  empty (or whose interest list becomes empty because file descriptors are closed
       or removed from the interest in another thread).  The call  will  block  until  some  file
       descriptor  is  later  added  to  the  interest  list  (in  another  thread) and that file
       descriptor becomes ready.


       In kernels before 2.6.37,  a  timeout  value  larger  than  approximately  LONG_MAX  /  HZ
       milliseconds  is  treated  as  -1  (i.e., infinity).  Thus, for example, on a system where
       sizeof(long) is 4 and the kernel HZ value is 1000, this means that timeouts  greater  than
       35.79 minutes are treated as infinity.

   C library/kernel differences
       The raw epoll_pwait() system call has a sixth argument, size_t sigsetsize, which specifies
       the size in bytes of the sigmask  argument.   The  glibc  epoll_pwait()  wrapper  function
       specifies this argument as a fixed value (equal to sizeof(sigset_t)).


       epoll_create(2), epoll_ctl(2), epoll(7)


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