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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 buffer pointed to by events is used to return information  from
       the  ready  list  about  file  descriptors  in  the  interest  list  that have some events
       available.  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.

       A call to epoll_wait() 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 epoll_event  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 descriptor.

       The events field is a bit mask that indicates  the  events  that  have  occurred  for  the
       corresponding  open  file  description.   See epoll_ctl(2) for a list of the bits that may
       appear in this mask.

       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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