epoll_pwait(2)



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

NAME
       epoll_wait,  epoll_pwait  -  wait for an I/O event on an epoll file de-
       scriptor

SYNOPSIS
       #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);

DESCRIPTION
       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 de-
       scriptors  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:

       o a file descriptor delivers an event;

       o the call is interrupted by a signal handler; or

       o 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 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
       available.

       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 descrip-
       tor.

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

   epoll_pwait()
       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().

RETURN VALUE
       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 appropriately.

ERRORS
       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.

VERSIONS
       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.

CONFORMING TO
       epoll_wait() is Linux-specific.

NOTES
       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  de-
       scriptors  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  de-
       scriptor becomes ready.

BUGS
       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 sigset-
       size, 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)).

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

COLOPHON
       This page is part of release 5.07 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                             2020-04-11                     EPOLL_WAIT(2)

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