epoll_wait

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 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 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 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.

   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 5.05 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                             2019-03-06                     EPOLL_WAIT(2)