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NAME

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

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 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 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 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.

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 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)).

SEE ALSO

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

COLOPHON

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