epoll_pwait(2) — Linux manual page


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

NAME         top

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

SYNOPSIS         top

       #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         top

       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 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 de‐

       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().

RETURN VALUE         top

       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         top

       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         top

       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         top

       epoll_wait() is Linux-specific.

NOTES         top

       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         top

       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         top

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

COLOPHON         top

       This page is part of release 5.09 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

Linux                            2020-04-11                    EPOLL_WAIT(2)

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