EPOLL_WAIT

NAME

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);
int epoll_pwait2(int epfd, struct epoll_event *events,
               int maxevents, const struct timespec *timeout,
               const sigset_t *sigmask);

DESCRIPTION

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 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.  

epoll_pwait()

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().  

epoll_pwait2()

RETURN VALUE

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_pwait() was added to Linux in kernel 2.6.19. Library support is provided in glibc starting with version 2.6.

epoll_pwait2() was added to Linux in kernel 5.11.  

CONFORMING TO

NOTES

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

C library/kernel differences

SEE ALSO

COLOPHON

Index

NAME

SYNOPSIS

DESCRIPTION

epoll_pwait()

epoll_pwait2()

RETURN VALUE

ERRORS

VERSIONS

CONFORMING TO

NOTES

BUGS

C library/kernel differences

SEE ALSO

COLOPHON