Manpage of EPOLL_WAIT

EPOLL_WAIT

Section: Linux Programmer's Manual (2)
Updated: 2015-05-07
Index
 

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 eventswill contain the events that will be available for the caller. Up to maxeventsare returned by epoll_wait(). The maxeventsargument must be greater than zero.

The timeoutargument specifies the number of milliseconds that epoll_wait() will block. 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 timeoutinterval 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 timeoutof -1 causes epoll_wait() to block indefinitely, while specifying a timeoutequal to zero cause epoll_wait() to return immediately, even if no events are available.

The struct epoll_eventis 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 dataof each returned structure will contain the same data the user set with an epoll_ctl(2) (EPOLL_CTL_ADD, EPOLL_CTL_MOD) while the eventsmember will contain 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 atomicallyexecuting 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 sigmaskargument 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 timeoutmilliseconds. When an error occurs, epoll_wait() returns -1 and errnois set appropriately.  

ERRORS

EBADF
epfdis not a valid file descriptor.
EFAULT
The memory area pointed to by eventsis 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 timeoutexpired; see signal(7).
EINVAL
epfdis not an epollfile descriptor, or maxeventsis 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_pwait(), it is possible for another thread to add a file descriptor to the waited-upon epollinstance. If the new file descriptor becomes ready, it will cause the epoll_wait() call to unblock.

For a discussion of what may happen if a file descriptor in an epollinstance being monitored by epoll_wait() is closed in another thread, see select(2).  

BUGS

In kernels before 2.6.37, a timeoutvalue larger than approximately LONG_MAX / HZmilliseconds is treated as -1 (i.e., infinity). Thus, for example, on a system where the sizeof(long)is 4 and the kernel HZvalue 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 sigmaskargument. 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)


 

Index

NAME
SYNOPSIS
DESCRIPTION
epoll_pwait()
RETURN VALUE
ERRORS
VERSIONS
CONFORMING TO
NOTES
BUGS
C library/kernel differences
SEE ALSO

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Time: 22:27:44 GMT, June 20, 2016