man-pages/man2/epoll_wait.2

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.\"  epoll by Davide Libenzi ( efficient event notification retrieval )
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.\"  Davide Libenzi <davidel@xmailserver.org>
.\"
.\" 2007-04-30: mtk, Added description of epoll_pwait()
.\"
.TH EPOLL_WAIT 2 2008-04-23 "Linux" "Linux Programmer's Manual"
.SH NAME
epoll_wait, epoll_pwait \- wait for an I/O event on an epoll file descriptor
.SH SYNOPSIS
.nf
.B #include <sys/epoll.h>
.sp
.BI "int epoll_wait(int " epfd ", struct epoll_event *" events ,
.BI "               int " maxevents ", int " timeout );
.BI "int epoll_pwait(int " epfd ", struct epoll_event *" events ,
.BI "               int " maxevents ", int " timeout ,
.BI "               const sigset_t *" sigmask );
.fi
.SH DESCRIPTION
The
.BR epoll_wait ()
system call waits for events on the
.B epoll
file descriptor
.I epfd
for a maximum time of
.I timeout
milliseconds.
The memory area pointed to by
.I events
will contain the events that will be available for the caller.
Up to
.I maxevents
are returned by
.BR epoll_wait ().
The
.I maxevents
parameter must be greater than zero.
Specifying a
.I timeout
of \-1 makes
.BR epoll_wait ()
wait indefinitely, while specifying a
.I timeout
equal to zero makes
.BR epoll_wait ()
to return immediately even if no events are available
(return code equal to zero).
The
.I struct epoll_event
is defined as :
.sp
.in +4n
.nf
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 */
};
.fi
.in

The
.I data
of each returned structure will contain the same data the user set with an
.BR epoll_ctl (2)
.RB ( EPOLL_CTL_ADD , EPOLL_CTL_MOD )
while the
.I events
member will contain the returned event bit field.
.SS epoll_pwait()
The relationship between
.BR epoll_wait ()
and
.BR epoll_pwait ()
is analogous to the relationship between
.BR select (2)
and
.BR pselect (2):
like
.BR pselect (2),
.BR epoll_pwait ()
allows an application to safely wait until either a file descriptor
becomes ready or until a signal is caught.

The following
.BR epoll_pwait ()
call:
.nf

    ready = epoll_pwait(epfd, &events, maxevents, timeout, &sigmask);

.fi
is equivalent to
.I atomically
executing the following calls:
.nf

    sigset_t origmask;

    sigprocmask(SIG_SETMASK, &sigmask, &origmask);
    ready = epoll_wait(epfd, &events, maxevents, timeout);
    sigprocmask(SIG_SETMASK, &origmask, NULL);
.fi
.PP
The
.I sigmask
argument may be specified as NULL, in which case
.BR epoll_pwait ()
is equivalent to
.BR epoll_wait ().
.SH "RETURN VALUE"
When successful,
.BR 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
.I timeout
milliseconds.
When an error occurs,
.BR epoll_wait ()
returns \-1 and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EBADF
.I epfd
is not a valid file descriptor.
.TP
.B EFAULT
The memory area pointed to by
.I events
is not accessible with write permissions.
.TP
.B EINTR
The call was interrupted by a signal handler before any of the
requested events occurred or the
.I timeout
expired; see
.BR signal (7).
.TP
.B EINVAL
.I epfd
is not an
.B epoll
file descriptor, or
.I maxevents
is less than or equal to zero.
.SH VERSIONS
.BR epoll_pwait ()
was added to Linux in kernel 2.6.19.

Glibc support for
.BR epoll_pwait ()
is provided starting with version 2.6.
.SH CONFORMING TO
.BR epoll_wait ()
is Linux-specific, and was introduced in kernel 2.5.44.
.\" The interface should be finalized by Linux kernel 2.5.66.
.SH "SEE ALSO"
.BR epoll_create (2),
.BR epoll_ctl (2),
.BR epoll (7)