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.\"  Copyright (C) 2003  Davide Libenzi
.\"  Davide Libenzi <davidel@xmailserver.org>
.\" and Copyright 2007, 2012, 2014, 2018 Michael Kerrisk <tk.manpages@gmail.com>
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.\" 2007-04-30: mtk, Added description of epoll_pwait()
.\"
.TH EPOLL_WAIT 2 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
epoll_wait, epoll_pwait, epoll_pwait2 \- wait for an I/O event on an epoll file descriptor
.SH SYNOPSIS
.nf
.B #include <sys/epoll.h>
.PP
.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 );
.BI "int epoll_pwait2(int " epfd ", struct epoll_event *" events ,
.BI "               int " maxevents ", const struct timespec *" timeout ,
.BI "               const sigset_t *" sigmask );
.\" FIXME: Check if glibc has added a wrapper for epoll_pwait2(),
.\"        and if so, check the prototype.
.\"        https://sourceware.org/bugzilla/show_bug.cgi?id=27359
.fi
.SH DESCRIPTION
The
.BR epoll_wait ()
system call waits for events on the
.BR epoll (7)
instance referred to by the file descriptor
.IR epfd .
The buffer pointed to by
.I events
is used to return information from the ready list
about file descriptors in the interest list that
have some events available.
Up to
.I maxevents
are returned by
.BR epoll_wait ().
The
.I maxevents
argument must be greater than zero.
.PP
The
.I timeout
argument specifies the number of milliseconds that
.BR epoll_wait ()
will block.
Time is measured against the
.B CLOCK_MONOTONIC
clock.
.PP
A call to
.BR epoll_wait ()
will block until either:
.IP \(bu 2
a file descriptor delivers an event;
.IP \(bu
the call is interrupted by a signal handler; or
.IP \(bu
the timeout expires.
.PP
Note that the
.I 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
.I timeout
of \-1 causes
.BR epoll_wait ()
to block indefinitely, while specifying a
.I timeout
equal to zero cause
.BR epoll_wait ()
to return immediately, even if no events are available.
.PP
The
.I struct epoll_event
is defined as:
.PP
.in +4n
.EX
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 */
};
.EE
.in
.PP
The
.I data
field of each returned
.I epoll_event
structure contains the same data as was specified
in the most recent call to
.BR epoll_ctl (2)
.RB ( EPOLL_CTL_ADD ", " EPOLL_CTL_MOD )
for the corresponding open file descriptor.
.PP
The
.I events
field is a bit mask that indicates the events that have occurred for the
corresponding open file description.
See
.BR epoll_ctl (2)
for a list of the bits that may appear in this mask.
.\"
.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.
.PP
The following
.BR epoll_pwait ()
call:
.PP
.in +4n
.EX
ready = epoll_pwait(epfd, &events, maxevents, timeout, &sigmask);
.EE
.in
.PP
is equivalent to
.I atomically
executing the following calls:
.PP
.in +4n
.EX
sigset_t origmask;

pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
ready = epoll_wait(epfd, &events, maxevents, timeout);
pthread_sigmask(SIG_SETMASK, &origmask, NULL);
.EE
.in
.PP
The
.I sigmask
argument may be specified as NULL, in which case
.BR epoll_pwait ()
is equivalent to
.BR epoll_wait ().
.\"
.SS epoll_pwait2()
The
.BR epoll_pwait2 ()
system call is equivalent to
.BR epoll_pwait ()
except for the
.I timeout
argument.
It takes an argument of type
.I timespec
to be able to specify nanosecond resolution timeout.
This argument functions the same as in
.BR pselect (2)
and
.BR ppoll (2).
If
.I timeout
is NULL, then
.BR epoll_pwait2 ()
can block indefinitely.
.SH RETURN VALUE
On success,
.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.
On failure,
.BR epoll_wait ()
returns \-1 and
.I errno
is set to indicate the error.
.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 either (1) any of the
requested events occurred or (2) 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_wait ()
was added to the kernel in version 2.6.
.\" To be precise: kernel 2.5.44.
.\" The interface should be finalized by Linux kernel 2.5.66.
Library support is provided in glibc starting with version 2.3.2.
.PP
.BR epoll_pwait ()
was added to Linux in kernel 2.6.19.
Library support is provided in glibc starting with version 2.6.
.PP
.BR epoll_pwait2 ()
was added to Linux in kernel 5.11.
.SH CONFORMING TO
.BR epoll_wait (),
.BR epoll_pwait (),
and
.BR epoll_pwait2 ()
are Linux-specific.
.SH NOTES
While one thread is blocked in a call to
.BR epoll_wait (),
it is possible for another thread to add a file descriptor to the waited-upon
.B epoll
instance.
If the new file descriptor becomes ready,
it will cause the
.BR epoll_wait ()
call to unblock.
.PP
If more than
.I maxevents
file descriptors are ready when
.BR epoll_wait ()
is called, then successive
.BR 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.
.PP
Note that it is possible to call
.BR epoll_wait ()
on an
.B 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.
.SS C library/kernel differences
The raw
.BR epoll_pwait ()
and
.BR epoll_pwait2 ()
system calls have a sixth argument,
.IR "size_t sigsetsize" ,
which specifies the size in bytes of the
.IR sigmask
argument.
The glibc
.BR epoll_pwait ()
wrapper function specifies this argument as a fixed value
(equal to
.IR sizeof(sigset_t) ).
.SH BUGS
In kernels before 2.6.37, a
.I timeout
value larger than approximately
.I LONG_MAX / HZ
milliseconds is treated as \-1 (i.e., infinity).
Thus, for example, on a system where
.I sizeof(long)
is 4 and the kernel
.I HZ
value is 1000,
this means that timeouts greater than 35.79 minutes are treated as infinity.
.SH SEE ALSO
.BR epoll_create (2),
.BR epoll_ctl (2),
.BR epoll (7)